Patent Application
20140196904
The present invention relates to methods for treating subterranean formations. More particularly, the present invention relates to the use of delayed release pH control agents in subterranean formations during fracturing operations.
Hydrocarbons (e.g., oil, natural gas, etc.) in a subterranean formation can be recovered by drilling a well into the subterranean formation. Hydrocarbons in the subterranean formation are driven into the well to be produced by, for example, pressure gradients that exist between the formation and the well, the force of gravity, displacement of the fluids using pumps or the force of another fluid injected into the well. The production of hydrocarbons is commonly increased by stimulating the subterranean formation.
One stimulation technique is fracturing which involves pumping a fluid down the well at a sufficient flow rate and pressure to overcome the fracture gradient pressure and create or enhance fractures in the rocks of the subterranean formation. In order to prevent the resulting fractures from closing upon release of the fluid pressure, typically a hard particulate material known as a proppant, is dispersed in the fracturing fluid to be deposited in the resulting fractures. To carry the proppant down into the fractures, it is desirable that the fracturing fluid have a fairly high viscosity, and preferably a gel-like consistency. Fracturing fluids also need to have a low leak-off rate and a low pumping friction loss. The rate of leak-off is dependent on the viscosity and wall-building properties of the fluid.
Aqueous gels are usually prepared by blending a polymeric gelling agent with an aqueous medium. Most frequently, the polymeric gelling agent of choice is a water-soluble biopolymer (e.g., polysaccharide), but may also be a synthetic polymer (e.g., polyacrylamide).
Suitable polysaccharides form a known class of compounds that include a variety of natural gums and certain cellulosic derivatives that have been rendered hydratable by hydrophilic substituents chemically attached to the polymer backbone. Examples of such polymers include guar, carboxyalkyl guar, hydroxyalkyl guar, carboxyalkyl hydroxyalkyl guar, galactomannan gums, glucomannan gums, xanthan gums and the like.
The water-soluble polysaccharides have a remarkable capacity to thicken aqueous liquids. Even small amounts are sufficient to increase the viscosity of such aqueous liquids from 10 to 100 times or more. In many instances, the viscosified fluid has sufficient viscosity to carry and deposit the proppant during the course of the fracturing process. In other instances, it is desirable to crosslink the polysaccharide in order to form a gel having sufficient strength and viscosity to carry the proppant. A variety of crosslinking agents have been developed to achieve this result.
Previously, a relatively high viscosity fracturing fluid would be prepared on the surface with proppant dispersed therein and then it would be pumped into the well according to the parameters of the fracturing operation. Such a procedure requires high pumping pressures due to the high viscosity of the gelled fluid as it is pumped down the wellbore, and results in high friction losses within the bore and pipes. In fact, the viscosity of the gel prepared on the surface might often be lower than the viscosity desired within the fracture due to limitations imposed by the pumping abilities of the equipment used. Further, fully crosslinked fluids may suffer irreparable viscosity degradation from the high shear environment during the pumping stages. Thus, it would be desirable that the fracturing fluid exhibit a relatively low viscosity to facilitate ease of pumping, but exhibit a relatively high viscosity downhole, e.g., when it is within the fracture itself.
The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to the use of delayed release pH control agents in subterranean formations during fracturing operations.
In some embodiments, the present invention provides a method comprising providing a fracturing fluid comprising: a base fluid, a viscosifying agent, a pH sensitive crosslinking agent, and a delayed release pH-adjusting material, the delayed release pH-adjusting material comprising a pH-adjusting agent and a binder; introducing the fracturing fluid into a subterranean formation; changing the pH of the fracturing fluid through at least partial degradation or disassociation of the delayed release pH-adjusting material; and allowing the viscosifying agent to be crosslinked through activation of the pH sensitive crosslinking agent.
In other embodiments, the present invention provides a method comprising providing a fracturing fluid comprising: a base fluid, a viscosifying agent, a pH sensitive crosslinking agent, and a delayed release pH-adjusting material, the delayed release pH-adjusting material comprising a basic pH-adjusting agent and a binder; introducing the fracturing fluid into a subterranean formation; raising the pH of the fracturing fluid through at least partial degradation of the delayed release pH-adjusting material; and allowing the viscosifying agent to be crosslinked through activation of the pH sensitive crosslinking agent.
In still other embodiments, the present invention provides a method comprising providing a fracturing fluid comprising: a base fluid, a viscosifying agent, a pH sensitive crosslinking agent, and a delayed release pH-adjusting material, the delayed release pH-adjusting material comprising an acidic pH-adjusting agent and a binder; introducing the fracturing fluid into a subterranean formation; lowering the pH of the fracturing fluid through at least partial degradation or disassociation of the delayed release pH-adjusting solid material; and allowing the viscosifying agent to be crosslinked through activation of the pH sensitive crosslinking agent.
The features and advantages of the present invention will be readily apparent to those skilled in the art upon a reading of the description of the preferred embodiments that follows.
The following figure is included to illustrate certain aspects of the present invention, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to those skilled in the art and having the benefit of this disclosure.
The present invention relates to methods and compositions for treating subterranean formations. More particularly, the present invention relates to the use of delayed release pH control agents in subterranean formations during fracturing operations.
One of the advantages of some embodiments of the present invention is the ability to delay the crosslinking of the fracturing fluid until after the fluid has been introduced into a subterranean formation. Such a delay may help to avoid difficulties in pumping a fracturing fluid into a subterranean formation due to high friction pressure and gel shear degradation prior to introduction into the formation. High friction pressures require increased power requirements which translate to greater costs. There are also other potential dangers and risks associated with operating at high treating pressures. Additionally, there will always be a maximum operating pressure, called a kick-out pressure, dictated by equipment or tubular requirements, which cannot be exceeded during a job. If the kick-out pressure is exceeded, the operation will be forced to shut down or reduce pumping rates and may not be able to finish the treatment. Shear degradation, in some cases, will ultimately damage the crosslinked gel beyond repair and may lead to a “screen-out” or treatment failure, which can be costly and timely to recover from. As a result of the present invention, these dangers may be minimized and the pumping rate of the fluid may be increased, allowing for more efficient placement of the fracturing fluid. Other advantages may be evident to one skilled in the art.
One method according to the present invention includes providing a fracturing fluid comprising: a base fluid, a viscosifying agent, a pH sensitive crosslinking agent, and a delayed release pH-adjusting material, the delayed release pH-adjusting material comprising a pH-adjusting agent and a binder.
Generally, suitable base fluids used in the fracturing fluids of the present invention may comprise fresh water, saltwater (e.g., water containing one or more salts dissolved therein), brine (e.g., saturated saltwater), seawater, or combinations thereof, and may be from any source, provided that they do not contain components that might adversely affect the stability and/or performance of the fracturing fluids of the present invention. In some embodiments, the density of the base fluid may be increased, among other purposes, to provide additional weight to the fluid for particle transport and suspension in the fracturing fluids of the present invention. Additionally, in some embodiments, the pH of the base fluid may be adjusted (e.g., by a buffer or other pH adjusting agent), among other purposes, to facilitate hydration of a gelling agent, activate a crosslinking agent, and/or reduce the viscosity of the fracturing fluid (e.g., activate a breaker, deactivate a crosslinking agent). With the benefit of this disclosure, one of ordinary skill in the art will recognize when such density and/or pH adjustments are appropriate.
In certain embodiments of the present invention, the base fluid is an aqueous base fluid. Examples of suitable aqueous base fluids in the present invention include at least one selected from fresh waters, municipal waters, wastewaters, saltwaters, seawaters, brines, produced waters, and flowback waters.
The fracturing fluid can contain a gelling agent, also known as a viscosifying agent. As used herein, the term “viscosifying agent” refers to a material capable of forming the fracturing fluid into a gel, thereby increasing its viscosity. Viscosifying agents, and their derivatives include semi-solid, jelly-like states assumed by some colloidal dispersions. In certain embodiments, the viscosifying agent may comprise one or more polymers that have at least two molecules that are capable of forming a crosslink in a crosslinking reaction in the presence of a suitable crosslinking agent, and/or polymers that have at least two molecules that are so crosslinked (i.e., a crosslinked viscosifying agent). The viscosifying agents may be naturally occurring viscosifying agents, synthetic viscosifying agents, or a combination thereof. The viscosifying agents also may be cationic viscosifying agents, anionic viscosifying agents, or a combination thereof.
Examples of suitable viscosifying agents include, but are not limited to, natural or derivatized compounds that are soluble, dispersible, or swellable in an aqueous liquid. Examples include modified celluloses and derivatives thereof, and biopolymers and derivatives thereof. Other examples include polysaccharides, biopolymers, and/or derivatives thereof that contain one or more of these monosaccharide units: galactose, mannose, glucoside, glucose, xylose, arabinose, fructose, glucuronic acid, or pyranosyl sulfate. Examples of suitable polysaccharides include, but are not limited to, guar gums (e.g., hydroxyethyl guar, hydroxypropyl guar, carboxymethyl guar, carboxymethylhydroxyethyl guar, and carboxymethylhydroxypropyl guar (“CMHPG”)), locust bean gums, gum ghattis, gum karayas, tamarind gums, and tragacanth gums; depolymerized gums such as depolymerized guar gum, and derivatives thereof; cellulose derivatives (e.g., hydroxyethyl cellulose, methylcellulose carboxyethylcellulose, carboxymethylcellulose, and carboxymethylhydroxyethylcellulose), xanthan, scleroglucan, succinoglycan, diutan, derivatives thereof, copolymers thereof, and combinations thereof.
In certain embodiments, the viscosifying agent may comprise a derivatized cellulose that comprises cellulose grafted with an allyl or a vinyl monomer, such as those disclosed in U.S. Pat. Nos. 4,982,793, 5,067,565, and 5,122,549, the entire disclosures of which are incorporated herein by reference.
Suitable synthetic polymers include, but are not limited to, acrylate/methacrylate/vinyl phosphate containing acrylamide copolymers, derivatives thereof, copolymers thereof, and combinations thereof.
The viscosifying agent may be present in the fracturing fluids useful in the methods of the present invention in an amount sufficient to provide the desired viscosity. In some embodiments, the viscosifying agents (i.e., the polymeric material) may be present in an amount in the range of from about 0.1% to about 10% by weight of the fracturing fluid. In certain embodiments, the viscosifying agents may be present in an amount in the range of from about 0.15% to about 2.5% by weight of the fracturing fluid.
In those embodiments of the present invention, the fracturing fluid may comprise one or more pH sensitive crosslinking agents. Examples of suitable crosslinking agents include, but are not limited to, those that comprise borate ions, magnesium ions, zirconium IV ions, titanium IV ions, aluminum ions, antimony ions, chromium ions, iron ions, copper ions, magnesium ions, and zinc ions. These ions may be provided by providing any compound that is capable of producing one or more of these ions. Examples of such compounds include, but are not limited to, ferric chloride, boric acid, disodium octaborate tetrahydrate, sodium diborate, pentaborates, ulexite, colemanite, magnesium oxide, zirconium lactate, zirconium triethanol amine, zirconium lactate triethanolamine, zirconium carbonate, zirconium acetylacetonate, zirconium malate, zirconium citrate, zirconium diisopropylamine lactate, zirconium glycolate, zirconium triethanol amine glycolate, zirconium lactate glycolate, titanium lactate, titanium malate, titanium citrate, titanium ammonium lactate, titanium triethanolamine, and titanium acetylacetonate, aluminum lactate, aluminum citrate, derivatives, and combinations thereof. An example of an alkali sensitive crosslinking agent is boric acid. An example of an acid sensitive crosslinking agent is aluminum citrate.
In certain embodiments, the crosslinking agent may be present in the fracturing fluid of the present invention in an amount in the range of from about 0.005% to about 1% by weight of the fracturing fluid. In certain embodiments, the crosslinking agent may be present in the fracturing fluids of the present invention in an amount in the range of from about 0.05% to about 1% by weight of the first fracturing fluid and/or second treatment fluid. One of ordinary skill in the art, with the benefit of this disclosure, will recognize the appropriate amount of crosslinking agent to include in a fracturing fluid of the present invention based on, among other things, the temperature conditions of a particular application, the type of viscosifying agents used, the molecular weight of the viscosifying agents, the desired degree of viscosification, and/or the pH of the fracturing fluid after the pH-adjusting agent has been released from the delayed release pH-adjusting material.
In the present invention, the pH sensitive crosslinking agent may be activated so as to crosslink at least a portion of the viscosifying agent by a change in pH of the fracturing fluid induced by the release of the delayed release pH-adjusting material. The delayed release pH-adjusting material may contain a pH-adjusting agent and a binder.
The pH-adjusting agent may include an acidic material for lowering the pH, or a basic material capable of raising the pH of the fracturing fluid, depending on the type of crosslinking agent used in the fluid. Suitable acidic pH-adjusting agents for use in the present invention include at least one selected from the group consisting of hydrochloric acid, hydrofluoric acid, hydrobromic acid, sulfuric acid, phosphoric acid, nitric acid, acetic acid, formic acid, and citric acid, derivatives thereof, and combinations thereof. Suitable basic pH-adjusting agents include at least one selected from the group consisting of: sodium hydroxide, ammonium hydroxide, potassium hydroxide, magnesium hydroxide, and a metal alkali salt, derivatives thereof, and combinations thereof.
The type of pH-adjusting agent used in the delayed release pH-adjusting material depends on the crosslinking agent used in the fracturing fluid. For example, most water soluble polysaccharides are typically crosslinkable in a basic aqueous medium (at a pH above 7) by a wide variety of organometallic compounds containing titanium or zirconium in a +4 oxidation (valance) state. The borate ion has been used extensively as a crosslinking agent for hydrated guar gums and other galactomannans to form aqueous gels used in fracturing and other areas. Some borate crosslinked systems require a basic pH, for example, 8.5 to 10, for crosslinking to occur.
Other crosslinking agents have been developed using certain transition metals. For example aqueous solutions of galactomannan gums have been crosslinked at pH 6-13 with antimony, bismuth, titanium, zirconium, chromium and iron compounds, derivatives thereof, and combinations thereof.
To form the delayed release pH-adjusting material, the pH-adjusting agent/s may be combined with a binder. The binder may be used to provide structure for which to hold the pH-adjusting agent together to allow for the material to be portioned out. Suitable binders for use in the present invention may include, but are not limited to, silica gel, aluminosilicate, chitosan, and cellulose, derivatives thereof, and combinations thereof.
The pH-adjusting agent and binder may be combined to form a slurry or paste, and then allowed to dry and harden to form a delayed release pH-adjusting material. Once in a hardened form, the delayed release pH-adjusting material may be cut or broken into small particles and sized with a sieve. Generally, the particles should have a size that allows for the particles to be transportable into a subterranean formation and be activated. In certain embodiments of the present invention, the particles are about 30 to about 80 mesh. These mesh sizes generally correspond to a particle size of from about 0.1 mm to 5 mm as measured along the longest axis.
In a particulate form, the delayed release pH-adjusting material may slowly degrade or disassociate in the fracturing fluid. This will allow for the release of the pH-adjusting agent, resulting in change in the pH of the fracturing fluid downhole, which causes delayed crosslinking by the pH sensitive crosslinking agent. To avoid premature degradation or disassociation of the delayed release pH-adjusting material, the delayed release pH-adjusting material can be added to the fracturing fluid immediately before introducing the fracturing fluid into the subterranean formation. In other examples, the delayed release pH-adjusting material could be added to the fracturing fluid once the fracturing fluid is in the formation.
Once down the well, the elevated temperatures may increase the rate of degradation or disassociation of the delayed release pH-adjusting material, depending on the composition of the delayed release pH-adjusting material, thereby allowing the viscosifying agent to be crosslinked at a time before placement of the fluid in the desired location of the well bore. In such instances, it may be desirable to delay activiation of the crosslinking agent by encapsulating the delayed release pH-adjusting material with an outer coating (e.g., a porous coating through which the crosslinking agent may diffuse slowly, or a degradable coating that degrades downhole) that delays the release of the pH-adjusting agent until a desired time or place. As used herein, the term “coating,” or “outer coating” and the like, does not imply any particular degree of coating on the particulate. In particular, the terms “coat” or “coating” do not imply 100% coverage by the coating on the particulate. In some embodiments, an outer coating, including degree of coating, may be used to control the rate of degradation of delayed release pH-adjusting material. For example, in one embodiment of the present invention, the outer coating may be configured to delay the degradation of the delayed release pH-adjusting material until the delayed release pH-adjusting material is in the portion of the subterranean formation to be fractured. The time period for degradation of the delayed pH-adjusting material may, in some embodiments, be from 1 to 10 minutes, depending on a number of factors, such as pump rate, pipe dimensions, and the like.
In some embodiments, the outer coating is formed of a water-insoluble material and has a melting point of from about 100° F. to about 500° F. A water insoluble material may prevent the outer coating from dissolving in the fracturing fluid until desired placement. However, once the fluid is down the well, elevated temperatures that occur down well can cause the outer coating to melt.
In the present invention, suitable outer coating materials include at least one selected from polysaccharides such as dextran and cellulose, chitins, lipids, latex, wax, chitosans, proteins, aliphatic polyesters, poly(lactides), poly(glycolides), poly(ε-caprolactones), poly(hydroxybutyrates), poly(anhydrides), aliphatic polycarbonates, orthoesters, poly(orthoesters), poly(amino acids), poly(ethylene oxides) and polyphosphazenes, derivatives thereof, copolymers thereof, and combinations thereof.
The concentration of the delayed release pH-adjusting material may be suitable to effectively alter the pH of the fracturing fluid to allow for a pH sensitive crosslinking agent to crosslink the viscosifying agent. In certain embodiments, the delayed release pH-adjusting material may be present in the fracturing fluid of the present invention in an amount in the range of from about 0.005% to about 0.1% by weight of the composition. In another example, as used in wellsite operations, a concentration of delayed release pH-adjusting material equivalent to about 0.1 to about 5 gallons of 25% sodium hydroxide per 1000 gallons by volume of the fracturing fluid is used. One of ordinary skill in the art, with the benefit of this disclosure, will recognize the appropriate amount of delayed release pH-adjusting material to include in a fracturing fluid of the present invention based on various conditions in the well.
In other embodiments, the fracturing fluid may also comprise a proppant material. Examples of suitable proppant materials include at least one selected from sand, bauxite, ceramic material, glass material, polymer material, polytetrafluoroethylene materials, nut shell pieces, cured resinous particulates comprising nut shell pieces, seed shell pieces, cured resinous particulates comprising seed shell pieces, fruit pit pieces, cured resinous particulates comprising fruit pit pieces, wood, and composite particulate. The mean particulate size generally may range from about 2 mesh to about 400 mesh on the U.S. Sieve Series; however, in certain circumstances, other mean particulate sizes may be desired and will be entirely suitable for practice of the present invention. In particular embodiments, preferred mean particulates size distribution ranges are one or more of 6/12, 8/16, 12/20, 16/30, 20/40, 30/50, 40/60, 40/70, or 50/70 mesh. It should be understood that the term “particulate,” as used in this disclosure, includes all known shapes of materials, including substantially spherical materials, fibrous materials, polygonal materials (such as cubic materials), and combinations thereof. Moreover, fibrous materials, that may or may not be used to bear the pressure of a closed fracture, may be included in certain embodiments of the present invention. In certain embodiments, the particulates may be present in the first treatment fluids of the present invention in an amount in the range of from about 0.5 pounds per gallon (“ppg”) to about 30 ppg by volume of the treatment fluid.
To facilitate a better understanding of the present invention, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.
3 g of 25% sodium hydroxide solution is added to 1 g of Sipernat® Silica Gel (Evonik Industries AG, Hanau-Wolfgang, Germany) in a beaker. The mixture is covered and allowed to react at room temperature for 10 days to obtain a solgel material.
An experiment was run to determine the delay in the change in pH of a test delayed release pH-adjusting material. A pH probe is calibrated using pH 4, 7 and 10 pH calibration standards. For each test sample, 50 ml of Deionized water is placed in a beaker, and a sample of the delayed release pH-adjusting material is added to the beaker while the pH of the solution is being measured continuously. 50 mg of 25% NaOH was used as a control. Two samples, a 25 mg sample of the NaOH sol gel and a 50 mg sample of the NaOH sol gel, were tested as delayed release pH-adjusting materials.
As can be seen in
Thus, the solgel/NaOH mixture allows for a delay in the rise of pH.
Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. The invention illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of “comprising,” “containing,” or “including” various components or steps, the compositions and methods can also “consist essentially of” or “consist of” the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, “from about a to about b,” or, equivalently, “from approximately a to b,” or, equivalently, “from approximately a-b”) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles “a” or “an,” as used in the claims, are defined herein to mean one or more than one of the element that it introduces. If there is any conflict in the usages of a word or term in this specification and one or more patent or other documents that may be incorporated herein by reference, the definitions that are consistent with this specification should be adopted.
