Patent Grant
7281580
The present invention relates to high porosity propped fractures and methods of creating high porosity propped fractures in portions of subterranean formations.
Subterranean wells (such as hydrocarbon producing wells, water producing wells, and injection wells) are often stimulated by hydraulic fracturing treatments. In hydraulic fracturing treatments, a viscous fracturing fluid, which also functions as a carrier fluid, is pumped into a portion of a subterranean formation at a rate and pressure such that the subterranean formation breaks down and one or more fractures are formed. Typically, particulate solids, such as graded sand, are suspended in a portion of the fracturing fluid are then deposited in the fractures. These particulate solids, or “proppant particulates,” serve to prevent the fractures from fully closing once the hydraulic pressure. By keeping the fracture from fully closing, the proppant particulates aid in forming conductive paths through which fluids may flow.
Commonly used proppant particulates generally comprise substantially spherical particles, such as graded sand, bauxite, ceramics, or even nut hulls. Generally, the proppant particulates are placed in the fracture in a concentration such that they formed a tight pack of particulates. Unfortunately, in such traditional operations, when fractures close upon the proppant particulates they can crush or become compacted, potentially forming non-permeable or low permeability masses within the fracture rather than desirable high permeability masses; such low permeability masses may choke the flow path of the fluids within the formation. Furthermore, the proppant particulates may become embedded in particularly soft formations, negatively impacting production.
The degree of success of a fracturing operation depends, at least in part, upon fracture porosity and conductivity once the fracturing operation is stopped and production is begun. Traditional fracturing operations place a large volume of proppant particulates into a fracture and the porosity of the resultant packed propped fracture is then related to the interconnected interstitial spaces between the abutting proppant particulates. Thus, the resultant fracture porosity from a traditional fracturing operation is closely related to the strength of the placed proppant particulates (if the placed particulates crush then the pieces of broken proppant may plug the interstitial spaces) and the size and shape of the placed particulate (larger, more spherical proppant particulates generally yield increased interstitial spaces between the particulates).
One way proposed to combat problems inherent in tight proppant particulate packs involves placing a much reduced volume of proppant particulates in a fracture to create what is referred to herein as a partial monolayer or “high porosity” fracture. In such operations the proppant particulates within the fracture may be widely spaced but they are still sufficient to hold the fracture open and allow for production. Such operations allow for increased fracture conductivity due, at least in part, to the fact the produced fluids may flow around widely spaced proppant particulates rather than just through the relatively small interstitial spaces in a packed proppant bed.
While this concept of partial monolayer fracturing has been investigated in the industry, the concept has not been successfully applied for a number of reasons. One problem is that successful placement of a partial monolayer of proppant particulates presents unique challenges in the relative densities of the particulates versus the carrier fluid. Another problem lies in the fact that placing a proppant that tends to crush or embed under pressure may allow the fracture to pinch or close in places once the fracturing pressure is released.
The present invention relates to high porosity propped fractures and methods of creating high porosity propped fractures in portions of subterranean formations.
One embodiment of the present invention provides a method of fracturing a portion of a subterranean formation so as to form a high porosity propped fracture comprising providing a slurry comprising a fracturing fluid and high density plastic particulates coated with an adhesive substance; introducing the slurry into a portion of a fracture within the subterranean formation; and, depositing the high density plastic proppant particulates into a portion of at least one fracture so as to form a high porosity propped fracture.
Another embodiment of the present invention provides a method of forming a high porosity propped fracture in a subterranean formation, comprising providing a slurry comprising a fracturing fluid and proppant particulates coated with an adhesive substance; introducing the slurry into a portion of a fracture within the subterranean formation; and, depositing the proppant particulates into the portion of the fracture within the subterranean formation so as to form a high porosity propped fracture.
Another embodiment of the present invention provides a high porosity propped fracture comprising high density plastic particulates substantially coated with an adhesive substance wherein the propped fracture has a porosity of at least about 50%.
Another embodiment of the present invention provides a high porosity propped fracture comprising proppant particulates substantially coated with an adhesive substance wherein the propped fracture has a porosity of at least about 50%.
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 present invention relates to high porosity propped fractures and methods of creating high porosity propped fractures in portions of subterranean formations.
The present invention provides methods of creating high porosity fractures. In certain methods of the present invention, proppant particulates coated with an “adhesive substance” are placed at relatively low concentrations within a subterranean fracture to create a high porosity propped fracture. As used herein, the term “adhesive substance” refers to a material that is capable of being coated onto a particulate and that exhibits a sticky or tacky character such that the proppant particulates that have adhesive thereon have a tendency to create clusters or aggregates. As used herein, the term “tacky,” in all of its forms, generally refers to a substance having a nature such that it is (or may be activated to become) somewhat sticky to the touch. As used herein, the term “high porosity fracture” refers to a proppant fracture having a porosity greater than about 40%.
I. High-Porosity Propped Fractures
Porosity values expressed herein are unstressed porosities, that is, the porosity before the fracture has closed or applied any substantial mechanical stress. By way of example, one embodiments of the present invention created a 70% porosity fracture with Nylon 6 proppant and, once 4,000 psi of stress was applied and the system was allowed to come to rest, the resultant porosity was 58%.
The methods of the present invention may be used, inter alia, to create high porosity fractures having increased conductivity as compared to a traditional packed propped fracture. The greater conductivity is believed to be due, at least in part, to a high porosity fracture that may be formed using a lower than traditional proppant loading in combination with an adhesive substance that encourages the formation of aggregates of proppant particulates that may then form pillars of proppant particulates within the fracture.
The use of lower than traditional proppant loading in combination with an adhesive substance may facilitate the formation of a conductive fracture with porosity much greater than about 40% while still maintaining enough conductive channels for production. Some embodiments of the present invention may be used to form a fracture exhibiting a porosity of at least about 50%. Other embodiments of the present invention may be used to form a fracture exhibiting a porosity of at least about 60%. Other embodiments of the present invention may be used to form a fracture exhibiting a porosity of at least about 70%. Other embodiments of the present invention may be used to form a fracture exhibiting a porosity of at least about 80%. Other embodiments of the present invention may be used to form a fracture exhibiting a porosity of at least about 90%.
The lower than traditional proppant loading in combination with an adhesive substance as used in the present invention may allow for increased conductivity and increased proppant particulate performance, at least in part, because the high porosity fractures they form allow for increased levels of open channels. With a high porosity fracture there may be more open spaces in the propped fracture that may remain open, even under severe closure stresses than found in traditional, high proppant loading applications.
By increasing the percentage of open spaces within a propped fracture, the methods of the present invention may act not only to increase the available space for production but also to eliminate non-darcy effects during production. Generally, non-Darcy effects are caused by inertial forces due to expansion and contraction of the local flow inside flow channels found in typical proppant packs. The high porosity propped fractures, decrease or eliminate the cycles of expansion and contraction because the interstitial spaces found in traditional propped fractures are not present. The article, Recent Advances in Hydraulic Fracturing, Gidley, J. L., et al. (ed.), Society of Petroleum Engineers, Richardson, Tex. (1989) discusses non-Darcy flow and its effects on conductivity of proppant beds and fractures, its relevant teachings are hereby incorporated by reference.
In some embodiments, once the proppant particulates are coated with an adhesive substance, the coated proppant particulates have the tendency to adhere to each other when they are in contact with one another. The adhesive should be strong enough that the proppant particulates remain clustered together while under static condition or under low shear rates. As the shear rate increases, the proppant clusters or aggregates may become dispersed into smaller clusters or even individual proppant particulates. This phenomenon may repeat again and again from the time the coated proppant is introduced into the fracturing fluid, pumped into the well bore and fracture, and even after being placed inside the fracture.
The present invention describes reduced particulate loadings to create a high porosity fracture compared to traditional fracturing applications that create packed fractures. Tables 1 and 2 provide example proppant loading schedules for a fracturing treatment. As will be understood by one skilled in the art, each operation is unique, and thus, may require its own unique proppant addition schedule. However, the example in Table 1 shows one possible addition schedule for achieving a high porosity fracture having a porosity in excess of about 75% for most of the propped fracture area. By contrast, Table 2 shows the proppant addition schedule for an operation placing a traditional packed proppant bed within a fracture that results in a packed fracture with porosity around 40% for most of the propped fracture area.
II. Suitable Proppant Particulates
A. Proppant Particulates—Size and Shape
Proppant particulates suitable for use in the methods of the present invention may be of any size and shape combination known in the art as suitable for use in a fracturing operation. Generally, where the chosen proppant is substantially spherical, suitable proppant particulates have a size in the range of from about 2 to about 400 mesh, U.S. Sieve Series. In some embodiments of the present invention, the proppant particulates have a size in the range of from about 8 to about 120 mesh, U.S. Sieve Series.
In some embodiments of the present invention it may be desirable to use substantially non-spherical proppant particulates. Suitable substantially non-spherical proppant particulates may be cubic, polygonal, fibrous, or any other non-spherical shape. Such substantially non-spherical proppant particulates may be, for example, cubic-shaped, rectangular shaped, rod shaped, ellipse shaped, cone shaped, pyramid shaped, or cylinder shaped. That is, in embodiments wherein the proppant particulates are substantially non-spherical, the aspect ratio of the material may range such that the material is fibrous to such that it is cubic, octagonal, or any other configuration. Substantially non-spherical proppant particulates are generally sized such that the longest axis is from about 0.02 inches to about 0.3 inches in length. In other embodiments, the longest axis is from about 0.05 inches to about 0.2 inches in length. In one embodiment, the substantially non-spherical proppant particulates are cylindrical having an aspect ratio of about 1.5 to 1 and about 0.08 inches in diameter and about 0.12 inches in length. In another embodiment, the substantially non-spherical proppant particulates are cubic having sides about 0.08 inches in length. The use of substantially non-spherical proppant particulates may be desirable in some embodiments of the present invention because, among other things, they may provide a lower rate of settling when slurried into a fluid as is often done to transport proppant particulates to desired locations within subterranean formations. By so resisting settling, substantially non-spherical proppant particulates may provide improved proppant particulate distribution as compared to more spherical proppant particulates.
In poorly consolidated formations (that is, formations that, when assessed, fail to produce a core sample that can be satisfactorily drilled, cut, etc.) the use of substantially non-spherical proppant particulates may also help to alleviate the embedment of proppant particulates into the formation surfaces (such as a fracture face). As is known by one skilled in the art, when substantially spherical proppant particulates are placed against a formation surface under stress, such as when they are used to prop a fracture, they are subject to point loading. By contrast, substantially non-spherical proppant particulates may be able to provide a greater surface area against the formation surface and thus may be better able to distribute the load of the closing fracture.
B. Proppant Particulates—Materials of Manufacture
Proppant particulates suitable for use in the present invention include graded sand, resin coated sand, bauxite, ceramic materials, glass materials, walnut hulls, polymeric materials, resinous materials, rubber materials, and the like. In some embodiments of the present invention, the proppant particulates suitable for use in the present invention are composed of at least one high density plastic. As used herein, the term “high density plastic” refers to a plastic having a specific gravity of greater than about 1. The preferable density range is from about 1 to about 2. More preferably the range is from about 1 to about 1.3. The most preferable is from about 1.1 to 1.2. In addition to being a high density plastic, plastics suitable for use in the present invention generally exhibit a crystallinity of greater than about 10%. In some embodiments, the high density plastic used to form the proppant particulates of the present invention exhibits a crystallinity of greater than about 20%. While the material is referred to as “high density,” it will be readily understood by one skilled in the art that the density is “high” relative to other plastics, but may be low as compared to traditional proppant particulate densities. For example, Ottawa sand may exhibit a specific gravity of about 2.65 whereas man-made ceramic proppants generally have specific gravities ranging from about 2.7 to about 3.6. The relatively low density of the high density plastics used to create the proppant particulates of the present invention may be beneficial to an even distribution when the proppant particulates are slurried into a fluid such as a fracturing fluid. Such even distribution may be particularly helpful in forming a high porosity proppant pack that is capable of holding open the majority of a fracture. Uneven distribution could result in a situation wherein a portion of a fracture is propped while another portion is substantially void of proppant particulates and thus, does not remain open once the hydraulic pressure is released.
Some well-suited high density plastic materials include polyamide 6 (Nylon 6), polyamide 66 (Nylon 6/6), acrylic, acrylonitrile butadiene styrene (ABS), ethylene vinyl alcohol, polycarbonate/PET polyester blend, polyethylene terephthalate (PET), unreinforced polycarbonate/polybutylene terephthalate (PC/PBT) blend, PETG copolyester, polyetherimide, polyphenylene ether, molded polyphenylene sulfide (PPS), heat resistant grade polystyrene, polyvinylbenzene, polyphenylene oxide, a blend of polyphenylene oxide and nylon 6/6, acrylonitrile-butadiene-styrene, polyvinylchloride, fluoroplastics, polysulfide, polypropylene, styrene acrylonitrile, polystyrene, phenylene oxide, polyolefins, polystyrene divinylbenzene, polyfluorocarbons, polyethers etherketones, polyamide imides, and combinations thereof. Some other well-suited high density plastic materials include oil-resistant thermoset resins such as acrylic-based resins, epoxy-based resins, furan-based resins, phenolic-based resins, phenol/phenol formaldehyde/furfuryl alcohol resins, polyester resins, and combinations thereof.
In some embodiments of the present invention it may be desirable to reinforce the proppant particulates made of high density plastic to increase their resistance to a crushing or deforming force. Suitable reinforcing materials include high strength particles such as bauxite, nut hulls, ceramic, metal, glass, sand, asbestos, mica, silica, alumina, and any other available material that is smaller in size than the desired, final high density plastic proppant particulate and that is capable of adding structural strength to the desired, final high density plastic proppant particulate. In some embodiments of the present invention the reinforcing material may be a fibrous material such as glass fibers or cotton fibers. Preferably, the reinforcing material is chosen so as to not unduly increase the specific gravity of the final proppant particulate.
One benefit of using proppant particulates formed from high density plastic is that they may be created on-the-fly during a fracturing or frac-packing operation. U.S. patent application Ser. No. 10/853,879 filed May 26, 2004 and titled “On-The-Fly Preparation of Proppant and its Use in Subterranean Operations,” the relevant disclosure of which is hereby incorporated by reference, describes methods of creating proppant particulates from thermoplastic materials on-the-fly. As described in that application, one example of a method for preparing proppant on-the-fly generally comprises providing a mixture comprising a thermoplastic/thermosetting polymer, and a filler, heating the resin mixture, extruding, atomizing, or spraying the mixture to particulate form into a well bore containing a treatment fluid; and allowing the extruded particulate to substantially cure and form proppant particles. This method relies, at least in part, on the ability of thermoplastic/thermosetting materials to be extruded from a liquid form at an elevated temperature, and then as the material cools, to then harden and form into a solid material. The thermoplastic or thermosetting proppant particulates can be prepared on-the-fly, according to the present invention, to a suitable size and shape.
Density and strength of proppant particulates formed from thermoplastic/thermosetting materials may be customized to meet the fracturing designs and well conditions. To help eliminate the problems that may be caused by large particle size, in one embodiment the on-the-fly thermoplastic proppant particulates may be introduced into the fracturing fluid at the discharge side of the pump. As will be recognized by one skilled in the art, during pumping of such on-the-fly proppant particulates (particularly where the flow passes through one or more perforations), the proppant particulates may break into smaller sizes as a result of high shear as they are being placed inside a portion of a subterranean formation.
C. Degradable Particles
Generally, the high density plastics suitable for use in forming the proppant particulates of the present invention are oil-resistant. That is, they do not degrade in the presence of hydrocarbon fluids and other fluids present in the subterranean formation; this allows the proppant particulates to maintain their integrity in the presence of produced hydrocarbon products, formation water, and other compositions normally produced from subterranean formations. However, in some embodiments of the present invention, a portion of the proppant particulates may be formed from degradable particles. One purpose of including degradable particulates in a high porosity propped fracture (be it a high porosity fracture or a packed fracture) is to ensure the permeability of the propped fracture. Despite of the preference in forming partial monolayer of lightweight, high strength particulates, the potential for forming a full monolayer or a packed potion in the fracture always exists due to, inter alia, to uneven distribution, particle accumulation, or particle settling.
In some embodiments the degradable particles used are oil-degradable materials. Where such oil-degradable proppant particulates are used, in the event the closure of the fracture undesirably compacts the proppant (thus undesirably reducing the permeability of the proppant pack) the oil-degradable proppant may be degraded by the produced fluids, thus restoring at least some of the lost permeability. The degradable proppant may also be degraded by materials purposely placed in the formation by injection, mixing the degradable particle with delayed reaction degradation agents, or other suitable means to induce degradation.
In some embodiments of the present invention, a high porosity propped fracture may be formed using proppant particulates and degradable particulates. Thus, as the degradable particulates are removed with time, the porosity of the propped fracture increases. The degradable particulates are preferably substantially uniformly distributed throughout the formed proppant pack. Over time, the degradable material will degrade in situ, causing the degradable material to substantially be removed from the proppant pack and to leave behind voids in the proppant pack. These voids enhance the porosity of the proppant pack, which may result, inter alia, in enhanced conductivity.
Suitable degradable materials include oil-degradable polymers. Oil-degradable polymers that may be used in accordance with the present invention may be either natural or synthetic polymers. Some particular examples include, but are not limited to, polyacrylics, polyamides, and polyolefins such as polyethylene, polypropylene, polyisobutylene, and polystyrene. Other suitable oil-degradable polymers include those that have a melting point which is such that the polymer will dissolve at the temperature of the subterranean formation in which it is placed such as a wax material.
In addition to oil-degradable polymers, other degradable materials that may be used in conjunction with the present invention include but are not limited to degradable polymers, dehydrated salts, and/or mixtures of the two.
As for degradable polymers, a polymer is considered to be “degradable” herein if the degradation is due to, inter alia, a chemical and/or radical process such as hydrolysis, oxidation, or UV radiation. The degradability of a polymer depends at least in part on its backbone structure. For instance, the presence of hydrolyzable and/or oxidizable linkages in the backbone often yields a material that will degrade as described herein. The rates at which such polymers degrade are dependent on the type of repetitive unit, composition, sequence, length, molecular geometry, molecular weight, morphology (e.g., crystallinity, size of spherulites, and orientation), hydrophilicity, hydrophobicity, surface area, and additives. Also, the environment to which the polymer is subjected may affect how it degrades, e.g., temperature, presence of moisture, oxygen, microorganisms, enzymes, pH, and the like.
It is desirable that the degradable particulate has similar particle size, shape, and specific gravity as those of the lightweight particulate to enhance the distribution of degradable particulate among the lightweight particulate and to minimize the segregation between the particulate materials.
In case the two particulate materials are different in shape, particle size, and specific gravity, a tackifying compound can be coated onto the particulate materials to enhance their uniform distribution as they are blended, pumped down hole, and placed inside the created fracture.
Suitable examples of degradable polymers that may be used in accordance with the present invention include but are not limited to those described in the publication of Advances in Polymer Science, Vol. 157 entitled “Degradable Aliphatic Polyesters” edited by A. C. Albertsson. Specific examples include homopolymers, random, block, graft, and star- and hyper-branched aliphatic polyesters. Polycondensation reactions, ring-opening polymerizations, free radical polymerizations, anionic polymerizations, carbocationic polymerizations, coordinative ring-opening polymerization, and any other suitable process may prepare such suitable polymers. Specific examples of suitable polymers include polysaccharides such as dextran or cellulose; chitins; chitosans; proteins; aliphatic polyesters; poly(lactides); poly(glycolides); poly(ε-caprolactones); poly(hydroxybutyrates); poly(anhydrides); aliphatic or aromatic polycarbonates; poly(orthoesters); poly(amino acids); poly(ethylene oxides); and polyphosphazenes. Of these suitable polymers, aliphatic polyesters and polyanhydrides are preferred.
Aliphatic polyesters degrade chemically, inter alia, by hydrolytic cleavage. Hydrolysis can be catalyzed by either acids or bases. Generally, during the hydrolysis, carboxylic end groups are formed during chain scission, and this may enhance the rate of further hydrolysis. This mechanism is known in the art as “autocatalysis,” and is thought to make polyester matrices more bulk eroding. Of the suitable aliphatic polyesters, poly(lactide) is preferred. Poly(lactide) is synthesized either from lactic acid by a condensation reaction or more commonly by ring-opening polymerization of cyclic lactide monomer. Since both lactic acid and lactide can achieve the same repeating unit, the general term poly(lactic acid) as used herein refers to formula I without any limitation as to how the polymer was made such as from lactides, lactic acid, or oligomers, and without reference to the degree of polymerization or level of plasticization.
Polyanhydrides are another type of particularly suitable degradable polymer useful in the present invention. Polyanhydride hydrolysis proceeds, inter alia, via free carboxylic acid chain-ends to yield carboxylic acids as final degradation products. The erosion time can be varied over a broad range of changes in the polymer backbone. Examples of suitable polyanhydrides include poly(adipic anhydride), poly(suberic anhydride), poly(sebacic anhydride), and poly(dodecanedioic anhydride). Other suitable examples include but are not limited to poly(maleic anhydride) and poly(benzoic anhydride).
Dehydrated salts may be used in accordance with the present invention as a degradable material. A dehydrated salt is suitable for use in the present invention if it will degrade over time as it hydrates. For example, a particulate solid anhydrous borate material that degrades over time may be suitable. Specific examples of particulate solid anhydrous borate materials that may be used include but are not limited to anhydrous sodium tetraborate (also known as anhydrous borax), and anydrous boric acid. These anhydrous borate materials are only slightly soluble in water. However, with time and heat in a subterranean environment, the anhydrous borate materials react with the surrounding aqueous fluid and are hydrated. The resulting hydrated borate materials are highly soluble in water as compared to anhydrous borate materials and as a result degrade in the aqueous fluid. In some instances, the total time required for the anhydrous borate materials to degrade in an aqueous fluid is in the range of from about 8 hours to about 72 hours depending upon the temperature of the subterranean zone in which they are placed. Other examples include organic or inorganic salts like acetate trihydrate.
Blends of certain degradable materials may also be suitable. One example of a suitable blend of materials is a mixture of poly(lactic acid) and sodium borate where the mixing of an acid and base could result in a neutral solution where this is desirable. Another example would include a blend of poly(lactic acid) and boric oxide. Other materials that undergo an irreversible degradation may also be suitable, if the products of the degradation do not undesirably interfere with either the conductivity of the proppant matrix or with the production of any of the fluids from the subterranean formation.
In choosing the appropriate degradable material, one should consider the degradation products that will result. These degradation products should not adversely affect other operations or components and may even be selected to improve the long term performance/conductivity of the propped fracture. The choice of degradable material also can depend, at least in part, on the conditions of the well, e.g., well bore temperature. For instance, lactides have been found to be suitable for lower temperature wells, including those within the range of 60° F. to 150° F., and polylactides have been found to be suitable for well bore temperatures above this range. Also, poly(lactic acid) may be suitable for higher temperature wells. Some stereoisomers of poly(lactide) or mixtures of such stereoisomers may be suitable for even higher temperature applications. Dehydrated salts may also be suitable for higher temperature wells.
In some embodiments a preferable result is achieved if the degradable material degrades slowly over time as opposed to instantaneously. Even more preferable results have been obtained when the degradable material does not begin to degrade until after the proppant matrix has developed some compressive strength. The slow degradation of the degradable material, inter alia, helps to maintain the stability of the proppant matrix.
The specific features of the degradable material may be chosen or modified to provide the high porosity fracture with optimum conductivity. Preferably, the degradable material is selected to have a size, and shape similar to the size and shape of the curable proppant particulates to help maintain substantial uniformity within the mixture. It is preferable if the proppant particulates and the degradable material do not segregate within the proppant composition. Whichever degradable material is used, the degradable materials may have any shape, depending on the desired characteristics of the resultant voids in the high porosity fracture including but not limited to particles having the physical shape of platelets, shavings, flakes, ribbons, rods, strips, spheroids, toroids, pellets, tablets, or any other physical shape. The physical shape of the degradable material should be chosen so as to enhance the desired shape and relative composition of the resultant voids within the high porosity fracture. One of ordinary skill in the art with the benefit of this disclosure will recognize the specific degradable material and the preferred size and shape for a given application.
In some embodiments of the present invention, from about 10% to about 90% of the total proppant particulates used to form the high porosity fracture are degradable. In other embodiments, from about 20% to about 70% of the total proppant particulates used to form the high porosity fracture are degradable. In still other embodiments, from about 25% to about 50% of the total proppant particulates used to form the high porosity fracture are degradable. One of ordinary skill in the art with the benefit of this disclosure will recognize an optimum concentration of degradable material that provides desirable values in terms of enhanced conductivity or permeability without undermining the stability of the high porosity fracture itself.
III. Adhesive Substances Suitable for Use in the Present Invention
Adhesive substances suitable for use in the present invention include non-aqueous tackifying agents; aqueous tackifying agents; silyl-modified polyamides; and curable resin compositions that are capable of curing to form hardened substances. In addition to encouraging the proppant particulates to form aggregates, the use of an adhesive substance may yield a propped fracture that experiences very little or no undesirable proppant flow back. As described in more detail above, the application of an adhesive substance to the proppant particulates used to create a high porosity fracture may aid in the formation of aggregates that increase the ability of a small amount of proppant particulates to effectively hold open a fracture for production. Adhesive substances may be applied on-the-fly, applying the adhesive substance to the proppant particulate at the well site, directly prior to pumping the fluid-proppant mixture into the well bore.
A. Adhesive Substances—Non-aqueous Tackifying Agents
Tackifying agents suitable for use in the consolidation fluids of the present invention comprise any compound that, when in liquid form or in a solvent solution, will form a non-hardening coating upon a particulate. A particularly preferred group of tackifying agents comprise polyamides that are liquids or in solution at the temperature of the subterranean formation such that they are, by themselves, non-hardening when introduced into the subterranean formation. A particularly preferred product is a condensation reaction product comprised of commercially available polyacids and a polyamine. Such commercial products include compounds such as mixtures of C36 dibasic acids containing some trimer and higher oligomers and also small amounts of monomer acids that are reacted with polyamines. Other polyacids include trimer acids, synthetic acids produced from fatty acids, maleic anhydride, acrylic acid, and the like. Such acid compounds are commercially available from companies such as Witco Corporation, Union Camp, Chemtall, and Emery Industries. The reaction products are available from, for example, Champion Technologies, Inc. and Witco Corporation. Additional compounds which may be used as tackifying compounds include liquids and solutions of, for example, polyesters, polycarbonates and polycarbamates, natural resins such as shellac and the like. Other suitable tackifying agents are described in U.S. Pat. No. 5,853,048 issued to Weaver, et al. and U.S. Pat. No. 5,833,000 issued to Weaver, et al., the relevant disclosures of which are herein incorporated by reference.
Tackifying agents suitable for use in the present invention may be either used such that they form non-hardening coating or they may be combined with a multifunctional material capable of reacting with the tackifying compound to form a hardened coating. A “hardened coating” as used herein means that the reaction of the tackifying compound with the multifunctional material will result in a substantially non-flowable reaction product that exhibits a higher compressive strength in a consolidated agglomerate than the tackifying compound alone with the particulates. In this instance, the tackifying agent may function similarly to a hardenable resin. Multifunctional materials suitable for use in the present invention include, but are not limited to, aldehydes such as formaldehyde, dialdehydes such as glutaraldehyde, hemiacetals or aldehyde releasing compounds, diacid halides, dihalides such as dichlorides and dibromides, polyacid anhydrides such as citric acid, epoxides, furfuraldehyde, glutaraldehyde or aldehyde condensates and the like, and combinations thereof. In some embodiments of the present invention, the multifunctional material may be mixed with the tackifying compound in an amount of from about 0.01 to about 50 percent by weight of the tackifying compound to effect formation of the reaction product. In some preferable embodiments, the compound is present in an amount of from about 0.5 to about 1 percent by weight of the tackifying compound. Suitable multifunctional materials are described in U.S. Pat. No. 5,839,510 issued to Weaver, et al., the relevant disclosure of which is herein incorporated by reference. Other suitable tackifying agents are described in U.S. Pat. No. 5,853,048 issued to Weaver, et al.
Solvents suitable for use with the tackifying agents of the present invention include any solvent that is compatible with the tackifying agent and achieves the desired viscosity effect. The solvents that can be used in the present invention preferably include those having high flash points (most preferably above about 125° F.). Examples of solvents suitable for use in the present invention include, but are not limited to, butylglycidyl ether, dipropylene glycol methyl ether, butyl bottom alcohol, dipropylene glycol dimethyl ether, diethyleneglycol methyl ether, ethyleneglycol butyl ether, methanol, butyl alcohol, isopropyl alcohol, diethyleneglycol butyl ether, propylene carbonate, d'limonene, 2-butoxy ethanol, butyl acetate, furfuryl acetate, butyl lactate, dimethyl sulfoxide, dimethyl formamide, fatty acid methyl esters, and combinations thereof. It is within the ability of one skilled in the art, with the benefit of this disclosure, to determine whether a solvent is needed to achieve a viscosity suitable to the subterranean conditions and, if so, how much.
B. Adhesive Substances—Aqueous Tackifying Agents
Suitable aqueous tackifier agents are capable of forming at least a partial coating upon the surface of a particulate (such as a proppant particulate). Generally, suitable aqueous tackifier agents are not significantly tacky when placed onto a particulate, but are capable of being “activated” (that is destabilized, coalesced and/or reacted) to transform the compound into a sticky, tackifying compound at a desirable time. Such activation may occur before, during, or after the aqueous tackifier compound is placed in the subterranean formation. In some embodiments, a pretreatment may be first contacted with the surface of a particulate to prepare it to be coated with an aqueous tackifier compound. Suitable aqueous tackifying agents are generally charged polymers that comprise compounds that, when in an aqueous solvent or solution, will form a non-hardening coating (by itself or with an activator) and, when placed on a particulate, will increase the continuous critical resuspension velocity of the particulate when contacted by a stream of water (further described in Example 7). The aqueous tackifier compound may enhance the grain-to-grain contact between the individual particulates within the formation (be they proppant particulates, formation fines, or other particulates), helping bring about the consolidation of the particulates into a cohesive, flexible, and permeable mass.
Examples of aqueous tackifier agents suitable for use in the present invention include, but are not limited to, acrylic acid polymers, acrylic acid ester polymers, acrylic acid derivative polymers, acrylic acid homopolymers, acrylic acid ester homopolymers (such as poly(methyl acrylate), poly (butyl acrylate), and poly(2-ethylhexyl acrylate)), acrylic acid ester co-polymers, methacrylic acid derivative polymers, methacrylic acid homopolymers, methacrylic acid ester homopolymers (such as poly(methyl methacrylate), poly(butyl methacrylate), and poly(2-ethylhexyl methacryate)), acrylamido-methyl-propane sulfonate polymers, acrylamido-methyl-propane sulfonate derivative polymers, acrylamido-methyl-propane sulfonate co-polymers, and acrylic acid/acrylamido-methyl-propane sulfonate co-polymers and combinations thereof. Methods of determining suitable aqueous tackifier agents and additional disclosure on aqueous tackifier agents can be found in U.S. patent application Ser. No. 10/864,061 and filed Jun. 9, 2004 and U.S. patent application Ser. No. 10/864,618 and filed Jun. 9, 2004 the relevant disclosures of which are hereby incorporated by reference.
C. Adhesive Substances—Silyl-Modified Polyamides
Silyl-modified polyamide compounds suitable for use as an adhesive substance in the methods of the present invention may be described as substantially self-hardening compositions that are capable of at least partially adhering to particulates in the unhardened state, and that are further capable of self-hardening themselves to a substantially non-tacky state to which individual particulates such as formation fines will not adhere to, for example, in formation or proppant pack pore throats. Such silyl-modified polyamides may be based, for example, on the reaction product of a silating compound with a polyamide or a mixture of polyamides. The polyamide or mixture of polyamides may be one or more polyamide intermediate compounds obtained, for example, from the reaction of a polyacid (e.g., diacid or higher) with a polyamine (e.g., diamine or higher) to form a polyamide polymer with the elimination of water. Other suitable silyl-modified polyamides and methods of making such compounds are described in U.S. Pat. No. 6,439,309 issued to Matherly, et al., the relevant disclosure of which is herein incorporated by reference.
D. Adhesive Substances—Curable Resins
Resins suitable for use in the consolidation fluids of the present invention include all resins known in the art that are capable of forming a hardened, consolidated mass. Many such resins are commonly used in subterranean consolidation operations, and some suitable resins include two component epoxy based resins, novolak resins, polyepoxide resins, phenol-aldehyde resins, urea-aldehyde resins, urethane resins, phenolic resins, furan resins, furan/furfuryl alcohol resins, phenolic/latex resins, phenol formaldehyde resins, polyester resins and hybrids and copolymers thereof, polyurethane resins and hybrids and copolymers thereof, acrylate resins, and mixtures thereof. Some suitable resins, such as epoxy resins, may be cured with an internal catalyst or activator so that when pumped down hole, they may be cured using only time and temperature. Other suitable resins, such as furan resins generally require a time-delayed catalyst or an external catalyst to help activate the polymerization of the resins if the cure temperature is low (i.e., less than 250° F.), but will cure under the effect of time and temperature if the formation temperature is above about 250° F., preferably above about 300° F. It is within the ability of one skilled in the art, with the benefit of this disclosure, to select a suitable resin for use in embodiments of the present invention and to determine whether a catalyst is required to trigger curing.
Any solvent that is compatible with the resin and achieves the desired viscosity effect is suitable for use in the present invention. Preferred solvents include those listed above in connection with tackifying compounds. It is within the ability of one skilled in the art, with the benefit of this disclosure, to determine whether and how much solvent is needed to achieve a suitable viscosity.
IV. Fracturing Fluids
Any fracturing fluid suitable for a fracturing or frac-packing application may be used in accordance with the teachings of the present invention, including aqueous gels, viscoelastic surfactant gels, oil gels, foamed gels and emulsions. Suitable aqueous gels are generally comprised of water and one or more gelling agents. Suitable emulsions can be comprised of two immiscible liquids such as an aqueous liquid or gelled liquid and a hydrocarbon. Foams can be created by the addition of a gas, such as carbon dioxide or nitrogen. In exemplary embodiments of the present invention, the fracturing fluids are aqueous gels comprised of water, a gelling agent for gelling the water and increasing its viscosity, and, optionally, a crosslinking agent for crosslinking the gel and further increasing the viscosity of the fluid. The increased viscosity of the gelled, or gelled and cross-linked, fracturing fluid, inter alia, reduces fluid loss and allows the fracturing fluid to transport significant quantities of suspended proppant particles. The water used to form the fracturing fluid may be salt water, brine, or any other aqueous liquid that does not adversely react with the other components. The density of the water can be increased to provide additional particle transport and suspension in the present invention.
A variety of gelling agents may be used, including hydratable polymers that contain one or more functional groups such as hydroxyl, carboxyl, sulfate, sulfonate, amino, or amide groups. Suitable gelling typically comprise polymers, synthetic polymers, or a combination thereof. A variety of gelling agents can be used in conjunction with the methods and compositions of the present invention, including, but not limited to, hydratable polymers that contain one or more functional groups such as hydroxyl, cis-hydroxyl, carboxylic acids, derivatives of carboxylic acids, sulfate, sulfonate, phosphate, phosphonate, amino, or amide. In certain exemplary embodiments, the gelling agents may be polymers comprising polysaccharides, and 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 polymers include, but are not limited to, guar gum and derivatives thereof, such as hydroxypropyl guar and carboxymethylhydroxypropyl guar, and cellulose derivatives, such as hydroxyethyl cellulose. Additionally, synthetic polymers and copolymers that contain the above-mentioned functional groups may be used. Examples of such synthetic polymers include, but are not limited to, polyacrylate, polymethacrylate, polyacrylamide, polyvinyl alcohol, and polyvinylpyrrolidone. In other exemplary embodiments, the gelling agent molecule may be depolymerized. The term “depolymerized,” as used herein, generally refers to a decrease in the molecular weight of the gelling agent molecule. Depolymerized gelling agent molecules are described in U.S. Pat. No. 6,488,091 issued Dec. 3, 2002 to Weaver, et al., the relevant disclosure of which is incorporated herein by reference. Suitable gelling agents generally are present in the viscosified treatment fluids of the present invention in an amount in the range of from about 0.1% to about 5% by weight of the water therein. In certain exemplary embodiments, the gelling agents are present in the viscosified treatment fluids of the present invention in an amount in the range of from about 0.01% to about 2% by weight of the water therein
Crosslinking agents may be used to crosslink gelling agent molecules to form crosslinked gelling agents. Crosslinkers typically comprise at least one ion that is capable of crosslinking at least two gelling agent molecules. Examples of suitable crosslinkers include, but are not limited to, boric acid, disodium octaborate tetrahydrate, sodium diborate, pentaborates, ulexite and colemanite, compounds that can supply zirconium IV ions (such as, for example, zirconium lactate, zirconium lactate triethanolamine, zirconium carbonate, zirconium acetylacetonate, zirconium malate, zirconium citrate, and zirconium diisopropylamine lactate); compounds that can supply titanium IV ions (such as, for example, titanium lactate, titanium malate, titanium citrate, titanium ammonium lactate, titanium triethanolamine, and titanium acetylacetonate); aluminum compounds (such as, for example, aluminum lactate or aluminum citrate); antimony compounds; chromium compounds; iron compounds; copper compounds; zinc compounds; or a combination thereof. An example of a suitable commercially available zirconium-based crosslinker is “CL-24” available from Halliburton Energy Services, Inc., Duncan, Okla. An example of a suitable commercially available titanium-based crosslinker is “CL-39” available from Halliburton Energy Services, Inc., Duncan, Okla. Suitable crosslinkers generally are present in the viscosified treatment fluids of the present invention in an amount sufficient to provide, inter alia, the desired degree of crosslinking between gelling agent molecules. In certain exemplary embodiments of the present invention, the crosslinkers may be present in an amount in the range from about 0.001% to about 10% by weight of the water in the fracturing fluid. In certain exemplary embodiments of the present invention, the crosslinkers may be present in the viscosified treatment fluids of the present invention in an amount in the range from about 0.01% to about 1% by weight of the water therein. Individuals skilled in the art, with the benefit of this disclosure, will recognize the exact type and amount of crosslinker to use depending on factors such as the specific gelling agent, desired viscosity, and formation conditions.
The gelled or gelled and cross-linked fracturing fluids may also include internal delayed gel breakers such as enzyme, oxidizing, acid buffer, or temperature-activated gel breakers. The gel breakers cause the viscous carrier fluids to revert to thin fluids that can be produced back to the surface after they have been used to place proppant particles in subterranean fractures. The gel breaker used is typically present in the fracturing fluid in an amount in the range of from about 0.5% to about 10% by weight of the gelling agent. The fracturing fluids may also include one or more of a variety of well-known additives, such as gel stabilizers, fluid loss control additives, clay stabilizers, bactericides, and the like.
To facilitate a better understanding of the present invention, the following examples of preferred embodiments are given. In no way should the following examples be read to limit or define the scope of the invention.
Table 3 illustrates the conductivity that may be achieved when forming high porosity propped fractures of the present invention. The data shown in Table 3 represents a high porosity propped fracture comprising proppant particulates having a flattened pillow shape (substantially non-spherical) at a surface area concentration of about 0.09 pounds per square foot versus substantially spherical 20/40 mesh Ottawa sand at about two pounds per square foot and not having an adhesive coating. At a closure stress of about 2000 psi and at 105° F., a high porosity fracture formed using proppant particulates of the present invention has about ten times the conductivity of a pack formed from 20/40 mesh Ottawa sand at about two pounds per square foot. At a closure stress of about 3000 psi and at 150° F., a high porosity fracture formed using proppant particulates of the present invention was over two and a half times as conductive as the pack formed from 20/40 mesh Ottawa sand at about two pounds per square foot. At a closure stress of about 4000 psi and at 150° F., a high porosity fracture formed using proppant particulates of the present invention was over two and a quarter times as conductive as the pack formed from 20/40 mesh Ottawa sand at about two pounds per square foot. The high porosity fracture formed using proppant particulates of the present invention shows a porosity of about 70% at the start and reduced to about 58% at a closure stress of about 4000 psi and at 150° F.
Table 4 shows data for another material that can be used (cylindrical particles) for the present invention. Here the created fracture porosity ranges from 80% to 88%. The higher porosity fracture provides the greatest conductivity values. The addition of an adhesive agent (Sandwedge®, commercially available from Halliburton Energy Services, Duncan, Okla.) to create clusters shows there is additional increased conductivity due to larger channels being created. The porosity remains at 80% but the conductivity is increased due to the large channels.
Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. While numerous changes may be made by those skilled in the art, such changes are encompassed within the spirit of this invention as defined by the appended claims.
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