Lost circulation material blend offering high fluid loss with minimum solids

Abstract

A lost-circulation-material includes: rubber particulates; crosslinked-copolymer particulates; cellulosic particulates; an expandable clay; a hydrogel material, and optionally a thickener. The average size of the particulates is selected so that each of the rubber particulates, crosslinked-copolymer particulates, and cellulosic particulates differ by at least a factor of 2 in diameter from one another. The components are admixed with fresh water having low salinity to form a remedial plug. In one embodiment, the copolymer particulates are pre-formed beads; the rubber particulates are finely ground butyl rubber compound; the expandable clay includes smectite; the hydrogel includes a partially hydrolyzed polyacrylamide compound; and the cellulosic particulates are wood flour. Methods of forming a slurry for plugging lost-circulation zones, and methods of treating wellbores with the slurry, are described.

Description

FIELD OF THE INVENTION

[0002] This invention relates to compositions and methods for fast acting and cost effective remediation of severe drilling fluid loss during drilling operations.

BACKGROUND OF THE INVENTION

[0003] Drilling fluids, or drilling muds as they are sometimes called, are slurries used in the drilling of wells in the earth for the purpose of recovering hydrocarbons and other fluid materials. Drilling fluids have a number of functions, the most important of which are: lubricating the drilling tool and drill pipe which carries the tool, removing formation cuttings from the well, counterbalancing formation pressures to prevent the inflow of gas, oil or water from permeable rocks which may be encountered at various levels as drilling continues, and holding the cuttings in suspension in the event of a shutdown in the drilling and pumping of the drilling fluid. The wellbore typically penetrates numerous potentially productive formations at different depths, and the drilling mud advantageously provides sufficient hydrostatic pressure to keep the formation fluids from entering the borehole. Drilling mud is typically a thixotropic composition comprising suspended solids in a water, oil, or stable water-oil emulsion. Generally, polymers, clays, and/or emulsifiers are present to provide the thixotropic properties. The suspended solids increase mud weight, and typically included barite, where the amount of barite is specified by mud weight in pounds per gallon (“ppg”). Water is about 8 ppg, but generally mud is weighted between 9 ppg and 21 ppg to so that the hydrostatic pressure exerted by the mud exceeds subsurface formation pressure. For example, a 14 ppg mud has 21 volume % barite. When the mud weight exerts the same pressure as exists in a subsurface formation, the mud weight is said to be balanced, and there is little fluid exchange between the formation and the drilling mud in the borehole. As the pressure of the next formation to be encountered is somewhat uncertain, drillers use over-balanced mud, such that the pressure generated in the borehole by the column of mud exceeds formation pressure. For a drilling fluid to perform these functions and allow drilling to continue, the drilling fluid must stay in the borehole. Most wells are drilled with the intent of forming a filter cake of varying thickness on the sides of the borehole. The primary purpose of the filter cake is to reduce the large losses of drilling fluid to the surrounding formation, in spite of the fact that the pressure in the borehole exceeds the formation pressure.

[0004] Generally, mud is circulated in a well, e.g., being pumped down a drill string in the well and flowing back up between the sides of the borehole and the outside of the drill string.

[0005] Unfortunately, formation conditions are frequently encountered which may result in unacceptable losses of drilling fluid to the surrounding formation despite the type of drilling fluid employed and filter cake created. In such a case the composition of the drilling mud may be altered to increase the ability of the mud to form a low-permeability filter cake. Less frequently, undesirable formation conditions are encountered in which substantial amounts or, in some cases, practically all of the drilling fluid may be lost to the formation. Drilling fluid can leave the borehole through large or small fissures or fractures in the formation or through a highly porous rock matrix surrounding the borehole. As drilling mud can continue to be pumped down the drill string, but does not flow out of the annulus between the drill string and the borehole, this condition is called lost circulation. As the amount of mud in the wellbore declines, the hydrostatic pressure exerted by the bud to counterbalance formation pressures can become insufficient, e.g., the mud becomes under-balanced, at which time formation fluids enter the borehole. Unless this condition can be rapidly and completely controlled, a catastrophic blow-out may occur, causing huge loss of property and an extremely unsafe working environment.

[0006] There is at such times a great emphasis on speed to remedy the problem, as the amount of drilling mud available to be pumped down a wellbore is for all practical circumstances limited to the mud stored on location. When such events occur, a variety of different substances are pumped down well bores in attempts to reduce the large losses of drilling fluid to fractures and the like in the surrounding formation. Under normal conditions, the most cost-effective method for preventing fluid loss to the formation is to add lost circulation materials to the drilling fluid, though some methods are, as described below, much more complicated. Listed below are exemplary treating materials suggested in the art.

[0007] Different forms of cellulose are the preferred materials employed, as described in for example U.S. Pat. No. 4,498,995. Some cellulosic substances which have been pumped into well bores to control lost circulation are: almond hulls, walnut hulls, bagasse, dried tumbleweed, and paper. This patent also describes adding coarse and fine mica, and even pieces of rubber tires. Unfortunately, such treatments are not always successful.

[0008] U.S. Pat. No. 3,082,823 describes a process that is employed to close off large lost circulation problems that is referred to as gunk squeeze, in which a quantity of a powdered bentonite is mixed in diesel oil and pumped down the well bore. Water injection follows the bentonite and diesel oil. If mixed well, the water and bentonite will harden to form a gunky semi-solid mass, which will reduce lost circulation. Problems frequently occur in trying to adequately mix the bentonite and water in the well, as the bentonite must also be kept dry until it reaches the desired point in the well.

[0009] Many of the methods devised to control lost circulation involve the use of a water expandable clay such as bentonite which may be mixed with another ingredient to form a viscous paste or cement. U.S. Pat. No. 2,890,169 describes a process for remedying lost circulation using a fluid made by forming a slurry of bentonite and cement in oil. The slurry is mixed with a surfactant and water to form a composition comprising a water-in-oil emulsion having bentonite and cement dispersed in the continuous oil phase. As this composition is pumped down the well bore, the oil expands and flocculates the bentonite which, under the right conditions, forms a filter cake on the well bore surface in the lost circulation area. Hopefully, the filter cake will break the emulsion causing the emulsified water to react with the cement to form a solid coating on the filter cake. But such a complex process can easily go wrong, for example when the process is interrupted with the gunk in the drill string, and the cement sets up in the drill string. U.S. Pat. No. 4,261,422 describes the use of an expandable clay such as bentonite or montmorillonite which is dispersed in a liquid hydrocarbon for injection into the well. After injection, the bentonite or montmorillonite will expand upon contact with water in the formation. Thus, it is hoped that the expanding clay will close off water producing intervals but not harm oil producing intervals.

[0010] U.S. Pat. No. 3,448,800 discloses another lost circulation treatment method wherein a water soluble polymer is slurried in a nonaqueous medium and injected into a well. An aqueous slurry of a mineral material such as barite, cement or plaster of paris is subsequently injected into the well to mix with the first slurry to form a cement-like plug in the well bore. Again, this is a costly, complicated process.

[0011] U.S. Pat. No. 3,078,920 suggests use of polymethacrylate in non-aqueous solvent to delay expansion until contacting formation water. It is known to mix an expandable clay, such as bentonite, with the proper amount of water in the presence of a water-soluble polymer which will flocculate and congeal the clay to create a much stronger paste can be made than when bentonite is merely mixed with water. U.S. Pat. No. 3,909,421 suggests using bentonite and water-soluble polymer, i.e., made by blending a dry powdered polyacrylamide with bentonite followed by mixing the powder blend with water.

[0012] U.S. Pat. Nos. 4,503,170, 4,475,594, 4,445,576, 4,442,241 and 4,391,925 suggest putting Wyoming bentonite in oil emulsion with surfactant and with polymers such as polyacrylamide. Nonionic or un-hydrolyzed polyacrylamides with molecular weight greater than about one million are preferred. U.S. Pat. No. 4,128,528 claims a powdered bentonite-polyacrylamide thickening composition prepared by mixing a water-in-oil emulsion with bentonite to form a damp, free-flowing powdered composition which rapidly forms a viscous, stiff material when mixed with water. A similar method is disclosed in U.S. Pat. No. 3,078,920 which uses a solution of polymerized methacrylate dissolved in a nonaqueous solvent such as acetic acid, acetic anhydride, propionic acid and liquid aliphatic ketones such as acetone and methyl-ethyl ketone. The methacrylate will expand upon contact with formation water in the water-producing intervals of the well.

[0013] U.S. Pat. No. 4,172,031 suggests polymers that swell in volume upon absorption of oil, e.g., polymers of styrenes, PVC/vinylacetate, vinylidene chloride/acrylonitrile, methylmethacrylate and ethylacrylate, and styrene and divinylbenzene copolymers. U.S. Pat. No. 6,518,224 suggests adding crumb rubber from 1-400 microns and hydrocarbon to swell same, cellulose fiber, mica, and calcite as bridging material.

[0014] U.S. Pat. Nos. 4,503,170; 4,475,594; 4,445,576; 4,442,241 and 4,391,925 teach the use of a water expandable clay dispersed in the oily phase of a water-in-oil emulsion containing a surfactant to stabilize the emulsion and a polymer dispersed in the aqueous phase. When the emulsion is sheared, it breaks and a bentonite paste is formed which hardens into a cement-like plug. The patent discloses the use of such polymers as polyacrylamide, polyethylene oxide and copolymers of acrylamide and acrylic or methacrylic acid.

[0015] U.S. Pat. No. 4,633,950 discloses the use of oil swelling polymers to reduce lost circulation in drilling fluids. In this patent, the polymers are introduced in an aqueous solution to prevent absorption of the hydrocarbon fluid until the polymers reach the well head. A group of oil absorbent polymers is disclosed in U.S. Pat. Nos. 4,191,813; 4,263,407; 4,384,095 and 4,427,793. U.S. Pat. No. 4,191,813 discloses lightly crosslinked copolymers containing at least 40% by weight of vinylbenzyl chloride, the balance of monomers, if any, comprising a major portion of aromatic monomers, with the copolymer being crosslinked in a swollen state by a Lewis acid catalyst. The preferred comonomers are one or more of styrene, divinylbenzene and acrylonitrile. U.S. Pat. No. 4,263,407 discloses similar copolymers wherein the copolymer is post-crosslinked in a swollen state in the presence of a Friedel-Crafts catalyst with a crosslinker selected from a polyfunctional alkylating or acylating agent and a sulfur halide.

[0016] Another group of highly hydrocarbon absorbent copolymers is disclosed in U.S. Pat. Nos. 4,384,095 and 4,427,793. They describe a crosslinked linear addition copolymer which contains repeating units of vinylbenzyl alcohol and at least one other alpha, beta-monoethylenically unsaturated monomer different from vinylbenzyl alcohol, wherein the vinylbenzyl alcohol units comprise about 0.5% to about 20% by weight of the linear polymer. The preferred comonomers are styrene, methylmethacrylate, vinyltoluene and vinylpyridine. The copolymers disclosed in all four of these patents absorb from two to ten times their weight in hydrocarbons and may swell up to ten times their original volume.

[0017] Oleophilic polymers for separating oil from water which show significant swelling in volume upon absorption of oil are described in U.S. Pat. No. 4,172,031. These polymers include polymers of styrenes and substituted styrenes, polyvinyl chloride copolymers of vinylchloride such as a copolymer of 60 wt % vinylchloride and 40 wt % vinylacetate, polymers and copolymers of vinylidene chloride and acrylonitrile, and acrylic polymers such as polymers of methylmethacrylate and ethylacrylate, styrene and divinylbenzene copolymers and alkyl styrene polymers and copolymers. The reference discloses that these polymers show significant swelling in volume upon absorption of oil.

[0018] While the above inventions purport to be effective in reducing loss of drilling fluids, there continues to be a need for effective and inexpensive additives for well bore fluids which can prevent or stop the loss of the fluids into the subterranean formation, which can be readily and easily prepared and pumped into the borehole, and which can be used in a variety of wellbore conditions including depth and temperature.

SUMMARY OF THE INVENTION

[0019] This invention includes a lost circulation material, hereafter a “dry plug composition”, that is 1) easy to mix and pump, 2) that is useful in for a wide variety of fluid loss situations such as high permeability zones, fractures, micro-fractures, and induced fractures, 3) that provides a high strength plug having sufficient integrity without the use of cross-linkers or cements that may set up in the drill string or injection tubing, and 4) is useful at a wide variety of depths and wellbore temperatures. Advantageously, the dry plug composition is stored in a dry state as a single “dry mix” composition or “dry plug” composition, and as the need arises the dry plug composition is admixed with water or a water-based composition to form an pumpable, injectable “remedial plug”. Also encompassed in this invention are methods to treat wells having lost circulation, or wells expecting to encounter lost circulation. The remedial plug may be used to remedy lost circulation in an open hole, and may be used in cased holes for sealing perforations or casing leaks. The remedial plug, once mixed with water as described infra, is compatible with all fluid systems, including water-based muds, briny muds, oil-based muds, synthetic muds, brines, oil, and the like. Without being bound by theory, we believe one reason is that certain components that would otherwise be reactable with various fluids are first admixed into a low-salinity water, and is saturated/hydrated therewith before being exposed to salty muds or brines.

[0020] In one embodiment the invention is a dry lost-circulation-composition useful for preparing a slurry to remediate lost circulation in a wellbore, the slurry comprising: rubber particulates; copolymer particulates; cellulosic particulates; between about 2% to about 12% by weight of expandable (water-swelling) clays; and between about 1% and about 10% by weight of hydrogel. In a variant of this embodiment, the expandable clay comprises Wyoming bentonite (alternatively, sodium montmorillinite), the rubber particulates comprise butyl rubber, and the copolymeric particulates are formed from a highly crosslinked copolymer compound and have a weight average size smaller than 12 mesh, for example between 16 and 40 mesh. In another variant of this embodiment, the rubber, e.g., butyl rubber, and copolymer e.g., styrene/divinylbenzene, particulates are present in a total amount between about 40% and about 80% by weight. The cellulosic particulates are present in an amount between about 10% and about 40% by weight; and preferably both the cellulosic and butyl rubber particulates have a weight average size smaller than about 100 mesh. In yet another variant of this embodiment, the polymeric material particulates are present in an amount between about 40% and about 80% by weight; the cellulosic particulates are present in an amount between about 10% and about 40% by weight; the expandable clay is present in an amount between about 2% and about 10% by weight and comprises Wyoming bentonite; and the hydrogel is present in an amount between about 2% and about 8% by weight and comprises partially hydrolyzed polyacrylamide. In yet another variant of every dry mix embodiment, the lost-circulation material further comprises 0.2% to 3%, e.g., 0.5% to 1.5% of a hydrophyllic modified cellulosic material, eg., polyamionic cellulose. In yet another variant of this embodiment, based on the weight of the dry lost-circulation-composition, the rubber particulates are present in an amount between about 35% and about 55% by weight, and the copolymer particulates between about 10% and about 30% by weight; the cellulosic particulates are present in an amount between about 15% and about 20% by weight; the expandable clay comprise smectite clays present in an amount between about 3% and about 9% by weight; and the hydrogel comprises partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof.

[0021] In a preferred variant of this embodiment, the rubber-based particulates have a weight average size between about 300 mesh and about 600 mesh and comprise between about 40% and about 50% by weight ground butyl rubber, and the copolymer material is present at between about 15% and about 20% by weight and are copolymer particulates having a size between about 12 and about 100 mesh, preferably between 16 and 40 mesh; the cellulosic particulates have a weight average size between about 300 mesh and about 600 mesh, and are present in an amount between about 15% and about 20% by weight; the expandable clay comprise smectite clays having a weight average size smaller than 300 mesh, and are present in an amount between about 2% and about 8% by weight; and the hydrogel are slightly cross-linked in soluble powder between 60 and 400 mesh, and comprises polyacrylamide, poly(acrylamide-acrylate) copolymers, poly(acrylamide-methacrylate) copolymers, poly(acrylamide-acrylate-methacrylate) copolymers, partially hydrolyzed versions of the above, or mixture thereof. A preferred variant of all of the above dry lost-circulation-compositions requires the dry lost-circulation-composition be substantially free of active crosslinkers and cements. In preferred embodiments, a small amount of a thickener, for example polyanionic cellulose, poly(meth)acrylate, or water-soluble gum, e.g., quar gum, in powder or microcrystalline form, for example at about 0.2% to 3% by weight, or between about 0.5% and 1.5% by weight.

[0022] Another aspect of this invention is a remedial plug comprising a slurry of between 40 pounds and 120 pounds of a dry mix composition per barrel of water having less than 1500 ppm chlorides, wherein the dry mix composition comprises: rubber particulates; copolymer particulates; cellulosic particulates; between about 2% to about 10% by weight of expandable clays; and between about 1% and about 10% by weight of hydrogel, wherein the weight is based on the dry weight of the dry mix composition. The ingredients can be added separately or together, but are preferably single dry mix compositions. The dry mix composition can be any of the dry lost-circulation-compositions described. In a preferred variant of this embodiment, the slurry comprises between about 50 pounds and about 100 pounds of the dry mix composition per barrel of water, and the dry mix composition comprises: polymeric material particulates including both ground butyl rubber and copolymer particulates, present in an amount between about 40% and about 80% by weight; cellulosic compound particulates which are present in an amount between about 10% and about 40% by weight; wherein the weight is based on the dry weight of the dry mix composition, and wherein all cellulosic particulates have a weight average size smaller than about 200 mesh. In yet another variant of this embodiment, the slurry comprises between about 60 pounds and about 80 pounds of the dry mix composition per barrel of water. In a preferred variant of this embodiment, the water has less than 1000 ppm chlorides. In another variant of this embodiment, the remedial plug further comprises between about 0.1 pounds and about 12 pounds of bentonite that is hydrated and dispersed per barrel of water and optionally barite, prior to mixing in the dry mix composition. In a preferred variant, the remedial plug further comprises between 0.2 and 3 pounds of a modified polyanionic cellulose or other water-soluble polymer thickener. In another variant of this embodiment, the remedial plug further comprises between about 8 pounds and about 12 pounds of bentonite that is hydrated and dispersed per barrel of water, and also further comprises barite particulates in an amount such that the density of the remedial plug is between 10 pounds per gallon and 20 pounds per gallon. In a preferred embodiment, the remedial plug is weighted to within 1 ppg of the weight of the fluid in the well bore. In a preferred variant of the above-described embodiments, the remedial plug slurry comprises between about 50 pounds and about 100 pounds of the dry mix composition per barrel of water, and wherein the dry mix composition comprises: rubber particulates present at between about 35% and about 55% by weight, e.g., ground butyl rubber and between about 10% and about 30% by weight of copolymer particulates, where the average diameter of the copolymer particulates is at least twice the average diameter of the rubber particulates, based on the weight of the dry mix composition; cellulosic particulates present in an amount between about 15% and about 20% by weight, based on the weight of the dry mix composition; the expandable clay comprise smectite clays present in an amount between about 4% and about 10% by weight, based on the weight of the dry mix composition; and the hydrogel particles which are present in an amount between 3% and 10% by weight and comprises one or more of partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof. In another preferred variant of the above-described embodiments, the slurry is substantially free of active crosslinkers and cements. Note, that while the dry mix and the remedial; plug are advantageously substantially free of crosslinkers, the rubber particles, the copolymer beads, and even the hydrogel may each be at least partially cross-linked.

[0023] Another aspect of this invention is a method of treating a wellbore that has undesirable fluid loss therefrom caused by leakage of fluid from the wellbore into a subsurface formation responsible for the undesirable fluid loss, said method comprising: providing a remedial plug comprising a slurry of between 40 pounds and 120 pounds of a dry mix composition per barrel of water having less than 2000 ppm, preferably less than 1500 ppm, of chlorides, for example having from 50 to 500 ppm chlorides, wherein the dry mix composition comprises: polymeric material particulates; cellulosic particulates; between about 2% to about 10% by weight of expandable clays; and between about 1% and about 10% by weight of hydrogel, wherein the weight is based on the dry weight of the dry mix composition; and pumping the remedial plug down tubing or a drill string, wherein the remedial plug is pumped from the tubing or drill string into the wellbore at a depth near the subsurface formation responsible for the undesirable fluid loss, so that at least a portion of the remedial plug penetrates and thereby reduces the fluid loss to the subsurface formation responsible for the undesirable fluid loss. In a variant of this embodiment, the slurry comprises between about 50 pounds and about 100 pounds of the dry mix composition per barrel of water, and the dry mix composition comprises: polymeric material particulates present in an amount between about 40% and about 80% by weight and comprising ground butyl rubber, copolymer particulates, or mixture thereof; and cellulosic particulates present in an amount between about 10% and about 40% by weight; wherein the weight is based on the dry weight of the dry mix composition, and wherein all particulates except the particulates formed from copolymer beads have a weight average size smaller than about 200 mesh. In another variant of this embodiment, the slurry comprises between about 60 pounds and about 80 pounds of the dry mix composition per barrel of water. In a preferred variant of this embodiment, the water has less than 1000 ppm chlorides, for example between 50 and 500 ppm chlorides. In an alternate variant of this embodiment, the slurry further comprises between about 0.1 pounds and about 12 pounds of bentonite, e.g., sodium montmorillinite, that is hydrated and dispersed per barrel of water in addition to the aforementioned dry mix composition, for example between about 8 pounds and about 12 pounds of bentonite that is hydrated and dispersed per barrel of water, and optionally also further comprises barite particulates, e.g., in an amount such that the density of the remedial plug is between 10 pounds per gallon and 20 pounds per gallon and is preferably within 1 ppg of the weight of fluid present in the well. In a preferred variant of the above-described embodiments, the slurry comprises between about 50 pounds and about 100 pounds of the dry mix composition per barrel of water, and the dry mix composition comprises: polymeric material particulates comprising between about 35% and about 55% by weight ground butyl rubber and between about 10% and about 30% by weight copolymer particulates, based on the weight of the dry mix composition; cellulosic particulates present in an amount between about 15% and about 20% by weight, based on the weight of the dry mix composition; the expandable smectite clays present in an amount between about 6% and about 10% by weight, based on the weight of the dry mix composition; and the hydrogel particles present in an amount between about 3% and about 12% based on the dry weight of the dry mix composition, wherein said hydrogel comprising, for example, substantially insoluble water-absorbing polymine particles partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof. In a preferred embodiment, the hydrogel is pressure sensitive, and absorbs at least 10 times its bulk volume of water at zero pressure. In another preferred variant of the above-described embodiments, the remedial plug is substantially free of active crosslinkers and cements.

[0024] The lost-circulation-material of the invention comprises a variety of ingredients, including: one or more polymeric compounds; a cellulosic solid material; a hydrolyzed (expanded) clay; partially or a hydrogel material, and optionally a modified cellulose. The hydrogel component in one embodiment is a stand-alone powder. Alternatively, the hydrogel partially or substantially coats the expandable clays and/or one or more of the other particulate components. The encapsulation may occur during the mixing of the dry mix with water. In one preferred embodiment of the invention, the dry plug composition comprises: 1) a polymeric material, e.g., a copolymer compound that is ground or pre-formed into beads, a ground butyl rubber compound, or preferably a mixture thereof; 2) an expandable clay, 3) hydrogel particles comprising partially hydrolyzed polyacrylamide compound; and 4) powdered cellulosic material. The composition may further comprise a minor amount of a water-soluble polymer or gum. A mixture of sizes and types of lost-circulation materials is more effective than a single, un-graded material. However, it is generally preferred that at least some particulate components of the remedial plug composition be smaller than 100 mesh and preferably smaller than 200 mesh.

DETAILED DESCRIPTION OF SELECTED EMBODIMENTS

[0025] As described herein, the dry plug composition is described in terms of weight percent of dry components. Mesh sizes are approximate average mesh sizes for U.S. Standard sieves, i.e., the largest mesh size (in 100 mesh increments) where at least half of the particulates of interest are caught. U.S. Standard sieve can be roughly correlated to particle diameter, wherein 100 mesh is about 75-150 microns, 200 mesh is about 50-75 microns, 300 mesh is about 35-50 microns, 400 mesh is about 30-35 microns, 500 mesh is about 22-30 microns, 600 mesh is about 22 microns, and 800 mesh is about 20 microns. A weight average size is the size wherein half the weight of the particles are larger than the weight average size, and half the weight of particles are smaller than the weight average size. Generally, while a size may be given over a broad range, it is preferred that the actual particle size distribution be narrow, e.g., with at least 80% by weight of the particles having a size withing a factor of three, preferably within a factor of two, of the weight average size.

[0026] In one embodiment, the invention comprises, on a dry basis: between about 40% and about 80% of copolymer particulates, and rubber particulates of different average particle sizes (by at least 50%); between about 10% to about 40% of one or more cellulosic particulates, e.g., wood flour; between about 2% to about 10% of one or more expandable clays, e.g., Wyoming bentonite; and between about 1% and about 10% hydrogel, e.g., partially hydrolyzed polyacrylamide.

[0027] The remedial plug is advantageously substantially free of active crosslinkers and cements which can set up in undesired locations despite high wellbore temperatures and long pumping time that may be encountered in deep holes. This allows a longer period of time to pump, allowing the plug to be pumped for example down a drill string or down tubing. As the plug has a long exposure window prior to setting up, the remedial plug can be pumped through MWD and Mud motors.

[0028] The remedial plug is advantageously substantially free of hydrocarbons, heavy metals, and other environmentally deleterious materials. The dry mix and the remedial plug each preferably contain less than 100 ppm environmentally undesirable metals including lead, chromium, copper, cadmium, mercury, and the like, and less than 500 ppm of aromatic hydrocarbons. Indeed, the preferred dry plug composition is substantially environmentally benign.

[0029] In one embodiment the lost circulation material may be used in an open hole as a preventative against lost circulation. The remedial plug in this instance can either be a spotted and squeezed as a remedial plug, or if a zone is being drilled into wherein the drilling engineer suspects a lost circulation event will be experienced, in another embodiment the remedial plug can be formed and then this plug is subsequently admixed with drilling mud, wherein after admixing with mud the amount of lost circulation material is between about 40 to about 120 pounds (dry basis) per barrel of mud. The low concentration end will be less effective at bridging fractures, but it will also not be prone to setting up than will remedial plugs having higher concentrations of components.

[0030] Method of use—The dry plug composition is admixed with water or a water-based composition as described infra to form a “remedial plug”, wherein it is the remedial plug that is injected into the wellbore to remediate a fluid loss problem. The remedial plug may be used for open hole remediation of lost circulation and/or for open hole preventative against lost circulation. The remedial plug may be used for cased holes for sealing perforations or casing leaks. Advantageously, as it may not be practicable to perform detailed analysis of a lost-circulation problem before action is required, it is particularly advantageous to have pre-mixed “single bag” dry plug composition that is able to be rapidly prepared and easily prepared, that is amenable to almost all down-hole conditions including temperature and the types of fluids in the wellbore, that can be pumped as a single batch, and that does not have a narrow time window wherein the material must be placed before it becomes set, and that plugs lost circulation zones including high permeability sandstones and dimestones and small fractures. The compositions of the present invention meet all these objectives.

[0031] Typically, in its application, the dry plug composition described in one embodiment of the invention is mixed with fresh water to form a slurry. The mixture ratio is generally described in terms of pounds per barrel (“ppb”), where the barrel is 55 gallons. Generally, the dry plug composition is admixed with fresh water using conventional equipment, e.g. utilizing a mud hopper. Such equipment is always available during drilling, but may not be available in subsequent treatments, e.g., plugging perforations or other breaches in the casing, for example. In such case virtually any mixer can be employed. Many preferred embodiments of this invention are a dry pre-mix, wherein most or preferably every component except water is present in the dry ready-to-mix formulation, and the dry formulation composition is free-flowing and hydrophilic so that adequate mixing can be obtained in a reasonable time using for example a standard chemical treatment truck having a tank and a circulating pump.

[0032] While all of the dry mix compositions of the invention are advantageously mixed with fresh water (e.g., between about 50 and 400 ppm chlorides), any water with less than 3000 ppm chlorides is useful, water with less than 1500 ppm chlorides is preferred, and any water with less than 1000 ppm chlorides is more preferred. Additionally, it is preferred that the water used to mix with the dry mix comprise less than about 400 ppm of dissolved alkaline earth metals, e.g., calcium and magnesium. While the dry plug composition can of course be admixed with saltier waters and such a plug will remediate many lost circulation problems, if the water is saltier than 3000 ppm chlorides, the hydrogel functionality is progressively impaired and the plug performance is likewise impaired. Additionally, a remedial plug can be formed by mixing at least the most sensitive dry ingredient (hydrogel) with fresh water, then admixing other ingredients and salty brines wherein the final salinity of the water may be greater than 3000 ppm.

[0033] The dry plug composition is added to the water in an amount between about 40 ppb and 120 ppb, preferably between about 50 ppb and about 100 ppb, more preferably between about 60 ppb and about 85 ppb. One potential problem to be addressed by the drilling engineer is there must be sufficient water to hydrate/saturate the hydrogel, clay and other water-absorbing components of the composition. Otherwise, there is an increased potential for gelling or pasting up, leading to problems such as plugging down-hole tools. Therefore, the lower fraction of the recited range of amounts of dry plug composition are added per barrel of water are preferred in embodiments of the invention containing the higher ranges of the amount of clay and hydrogel. For example, in one above-described embodiment the dry plug composition comprises between 2% to 10% of Wyoming bentonite and between 1% to 10% hydrogel. If the dry plug composition comprises about 2% of Wyoming bentonite and about 1% hydrogel, it may be beneficial to mix the dry plug composition with water at 120 ppb or more. Conversely, if the dry plug composition comprises about 10% of Wyoming bentonite and about 10% hydrogel, it may be beneficial to mix the dry plug composition with water at 80 ppb or less.

[0034] In some embodiments a more firmly gelled remedial plug may be desired. In such a case, some, e.g., between about 1 ppb and about 12 ppb of bentonite can be admixed with the water. This composition is mixed until the clay is hydrated and dispersed (often called “allowed to yield”). The dry plug composition is added in an amount between about 30 ppb and 120 ppb, preferably between about 50 ppb and about 100 ppb, more preferably between about 60 ppb and about 80 ppb. Advantageously the components of the dry mix are added simultaneously. Admixing all ingredients at one time is encouraged, as some components may have undesirable reactions, e.g., and getting complete admixing is important or the unhydrated and undispersed bentonite hydrogel may form a solid composition that has increased probability of plugging down-hole tools and increased probability of setting up before the plug is properly placed.

[0035] It may be beneficial to substantially balance (at least within 1 ppg) the weight of the remedial plug to the weight of the drilling mud or other fluid in the wellbore, especially if pumping the remedial plug down a large interior diameter drill string. One reason is that, if the remedial plug is substantially lighter than the drilling fluid, portions of the remedial plug may be dispersed in and float away from the remainder of the remedial plug during transit downhole, creating difficulty in spotting the plug and in reliably treating the entire interval. Therefore, in another embodiment, a weighted remedial plug is formed as follows. Bentonite is added to fresh water in an amount between about 4 ppb and about 16 ppb, typically around 10 ppb. This composition is mixed until the clay is hydrated and dispersed (often called “allowed to yield”), thereby forming a thixotropic composition. Suspended solids, e.g., barite, is added until the desired weight is achieved. The dry plug composition is added in an amount between about 30 ppb and 110 ppb, preferably between about 50 ppb and about 90 ppb, more preferably between about 60 ppb and about 80 ppb. Generally, the barite and bentonite in the base material complement the plugging capability of the dry plug composition, and as a result less dry plug composition is needed on a ppb basis for a remedial plug made by admixing a dry mix composition into mud as opposed to that needed to get the same plugging effect when the dry mix id admixed with water. Empirically, a 12 ppg base having 60 ppb of dry plug composition will in many situations have a lost circulation remedial performance equivalent to 80 ppb of dry plug composition admixed in water.

[0036] In yet another embodiment of the invention, a weighted remedial plug can be more quickly prepared by admixing a predetermined quantity of available water-based drilling mud, optionally but preferably also a quantity of fresh water, and the dry plug composition. Again, there must be sufficient water to hydrate the hydrogel, and the water must be sufficiently fresh. If the water comprises at least one half by volume of the fluid, it is advantageous to mix the dry plug composition with the water prior to adding the mud. The volume ratio of mud to water can range 100:1 to 1:100, but to obtain a plug that approximately matches the mud weight it is advantageous to have a volume ratio of mud to water between about 3:1 to about 1:2.

[0037] After mixing, the remedial plug is compatible with all fluid systems, including water-based muds, briny muds, oil-based muds, synthetic muds, and the like. For example, pre-mixed lost circulation made from water may be used in a wellbore containing oil-based drilling mud. By compatible with we mean the premixed lost circulation plug can be injected into a drill pipe or tubing having any of the aforesaid compositions (muds, brines, and the like) therein, and the premixed lost circulation plug will not have undesired reactions with those fluids 1) in a time frame where placing the plug are impeded, and 2) that adversely affects the performance of the lost circulation plug. Without being bound by theory, we believe one reason is that certain components that would otherwise be reactable with various fluids are first dissolved in a fresh water and absorb/hydrate with water until near saturation/hydration, and subsequent absorption or reaction with brines and other materials is reduced because of this saturation and also by the isolation of the material by the bounds of the remedial plug itself. That is, the pre-mixing with fresh water provides a material that carries with it its own “spacer”, i.e., a boundary layer that prevents or delays interaction of reactable components in the remedial plug with various components in the drilling muds formation fluids and the like.

[0038] The remedial plug of the current invention has a window of time wherein the plug should be spotted (placed) in the wellbore adjacent to the thief zone and advantageously subsequently squeezed therein. One advantage of the compositions of this invention is that they have a reduced tendency to set up and plug a tubing or drill string. Additionally, the window of time prior to “setting up” is long. For example, at ambient temperature, the remedial plug is “pumpable”, i.e., has not set up or otherwise changed to become inoperative, for about 8 to 10 hours (with some margin of safety). At higher temperatures such as are encountered downhole, the time is reduced. For a preferred embodiment of this invention, at a temperature between 100 degrees F. and 150 degrees F., the remedial plug is pumpable for about 6 to 8 hours. At a temperature between 150 degrees F. and 200 degrees F., the remedial plug is pumpable for about 4 to 6 hours. At a temperature between 200 degrees F. and 400 degrees F., the remedial plug is pumpable for about 3 to 5 hours.

[0039] Generally, when lost circulation is noted, it is advisable before making the remedial plug, the location of the loss zone or thief zone should be determined. The remedial plug should then be prepared, wherein the volume of the remedial plug should be between 20 and 60% excess of the volume of the wellbore over which the thief zone is believed to extend. The remedial plug then spotted as close to the thief zone as possible, preferably extending over the thief zone. Advantageously, the operator may then wish to pull above pill 200 to 250 feet, at the same time pumping down drilling mud or additional pill material, to make sure all of the remedial plug is displaced from the drill string or tubing. Begin at a low pump rate to start circulation and increase pump rates as well bore condition allow. If no returns is accomplished while spotting the remedial plug, allow well bore to remain static for 4 to 8 hours and re-treat as necessary. It may be advantageous in such a case to add between 1 and 5 ppb of larger diameter, e.g., between 40 and 160 mesh, cellulosic material in addition to the components listed herein into the subsequent remedial plug. Beneficially, as re-circulation is achieved, the Hydrill can be closed in order to squeeze pill into formation, applying for example a 200 psi to 300 psi squeeze to force at least a portion of remedial plug further into the thief zone. If full re-circulation is accomplished while spotting the remedial plug, a squeeze may be unnecessary, and the operator can circulate mud for 4 to 8 hours.

[0040] The individual components of the dry plug composition are now described in greater detail.

[0041] Expandable clay—One component of the dry plug composition is expandable clay. The principal purpose of the clay is to help suspend the other components into the plug. The clay material also has an important plugging function. Finally, the clay material will on long term exposure to reservoir fluids further expand and form a cojoined mass, increasing the plug strength and integrity, i.e., ability to prevent undesired fluid flow through or around the plug. Generally, when water contacts a hydrophilic clay such as Wyoming bentonite, present in sufficient concentration, a strong and/or pasty material forms. Prior art encapsulates bentonite in for example oil, as bentonite does not hydrate in oil, but when water contacts a bentonite/oil mixture, a solid strong material forms. In the instant invention, the formulation in fresh water, the low concentration of expandable clay, and the other constituents (particularly the saturated hydrogel) keep individual bentonite particles from binding together, at least for a time sufficient to pump the pill.

[0042] Clays useful in this invention include any water-expandable clay, especially the smectite clays, more particularly the Wyoming bentonite-type, and expandable (e.g., sodium) montmorillonite clays. The clay need not be natural, e.g., smectite Na fluorohectorite (Na0.3Mg2LiSi4O10F2) is a useful synthetic clay, but generally the costs of other clays are prohibitive. The dry composition comprises between about 2% to about 12%, for example between about 4% and about 10%, e.g., between about 6% and about 8%, of one or more expandable clays. When the amount of expandable clay exceeds about 8%, the composition on mixing with water can develop rheology problems. If the amount is less than 2%, the composition on mixing with water will have insufficient reological properties to prevent settling. In preferred embodiments, the clay is powdered clay, e.g., between about 150 mesh and about 500 mesh, preferably between about 300 mesh and about 400 mesh.

[0043] Hydrogel—The composition advantageously comprises hydrogel particles, for example between 20 and 200 mes, (dry) e.g., between 40 and 100 mesh or in a preferred embodiment, between about 60 to 80 mesh particles. To prepare the dry plug composition, in one embodiment a source of hydrogel is hydrated and then coated onto the other solids and dried. In another embodiment, hydrogel particles are admixed in dry form with the particulate clay and other components, and the resulting product is processed as needed to break up aggregates. Any method of mixing the dry components is envisioned, though in one embodiment the clay is first mixed with the dried hydrogel, and optionally the cellulosic particulates and/or polyanionic cellulose, and then the remaining solids, e.g., the rubber particulates and copolymer particulates, are admixed. The principal purposes of the hydrogel are to 1) prevent the expandable clays from interacting and setting up before the pill is formed, and 2) provide a material that has an expanded volume which is changeable with exerted force, allowing the material to shrink as it is being squeezed into a formation and to re-expand thereafter. Additionally, the hydrogel is also believed to lubricate and suspend the other solid material in the remedial plug.

[0044] In one embodiment the dry plug composition comprises between about 1% and about 12% hydrogel, for example, between about 5% and about 9% of hydrogels. Advantageously, most of the hydrogel is in a powdered state. Additionally, in some preferred embodiments, the hydrogel encapsulates the cellulosic material and/or the expandable clay. In one embodiment, the dry plug composition comprises between about 1% to 5% of hydrogel. In another embodiment, the dry plug composition comprises more than 5% to about 12% of hydrogel.

[0045] Hydrogels are widely used in absorbent articles, such as disposable diapers, sanitary napkins, and incontinent pads, for the purpose of causing the water-absorbent resins absorb body fluids. A hydrogel upon contact with aqueous liquids has excellent water absorption amount and speed. Examples of hydrogels include: crosslinked products of partially neutralized polyacrylic acids; hydrolyzed products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic acid ester copolymers; hydrolyzed products of acrylonitrile- or acrylamide copolymers, and their crosslinked products; and crosslinked polymers of cationic monomers, or mixtures thereof.

[0046] One class of hydrogels for use in this invention are those that are capable of absorbing at least 10 grams of aqueous liquid, e.g., distilled water, per gram of absorbent material while immersed in the liquid for 4 hours and holding substantially all of the absorbed aqueous liquid while under a compression force of up to about 1.5 psi. Suitable hydrogels are available from various commercial vendors, such as Dow Chemical Company (Drytech 2035 LD), Hoechst-Celanese Corporation and Allied-Colloid. In one preferred embodiment the hydrogel is capable of absorbing at least about 15 times its weight in water, and preferably is capable of absorbing more than about 2 times its weight in water. Such hydrogel polymers may include, for example, carboxymethylcellulose, alkali metal salts of polyacrylic acids, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, polymers and copolymers of vinyl sulfonic acid, polyacrylates, polyacrylamides, polyvinyl pyridine or the like. Other suitable polymers can include hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, and isobutylene maleic anhydride copolymers, and mixtures thereof. The hydrogel can comprise acrylonitrile copolymers with hydrophilic comonomers, e.g., derivatives of acrylic acid, such as salts, esters, amides, amidines, hydrazidines and the like. Suitable high-absorbency materials are described in U.S. Pat. No. 4,699,823 issued Oct. 13, 1987, to Kellenberger et at. And U.S. Pat. No. 5,147,343 issued Sep. 15, 1992 to Kellenberger, which are incorporated herein by reference. U.S. Pat. No. 6,660,247 describes hydrogels comprising a copolymer of 2-(N,N-dimethylamino)-ethylacrylate and N-isopropylacry lamide.

[0047] The preferred hydrogels comprise polyacrylamide, poly(acrylamide-acrylate) copolymers, poly(acrylamide-methacrylate) copolymers, poly(acrylamide-acrylate-methacrylate) copolymers, partially hydrolyzed versions of the above, or mixture thereof.

[0048] The high absorbency hydrogel polymers in one embodiment are sufficiently cross-linked to render the hydrogel at least partially or substantially water-insoluble. However, the crosslinking can not be so extensive that the function of the hydrogel, i.e., to swell in size with absorbed water, is unduly impaired. The method of crosslinking is not important, and the cross-linking can be done for example by irradiation or by covalent, ionic, van der Waals, or hydrogen bonding. However, the crosslinking material is advantageously incorporated into the hydrogel in an amount sufficient to retard water solubility, but preferably substantially no residual active crosslinking agent remains.

[0049] The dry plug composition and the remedial plug are both advantageously substantially free of active or activatable crosslinker, for example, the crosslinkers mentioned in the prior art. The reason is that if such active crosslinker is present in the lost circulation material, then the remedial plug may set up (crosslink) the polymers at very inconvenient times, e.g., while the remedial plug is still in and is transiting down the drill pipe or tubing, for example. The prior art formulations included crosslinkers and/or cements to make the plug tougher, and depended on crosslinkers to transform thermoplastic hydrogels to thermoset hydrogels once the hydrogels were positioned against and/or in the formation causing the lost circulation. However, when there is lost circulation, there is usually one or two other related problems that are occurring simultaneously. Therefore, there is a higher likelihood of not pumping the material to the required location before the material begins to set. If a plug sets in the drill pipe or tubing, then it may become impossible to continue to pump down that drillpipe or tubing. In such a case, the situation is made drastically worse, as the ability to place remedial compositions in the wellbore are substantially impaired.

[0050] Hydrogels can be either of a conventional “thermoset” type with a covalent network, or “thermoplastic” hydrogels with physical network formed by interactions between hydrophobic groups. Therefore, the preferred embodiments of the current invention comprise no cements and no activatable crosslinkers. Therefore, in preferred embodiments, at least one half, more preferably at least 70% by dry weight of the hydrogel is of the thermoset type.

[0051] To prevent undesired set-up of the plug, in one embodiment the hydrogel is substantially free of reversible, thermally gelling polymers such as block copolymers of poly(ethylene oxide-b-propylene oxide-b-ethylene oxide) which form reversible gels at high temperatures.

[0052] Without being bound by theory, it is believed that as the remedial plug material enters the thief zone, impairment of progress due to bridging and/or progressive plugging of the thief zone creates a restriction. As the components of the remedial plug encounter the restriction, the fluid component of the remedial plug is relatively un-impeded, while the particulates form a cake that exerts a solid-to-solid force or solid-to-solid pressure on the hydrogel constituents of the remedial plug. This solid-to-solid force or solid-to-solid pressure squeezes water from the hydrogel, making the effective size of the particles reduce and allowing closer packing. Note that this solid-to-solid force or solid-to-solid pressure is distinguishable from hydrostatic pressure, which does not result in the desired dehydration of the hydrogel. Finally, as the pumping slows and the plug becomes impermeable, the solid-to-solid force or solid-to-solid pressure reduces, and the hydrogel coating can rehydrate and expand. This creates a highly packed impermeable plug. For this reason, even if simply spotting the remedial plug on the thief zone stops flow, it is advantageous to perform a squeeze to induce solid-to-solid force or solid-to-solid pressure on the particulate material the thief zone, thereby forcing additional plugging material into the reservoir volume. After the squeeze is terminated, the hydrogel re-expands to fill all available space.

[0053] Polymeric materials—In one embodiment the dry plug composition comprises between about 40% and about 80% of one or more polymeric materials, e.g., a copolymer compound, a butyl rubber compound, or mixture thereof. Advantageously the polymeric material comprises at least two distinct solids with mesh sizes differing by at least 30%, e.g., at least 50%. Different particle sizes aid in bridging fractures. In a preferred embodiment, the polymeric materials comprise between about 10% and about 30%, preferably between about 15% and about 25%, by weight of copolymer particulates, and between about 35% to about 55%, preferably between about 40% to about 50% of rubber particulates, e.g., butyl rubber, based on the weight of the dry plug composition.

[0054] Copolymer Particulates: The copolymer particulates are advantageously the largest particulates in the dry plug composition. They are therefore useful in starting the plugging of larger flow paths, for example microfractures. In addition to its plugging function, and swelling ability, this material is also believed to lubricate the remedial plug, impairing setting up in the drill string or tubing. Additionally, certain classes of the copolymeric material will on long term exposure to hydrocarbons in a formation or in treating fluid expand, increasing the plug integrity, i.e., ability to prevent undesired fluid flow through or around the plug. These copolymer particulates are advantageously tough, resilient, and partially rigid. The geometry of the copolymer particulates can be beads, spheroids, seeds, pellets, granules, and mixtures thereof, but are preferably beads and/or spheroids.

[0055] The copolymer is advantageously formed from highly-crosslinked olefinic monomers, e.g., polymers and/or copolymers of styrenes, PVC/vinylacetate, vinylidene chloride/acrylonitrile, acrylate esters and/or methacrylate esters such as methylmethacrylate/ethylacrylate, and styrene/divinylbenzene, as disclosed for example in U.S. Pat. Nos. 4,172,031, 4,384,095 and 4,427,793, the disclosures of which are incorporated by reference, and/or copolymers of styrene, vinyl acetate, vinylnaphthalene, vinyltoluene, allyl esters, olefins, vinyl chloride, allyl alcohol, acrylonitrile, acrolein, vinyl fluoride, vinylidene difluoride, and/or polyamides, for example acrylamides and/or methacrylamides, preferably highly branched polyamides and partially aliphatic highly branched polyamides as disclosed in U.S. Pat. No. 6,541,599.

[0056] The preferred beads are made rigid and nonelastic by the physical crosslinking of chain entanglement in addition to the chemical crosslinking of polyfunctional monomers. The preferred copolymer compound is formed of divinylbenzene, divinylbenzene/styrene, and/or polystyrene that is highly cross-linked and, most preferably, also exhibits significant chain entanglement as disclosed in co-owned U.S. Pat. No. 6,451,953, the disclosure of which is incorporated herein by reference. Other preferred copolymer bead products are those disclosed in co-owned U.S. Pat. Nos. 6,541,579, 6,248,838, and 6,348,629 the disclosures of which are incorporated herein by reference. This copolymer material exhibits high rigidity and has high levels of physical crosslinking properties resulting from chain entanglement. The physical strength of the polymer is a result of both the chain entanglement and the chemical crosslinking agents. The copolymer is polymerized in a manner to encourage chain entanglement, e.g., where the method comprises the step of generating chain entanglement by rapid rate polymerization. Chain entanglement has been shown to increase greatly during polymer formation by conducting the polymerization as rapidly as possible, where the faster the rate of the propagation step, the greater is the degree or the level of crosslinking by chain entanglement. The rapid rate polymerization procedure comprises the step of elevating a radical flux by employing large concentrations of an initiator within the range from about 1.0% to about 10% weight of monomer weight and elevating the polymerization temperature to a temperature greater than the ten-hour half-life temperature of the initiator. A preferred temperature ramping rate is one degree centigrade (Celsius) every three minutes. The temperature ramp of the radical flux can also be a series of step functions of temperature increases followed by plateaus of varying lengths so that the temperature ramp has the form of increasing steps.

[0057] Crosslinking agents are present initially and/or can be added during polymerization, at an amount between 1% to about 50% of the polymer weight and preferably from about 1% to about 10% of the polymer weight. Rigid, non-elastic particles are able to be made at moderately low levels of a chemical crosslinker (e.g., 20% DVB or less) by enhancing chain entanglement. There are many useful polyfunctional crosslinking agents, but the most preferred crosslinking monomer is divinylbenzene. The preferred copolymer compound comprises or consists essentially of styrene/divinylbenzene. The term divinylbenzene includes the derivatives normally found therein, e.g., ethylvinylbenzene. Other crosslinking monomers are polyfunctional acrylates, methacrylates, acrylamides, methacylamides and polyunsaturated hydrocarbons. Other exemplary crosslinking agents include trimethylolpropane trimethacrylate, trimethylolpropane triacrylate, trimethylolpropane dimethacrylate, trimethylolpropane diacrylate, pentaerythritol tetramethacryalate, pentaerythritol trimethacrylate, pentaerythritol dimethacrylate, pentaerythritol tetraacrylate, pentaerythritol triacrylate, pentaerythritol diacrylate, ethyleneglycol dimethacrylate, ethyleneglycol diacrylate, diethyleneglycol dimethacrylate, diethyleneglycol diacrylate, triethyleneglycol dimethacrylate, triethyleneglycol diacrylate, a bis(methacrylamide), a polyethyleneglycol dimethacrylate, a polyethyleneglycol diacrylate, divinyl benzene, or mixtures thereof.

[0058] The particulates are advantageously between about 12 mesh and about 160 mesh, preferably between about 16 mesh and about 80 mesh, e.g., about 16 to 40 mesh.

[0059] In a preferred embodiment the copolymer compound comprises Lubra-Glide® CE Copolymer Beads™, having a specific gravity of about 1.0 to about 1.1, which are commercially available from Sun Drilling Products, Inc.

[0060] Rubber Particulates: The rubber particulate material can comprise any commercially available rubber material, and advantageously comprises or consists essentially of recovered or recycled rubber, which is less costly and provides environmental advantages. The rubber particulates are slightly compressible and deformable, and advantageously have irregular surfaces, therefore assisting in bridging and plugging high permeability fluid loss zones and microfractures. Additionally, certain classes of the rubber material will on long term exposure to hydrocarbons expand, increasing the plug integrity, i.e., ability to prevent undesired fluid flow through or around the plug. Advantageously, the prepared remedial plug is substantially free of or contains only incidental amounts, e.g., less than 2% by weight, of hydrocarbons that may swell the one or more polymeric materials.

[0061] The rubber particulates can comprise or consist essentially of: ethylene propylene diene monomer rubber; neoprene rubber; buna rubber, for example styrene butadiene rubber, butyl rubber; butadiene rubber, and butyl rubber; nitrile rubber; latex rubber; or mixtures thereof, or combinations thereof.

[0062] In one embodiment the rubber is vulcanized, and can for example comprise recycled ground tire material. In another embodiment this rubber comprises the crumb rubber having a size of from 1-400 microns as described in U.S. Pat. No. 6,518,224, the disclosure of which is incorporated by reference. This rubber is particularly effective, when included with the described remedial plug, at plugging both high permeability zones and at bridging and plugging fractures. Additionally, this rubber on long term exposure to hydrocarbons swells, thereby increasing plug intergrity.

[0063] The rubber particulates preferably are obtained by grinding cured and/or vulcanized rubber material. The rubber particulates are advantageously between about 200 mesh and about 1000 mesh, preferably between about 400 mesh and about 800 mesh, e.g., about 600 to about 800 mesh.

[0064] The preferred rubber is very finely ground or powdered polybutadiene.

[0065] In less preferred embodiments, all of the one or more polymeric materials will be either the first particulate copolymer compound or the particulate butyl rubber. While the two components are somewhat interchangable in quantity, preferably both are present in the lost circulation material as the functionality of each is similar but not completely overlapping.

[0066] Cellulosic material—The dry plug composition additionally comprises cellulosic particulates, e.g., between about 10% to about 40%, preferably from about 15% to about 20%, of finely processed cellulose. The particle size of the cellulosic material can vary, and in some preferred embodiments there are two or three ranges of particle sizes, or alternately a broad distribution of particles sizes, wherein the sizes advantageously range from about 100 mesh to about 800 mesh, for example from about 300 to 600 mesh. In one embodiment the processed cellulose consists essentially or comprises ground wood flour, cottonseed hulls, walnut shells, bagasse, dried tumbleweed, paper, or mixture thereof, as described in for example U.S. Pat. No. 4,498,995, the disclosure of which is incorporated by reference. In a less preferred embodiment at least some of the cellulosic material is replaced by finely ground coal. In the preferred embodiment the processed cellulose consists essentially or comprises wood flour.

[0067] It is generally preferred to not have large cellulosic particulates present, e.g., cellulosic pariculates of size greater than 100 mesh. However, if a first treatment of a formation is unsuccessful due to insufficient plugging of the high permeability formation, then adding some larger sized cellulosic material to a remedial plug may be advantageous. By larger it is meant larger by at least 50% compared to the average diameter of the primary cellulosic particulates, e.g., between about 20 mesh to about 200 mesh.

[0068] Other additives—In alternate embodiments, the remedial plug may contain a variety of other materials, such as coarsely ground cottonseed hulls, coarsely ground walnut shells, ground or coarsely ground coal, coarsely ground wood, and/or coarsely ground rubber that act as bridging materials or filtrate control materials. As discussed in the above paragraph, it may be advantageous to add one or more of these other additives to remedial plugs wherein the first treatment did not result in circulation. In another embodiment, the remedial plug is substantially free of these above-listed other additives.

[0069] In a preferred embodiment of this invention the remedial plug dry mix comprises between about 0.2% to about 3%, more preferably between about 0.5% and about 1.5%, for example about 1%, of a water soluble cellulose, e.g. a polyanionic cellulose, or water soluble gum, or water soluble thickener such as polyacrylates, or mixture or combination thereof.

[0070] One important feature in the remedial plugs of the current invention is the particle size of the various components. The combination of products, each having its particular particle size distribution, is particularly suitable for plugging large pores and microfractures which are typically responsible for fluid loss. If too large a particle is used, the particle will not penetrate the thief zone, and any remediation will be effective only until the surface cake is removed. If too small a particle is used, the particles will not be able to bridge and plug the wider microfractures. A wide range of particle sizes makes sure there are appropriately sized particulates to penetrate the thief zone, bridge openings and start a filter cake, and also a substantial number of smaller sized particulates to pack into and severely reduce the permeability of the filter cake. Preferred mesh sizes for the various components are shown below.

				Exemplary
	Pref. Wt %	Preferred	More Pref.	embodiment
Copolymer	16 to 24	12 to 140	16 to 60	16 to 40
beads
Butyl rubber	35 to 48	200 to 1000	400 to 800	600
Wood flour	16 to 24	100 to 800	300 to 600	300 to 400
Hydrogel (dry)	4 to 8	20 to 40	40 to 200	80 to 160
Expandable	6 to 10	100 to 1000	200 to 400	200 to 400
clay (dry)

[0071] In one embodiment, the dry plug composition comprises about 18% to about 24% of one or more copolymer compounds in ground or bead form, e.g., Lubra-Glide® CE Copolymer Beads™ available from Sun Drilling Products, Inc.; about 35% to about 48% ground butyl rubber; about 16% to about 24% cellulosic powder; about 6% to about 10% hydrophilic clay; and about 4% to about 8% of partially hydrolyzed polyacrylamide. In less preferred embodiments, any suitable substitute component known in the art to be interchangable with the listed components may be partially or wholly substituted for each of the components listed.

[0072] The lost circulation material may be may be packaged in buckets, lined water-resistant bags in 20 to 100 pound increments and/or in 1000 to 4000 pound increments, or in barrels. Again, it is preferred that all components be packaged in a ready to use dry pre-mix.

EXAMPLE

[0073] An more preferred dry plug composition comprising the components represented in the table above, called Smart-Plug™, was prepared by Sun Drilling Products Corp. The dry plug composition was admixed with fresh water to form a slurry having about 80 pounds of Smart-Plug™ per barrel. The material was then bumped into six wells, each manifesting a significant fluid loss problem, so that the plug covered the lost-fluid zone. The resulting plug was found to have excellent capability to reduce fluid loss due to high permeability-high porosity formations and due to natural or induced fracturing. The resulting plug was also found to be stable during subsequent drilling and workover actions.

[0074] Although the present invention is described with reference to certain preferred embodiments, it is apparent that modification and variations thereof may be made by those skilled in the art without departing from the spirit and scope of this invention as defined by the appended claims. In particular, it will be clear to those skilled in the art that the present invention may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. One skilled in the art will appreciate that the invention may be used with many modifications of materials, methods, and components otherwise used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, and not limited to the foregoing description.

Claims

1. A dry lost-circulation-composition useful for preparing a slurry to remediate lost circulation in a wellbore, the slurry comprising: between about 40% to about 80% by weight total of rubber particulates and crosslinked copolymer particulates; between about 10% to about 40% by weight of cellulosic particulates; between about 2% to about 12% by weight of expandable clays; and between about 1% and about 10% by weight of dry hydrogel that on mixing with water absorbs at least 5 times its weight in water, wherein the weight percent is based on the weight of the dry components.

2. The dry lost-circulation-composition of claim 1, wherein the expandable clay comprises Wyoming bentonite, the composition further comprising between about 0.2% and about 3% of a water soluble cellulose, a water soluble gum, a water soluble polymeric thickener, or mixture thereof.

3. The dry lost-circulation-composition of claim 1, wherein the rubber particulates have a weight average size between 200 and 1000 mesh, the crosslinked copolymer particulates have a weight average size between 12 and 140 mesh, the cellulosic particulates have a weight average size between 100 and 800 mesh, the expandable clays have a weight average dry size between 100 and 1000 mesh, and the hydrogel are substantially water-insoluble particulates having a dry weight average size between about 20 and about 400 mesh, and wherein the dry lost-circulation composition is in the form of a pre-mixed dry composition.

4. The dry lost-circulation-composition of claim 1, wherein: the rubber particulates have a weight average size between 400 and 800 mesh and comprise ethylene propylene diene monomer rubber; neoprene rubber; buna rubber; styrene butadiene rubber; butyl rubber; butadiene rubber; nitrile rubber; latex rubber; and mixtures and combinations thereof; the copolymer particulates have a weight average size between 16 and 60 mesh and comprise crosslinked copolymers of styrenes, PVC, vinylacetate, vinylidene chloride, acrylonitrile, acrylates, methacrylates, methylmethacrylate, ethylacrylate, styrene/divinylbenzene, vinylnaphthalene, vinyltoluene, allyl esters, C1 to C8 olefins, vinyl chloride, acrolein, polyamides, and mixtures and combinations thereof; the cellulosic particulates have a weight average size between 300 and 600 mesh, the expandable clays comprise Wyoming bentonite and have a weight average dry size between 200 and 1000 mesh, and the hydrogel comprises partially crosslinked partially hydrolyzed polyacrylamide, polyacrylamides, carboxymethylcellulose, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof.

5. The dry lost-circulation-composition of claim 1, wherein the cellulosic particulates, the clays, or both are substantially encapsulated in hydrogel.

6. The dry lost-circulation-composition of claim 3, wherein: the rubber particulates comprise are present in an amount between about 35% and about 55% by weight, wherein the rubber particulates comprise ground butyl rubber, butadiene rubber, or styrene butadiene rubber; the crosslinked-copolymer particulates are beads present in an amount between about 10% and about 30% by weight, wherein the beads comprise a crosslinking agent crosslinking copolymers of one or more of styrene, C1 to C8 olefins, acrylonitrile, acrylates, methacrylates, methylmethacrylate, and ethylacrylate; the cellulosic particulates are present in an amount between about 10% and about 30% by weight; the expandable clay comprise smectite clays present in an amount between about 2% and about 8% by weight; and the hydrogel comprises partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof.

7. The dry lost-circulation-composition of claim 1, wherein: the rubber particulates comprise ground butyl rubber having a weight average size between about 300 mesh and about 600 mesh and are present in an amount between about 35% and about 48% by weight; the crosslinked-copolymer particulates are beads comprising divinyl benzene and styrene, wherein the beads have a weight average size between about 12 mesh and about 140 mesh and are present in an amount between about 16% and about 24% by weight; the cellulosic particulates comprise wood flour and are present in an amount between about 16% and about 24% by weight; and the expandable clay comprise smectite clays having a weight average size smaller than 200 mesh, and are present in an amount between about 3% and about 10% by weight.

8. The dry lost-circulation-composition of claim 7, wherein the dry lost-circulation-composition is substantially free of active crosslinkers and cements.

9. The dry lost-circulation-composition of claim 8, wherein the hydrogel comprises polyacrylamide, poly(acrylamide-acrylate) copolymers, poly(acrylamide-methacrylate) copolymers, poly(acrylamide-acrylate-methacrylate) copolymers, partially hydrolyzed versions of the above, or mixture thereof.

10. The dry lost-circulation-composition of claim 9, wherein the dry lost-circulation-composition further comprises between 0.2% to about 3% of polyanionic cellulose, a water soluble gum, a water soluble thickener, or mixture thereof.

11. The dry lost-circulation-composition of claim 10, wherein the dry lost-circulation-composition further comprises between 0.5% to about 1.5% of polyanionic cellulose, a water soluble gum, a water soluble thickener, or mixture thereof.

12. A dry lost-circulation-composition useful for preparing a slurry to remediate lost circulation in a wellbore, the slurry comprising: between about 40 parts to about 85 parts by weight total of rubber particulates and copolymer particulates; between about 10 parts to about 40 parts by weight of cellulosic particulates; between about 2 parts to about 20 parts by weight of expandable clays; and between about 2 parts and about 20 parts by weight of a hydrogel capable of absorbing at least 10 times its weight in water, wherein the dry lost-circulation composition is in the form of a dry mix.

13. The dry lost-circulation-composition of claim 12, wherein the expandable clay comprises Wyoming bentonite, the composition further comprising between about 0.2 parts and about 3 parts of a polyanionic cellulose, a water soluble gum, a water soluble polymeric thickener, or mixture thereof.

14. The dry lost-circulation-composition of claim 12, wherein the rubber particulates have an average size between 200 and 1000 mesh, the copolymer particulates have a weight average size between 12 and 140 mesh, the cellulosic particulates have a weight average between 100 and 800 mesh, the expandable clays have a weight average dry size between 100 and 1000 mesh, and the hydrogel are substantially water-insoluble particulates.

15. The dry lost-circulation-composition of claim 12, wherein: the rubber particulates are present in an amount between about 35 parts and about 55 parts by weight and comprise ground butyl rubber, butadiene rubber, or buna rubber; the copolymer particulates are present in an amount between about 10 parts and about 30 parts by weight; the cellulosic particulates are present in an amount between about 10 parts and about 30 parts by weight; the expandable clay comprises smectite clays; and the hydrogel comprises partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof.

16. The dry lost-circulation-composition of claim 12, wherein the expandable clay comprises Wyoming bentonite, the composition further comprising between about 0.5 parts and about 1.5 parts of polyanionic cellulose, water soluble gum, water soluble polymeric thickener, or mixture thereof.

17. A remedial slurry plug comprising: between about 21 and about 66 pounds of rubber particulates per barrel; between about 6 and about 36 pounds of crosslinked-copolymer particulates per barrel; between about 6 and about 36 pounds of cellulosic particulates per barrel; between about 2 and about 20 pounds of expandable clay per barrel; and between about 2 and about 14 pounds of hydrogel (dry weight) per barrel, wherein the total dry weight of the rubber particulates, copolymer particulates, cellulosic particulates, expandable clay, and hydrogel (dry weight) is between about 60 pounds and about 120 pounds per barrel; and water having less than 1500 ppm chlorides.

18. The remedial plug of claim 17, wherein the slurry comprises between about 50 pounds and about 100 pounds of the dry mix composition per barrel of remedial plug, and wherein the dry mix composition comprises: the rubber particulates and crosslinked-copolymer particulates present in a total amount between about 40% and about 80% by weight; the cellulosic particulates are present in an amount between about 10% and about 40% by weight; at least a portion of the expandable clay; and the hydrogel, wherein the weight is based on the dry weight of the dry mix composition, and the rubber particulates and the cellulosic particulates each have a weight average size smaller than about 200 mesh.

19. The remedial plug of claim 18, wherein the slurry comprises between about 70 pounds and about 90 pounds of the dry mix composition per barrel of remedial plug.

20. The remedial plug of claim 17, wherein the water has between 50 ppm and 1000 ppm chlorides.

21. The remedial plug of claim 17, wherein expandable clay fraction of the remedial plug comprises at least about pounds of Wyoming bentonite, the remedial plug further comprising barite particulates in an amount such that the density of the remedial plug is between 10 pounds per gallon and 20 pounds per gallon.

22. The remedial plug of claim 17, wherein the expandable clay comprises Wyoming bentonite, the remedial plug further comprising between about 0.1 pounds and about 2 pounds of polyanionic cellulose, water soluble gum, water soluble polymeric thickener, or mixture thereof per barrel of remedial plug.

23. The remedial plug of claim 20, wherein a barrel of the remedial plug comprises: between about 24 pounds and about 50 pounds of the rubber particulates; between about 7 pounds and about 27 pounds of the copolymer particulates; between about 10 pounds and about 18 pounds of the cellulosic particulates; between about 1.4 pounds and about 9 pounds of the expandable clay, wherein the expandable clay comprise smectite clays; and between about 2.8 pounds and about 7.2 pounds of the hydrogel, wherein the hydrogel comprises partially hydrolyzed polyacrylamide, carboxymethylcellulose, polyacrylamides, polyvinyl alcohol, ethylene maleic anhydride copolymers, polyvinyl ethers, hydroxypropyl cellulose, polyvinyl morpholinone, copolymers of vinyl sulfonic acid, polyacrylates, hydrolyzed acrylonitrile grafted starch, acrylic acid grafted starch, isobutylene maleic anhydride copolymers, acrylonitrile copolymers with hydrophilic comonomers, or mixtures thereof.

24. The remedial plug of claim 23, wherein the slurry is substantially free of active crosslinkers and cements, the remedial plug further comprising between about 0.3 pounds and about 1.4 pounds of polyanionic cellulose, water soluble gum, water soluble polymeric thickener, or mixture thereof per barrel of remedial plug.

25. The remedial plug of claim 17 wherein the rubber particulates have a weight average size between 200 and 1000 mesh, the crosslinked copolymer particulates have a weight average size between 12 and 140 mesh, the cellulosic particulates have a weight average size between 100 and 800 mesh, the expandable clays have a weight average (dry) size between 100 and 1000 mesh, and the hydrogel are substantially water-insoluble particulates having a dry weight average size between about 20 and about 400 mesh.

26. The remedial plug of claim 23 wherein the rubber particulates have a weight average size between about 400 and about 800 mesh, the crosslinked copolymer particulates have a weight average size between about 16 and about 60 mesh, the cellulosic particulates have a weight average size between about 300 and about 600 mesh, the expandable clays have a weight average (dry) size smaller than about 200 mesh, and the hydrogel are substantially water-insoluble particulates.

27. A method of treating a wellbore that has undesirable fluid loss therefrom caused by leakage of fluid from the wellbore into a subsurface formation responsible for the undesirable fluid loss, said method comprising: providing the remedial plug of claim 17; and pumping the remedial plug down a wellbore so that at least a portion of the remedial plug penetrates and thereby reduces the fluid loss to the subsurface formation responsible for the undesirable fluid loss.

28. The method of treating a wellbore of claim 27, wherein the remedial plug is pumped through a tubing or a drill string to a depth near the subsurface formation responsible for the undesirable fluid loss; and then the pressure in the wellbore is increased by at least 100 psi, thereby forcing at least a portion of the remedial plug into the subsurface formation responsible for the undesirable fluid loss and also driving water from the hydrogel.

29. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 18.

30. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 19.

31. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 20.

32. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 21.

33. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 22.

34. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 23.

35. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 24.

36. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 25.

37. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug of claim 26.

38. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug is formed by admixing between about 60 and about 100 pounds of the dry lost-circulation-composition of claim 1 per barrel of fresh water comprising less than 1500 ppm chlorides.

39. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug is formed by admixing between about 60 and about 100 pounds of the dry lost-circulation-composition of claim 12 per barrel of fresh water comprising less than 1500 ppm chlorides.

40. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug is formed by admixing between about 70 and about 90 pounds of the dry lost-circulation-composition of claim 1 per barrel of fresh water comprising less than 1000 ppm chlorides.

41. The method of treating a wellbore of claim 27, wherein the remedial plug is the remedial plug is formed by admixing between about 70 and about 90 pounds of the dry lost-circulation-composition of claim 12 per barrel of fresh water comprising less than 1000 ppm chlorides.

42. The method of treating a wellbore of claim 39, wherein the water has between about 50 and about 400 ppm chlorides

43. The method of treating a wellbore of claim 27, wherein the remedial plug further comprises between about 8 pounds and about 12 pounds of bentonite that is hydrated and dispersed per barrel of remedial plug, and also further comprises barite particulates in an amount such that the density of the remedial plug is between 10 pounds per gallon and 20 pounds per gallon, wherein the density of the remedial plug is within about 2 pounds per gallon of the weight of the fluid in the wellbore

44. The method of treating a wellbore of claim 27, wherein the remedial plug is substantially free of active crosslinkers and cements.

45. A method of treating a wellbore that has undesirable fluid loss caused by leakage of fluid from the wellbore into a subsurface formation responsible for the undesirable fluid loss, said method comprising: providing a remedial plug comprising plugging means, a means for suspending the plugging means in the remedial plug, and a means for preventing, for a period of time, the suspending means from setting up; pumping the remedial plug down tubing or a drill string, wherein the remedial plug is pumped from the tubing or drill string into the wellbore at a depth near said subsurface formation; and pumping at least a portion of the remedial plug into said subsurface formation, so that the remedial plug reduces the fluid loss to said subsurface formation.