WO2016093690A1

WO2016093690A1 - Method for treating coalbed methane formation

Info

Publication number: WO2016093690A1
Application number: PCT/MY2014/000269
Authority: WO
Inventors: Diankui Fu; Kong Teng Ling
Original assignee: Schlumberger Technology B.V.; Schlumberger Canada Limited; Services Petroliers Schlumberger; Schlumberger Holdings Limited; Prad Research And Development Limited; Schlumberger Technology Corporation
Priority date: 2014-12-12
Filing date: 2014-12-12
Publication date: 2016-06-16

Abstract

A method for treating a subterranean coalbed methane formation including injecting a composition containing an anionic viscoelastic surfactant and an acid- generating material, degrading the acid-generating material in the subterranean coalbed methane formation to release an acid, and changing a wettability of a surface of the subterranean coalbed methane formation from hydrophilic to hydrophobic due to a reaction of the acid with the anionic viscoelastic surfactant.

Description

METHOD FOR TREATING COALBED METHANE FORMATION

BACKGROUND

[1] Subterranean coal seams contain substantial quantities of natural gas, primarily in the form of methane, that are adsorbed into the solid matrix of the coal. Such reservoirs are commonly referred to as coalbed methane (CBM) reservoirs. The bulk of the methane is adsorbed onto the coal surface. Due to the water saturation of coal reservoirs, coalbed methane production generally includes first removing large amounts of water to sufficiently reduce the hydrostatic pressure in the subsurface so that methane can desorb from the coal. As such, the proportion of water to methane pumped is initially high and decreases over time with increasing coalbed methane production.

[2] Unlike conventional subterranean formations, for example, sandstone, the surface of which is hydrophilic, the surface of coalbed methane formations is hydrophobic or "oil-wet." The dewatering process may be affected by wettability changes in the coal surface. For example, hydraulic fracturing fluids may affect the coal surface and lead to changes in the relative permeability of the coal, which can affect both the water drainage rate and fluid saturation level of the coal. Wettability changes to the surface of coal formations from oil-wet to, for example, hydrophilic or water-wet, may inhibit the dewatering process as the oil-wet surface of the coal allows the de-watering process to reduce the reservoir pressure for methane production.

[3] Although coalbed formations generally have very low permeability, for example, 0.1 to 30 md, coalbed formations are generally highly fractured or cleated.

Therefore, hydraulic fracturing may be used to connect the fractures or cleats for coalbed methane production. Slick water, crosslinked, and foam fluids have been used during hydraulic fracturing of coalbed methane formations. Due to poor proppant transport, slick water treatments provide insufficient propped fracture length. Crosslinked fluids cause damage to the fracture and formation from the residual polymer. Surfactants are generally used in foam fluids for foam generation and stability. However, most surfactants change the hydrophobic or oil-wetting surface of coalbed into hydrophilic or water-wetting, which inhibits methane production.

[4] Viscoelastic surfactant fluids, which have been widely used in hydraulic fracturing and acid fracturing of conventional reservoirs, for example, sandstone, limestone, 69

2 and dolomite formations, have excellent proppant transport and cause less damage to the formation. However, viscoelastic surfactant fluids have been undesirable for use in hydraulic fracturing of coalbed because of the impact of wettability changes to the coal surface.

SUMMARY

[5] This summary is provided to introduce a selection of concepts that are further described below in the detailed description. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used as an aid in limiting the scope of the claimed subject matter.

[6] Disclosed herein is a method of treating a subterranean coalbed methane formation including injecting into the subterranean coalbed methane formation a composition including an anionic viscoelastic surfactant and an acid-generating material, degrading the acid-generating material in the subterranean coalbed methane formation to release an acid, and changing a wettability of a surface of the subterranean coalbed methane formation due to a reaction of the acid with the anionic viscoelastic surfacant.

BRIEF DESCRIPTION OF THE DRAWINGS

[7] Fig. 1 is a schematic figure illustrating the change in the wettability of the surface of a coalbed methane formation after exposure to a surfactant.

[8] Fig. 2 is a schematic figure illustrating the change in the wettability of the surface of a coalbed methane formation after reaction of the anionic viscoelastic surfactant with the acid released by the acid-generating material.

DETAILED DESCRIPTION

[9] At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions may be made to achieve the developer's specific goals, such as compliance with system related and business related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time consuming but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure. In addition, the composition used/disclosed herein can also comprise some components other than those cited. In the summary and this detailed description, each numerical value should be read once as modified by the term "about" (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the summary and this detailed description, it should be understood that a range listed or described as being useful, suitable, or the like, is intended to include support for any conceivable sub-range within the range at least because every point within the range, including the end points, is to be considered as having been stated. For example, "a range of from 1 to 10" is to be read as indicating each possible number along the continuum between about 1 and about 10. Furthermore, one or more of the data points in the present examples may be combined together, or may be combined with one of the data points in the specification to create a range, and thus include each possible value or number within this range. Thus, even if a specific data points within the range, or even no data points within the range, are explicitly identified or refer to a few specific, it is to be understood that inventors appreciate and understand that any

conceivable data point within the range is to be considered to have been specified, and that inventors possessed knowledge of the entire range and each conceivable point and subrange within the range.

[10] The following definitions are provided to aid those skilled in the art in understanding the detailed description.

[11] The term, "acid-generating material" refers to materials that degrade or react with another reactant to form or release acid.

[12] The term, "coalbed methane formation" refers to any hydrocarbon formation that includes coal and in which natural gas, for example, methane, is adsorbed onto the surface of or embedded in the coal. The coalbed methane formation may also include other hydrocarbons, such as lignite, sub-bituminous, bituminous, anthracite, peat, and the like, and may be saturated with water.

[13] The term, "coalbed methane" refers to a natural gas that is adsorbed onto the surface of or embedded in coal. Coalbed methane may be substantially comprised of methane, but may also include ethane, propane, and other hydrocarbons. Coalbed methane may include some amount of other gases, such as carbon dioxide, nitrogen, and H₂S.

[14] The term, "hydrophilic" or "water-wet" refers to a surface of the coalbed methane formation on which more polar groups are present than nonpolar groups.

[15] The term, "hydrophobic" or "oil-wet" refers to a surface of the coalbed methane formation on which more nonpolar groups are present than polar groups. 14 000269

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[16] The term, "fracturing" refers to the process and methods of breaking down a geological formation and creating a fracture in the formation, such as in the formation around a wellbore, by pumping fluid at a pressure.

[17] The term, "injecting" refers to the introduction of a new or different element into a first element. For example, it may refer to the introduction of a fluid, solid, or other composition by any form of physical introduction, including but not limited to pumping.

[18] The term, "wellbore" refers to any type of well, including, but not limited to, a producing well, a non-producing well, an injection well, a fluid disposal well, an experimental well, an exploratory well, and the like. Wellbores may be vertical, horizontal, deviated some angle between vertical and horizontal, and combinations thereof, for example, a vertical well with a non-vertical component.

[19] Disclosed herein is a method for treating a subterranean coalbed methane formation that includes injecting a composition containing an anionic viscoelastic surfactant 101 having at least one polar group depicted as a circle and at least one nonpolar or hydrocarbon group depicted as a line in Fig. 1, and an acid-generating material. The composition may be injected into a coalbed methane formation during hydraulic fracturing treatments. As shown in Fig. 1, due to the presence of the anionic viscoelastic surfactant 101 in the subterranean coalbed methane formation, the wettability of the surface of the coalbed methane formation may change from hydrophobic 102 to hydrophilic 103. In some embodiments, the anionic viscoelastic surfactant may increase the pH of the coalbed surface from a pH in a range of from about 6 to about 8 to a pH of in a range of from about 1 1 to about 12. The method may further include degrading the acid-generating material in the subterranean coalbed methane formation over time to release an acid that reacts with the anionic viscoelastic surfactant to produce a hydrophobic oily liquid. One example of the reaction between the anionic viscoelastic surfactant and the acid is shown below.

VES fracturing fluid Hydrophobic liquid

The hydrophobic oily liquid may be fat and may be beneficial in that it can "glue" fine coal particles together to prevent them from plugging the fracture.

[20] As shown in Fig. 2, due to the reaction and reduction in the amount surfactant, the pH of the surface of the coalbed methane formation decreases from a higher pH, for example, in a range of from about 11 to about 12, to a lower pH, for example, in a range of from about 6 to about 7, and the wettability of the coalbed surface returns from a hydrophilic or water- wet state 201 to a hydrophobic or oil-wet state 202. By returning the coalbed surface 201, 202 to a hydrophobic or oil-wet state, a high rate of water production is enabled and, subsequently, a high rate of methane production is enabled.

[21] In some embodiments, when the pH of the surface of the coalbed methane formation is adjusted to be higher, such as a basic pH or a pH in a range of from about 11 to about 12, for example due to the injected surfactant, the surface is hydrophilic or water- wet, which may inhibit methane production. In some embodiments, when the pH is adjusted to be lower, such as an acidic pH or a pH in a range of from about 6 to about 7, for example due to the reaction between the surfactant and the acid released by the acid- generating material, the coalbed surface may be made to be hydrophobic or oil-wet, such that a high rate of methane production may be achieved.

[22] In the process of recovering natural gas, principally methane, from subterranean coal reservoirs, a wellbore may be drilled to the subterranean coal seam and the composition may be pumped into the reservoir. To dewater and produce gas from subterranean coal reservoirs, the formation is fractured. There are numerous variations of hydraulic fracturing of coal formations. Generally, the techniques involve injecting a fluid into the formation at sufficient pressure to initiate and propagate a hydraulic fracture, filling the fracture with proppant by continuing injection of a proppant laden fluid, and then flushing the treatment so that the proppant fills the fracture but not the wellbore.

Once the appropriate borehole is completed, dewatering may be carried out to reduce the pressure in the formation. Pressure drop in turn promotes methane release from within the coal into cleats. If the cleats contain a high enough permeability, that is, inter-connectivity, then the methane will flow from the coal into the wellbore and can be extracted.

[23] Without wishing to be bound by any theory, viscoelastic surfactants are believed to provide fluid viscosity by forming rod-like micelles. Entanglement of the micelles in the fluid is thought to create internal flow resistance that is in turn translated into viscosity. A thorough description of viscoelastic surfactants and the mechanisms by which they provide viscosity is given in the following publications. Zana R and Kaler E W (eds.): Giant Micelles, CRC Press, New York (2007). Abdel-Rahem V and Hoffmann H: "The distinction of viscoelastic phases from entangled wormlike micelles and of densely packed multilamellar vesicles on the basis of rheological measurements," Rheologica Acta, 45 (6) 781-792 (2006).

[24] The viscosity provided by the viscoelastic surfactants may allow optimal transport of fibers and solids and prevent bridging or plugging as the fluid is pumped to its destination through tubulars, tools or annuli. Viscoelastic surfactant fluids are well known and used for various oilfield applications such as hydraulic fracturing, diversion in acidizing, and leakoff control. Further viscoelastic surfactant fluids useful as base fluids in the embodiments may include, but are not limited to, those available under the tradename CLEARFRAC™, available from Schlumberger Limited.

[25] Carboxylate-based anionic viscoelastic surfactants generally have excellent gelling properties in high pH environments, but their viscosity decreases dramatically at lower pH conditions. For example, the anionic viscoelastic surfactant may include any surfactant having a fluid pH greater than about 10, such as a pH in a range of from about 11 to about 12. Suitable anionic surfactants aggregate into three-dimensional structures that substantially increase the viscosity at such high pH values but not at low pH. As such, the viscosity of the anionic viscoelastic surfactant at these high pH values may enable optimal transport of fibers and solids. The anionic viscoelastic surfactant may increase the pH of the coalbed methane formation, for example, from a pH in a range of from about 6 to about 8 to a pH in a range of from about 11 to about 12 and may adjust the wettability of the surface of the coalbed methane formation, for example, from a hydrophobic state to a hydrophilic state, from a hydrophilic state to a more hydrophilic state, or from a hydrophobic state to a less hydrophobic state. Then, upon release of the acid from the acid-generating material and its reaction with the surfactant, the pH of the coalbed methane formation may be reduced from a pH in a range of from about 11 to about 12 to a pH in a range of from about 6 to about 7 and the wettability may be adjusted, for example, from a hydrophilic state to a hydrophobic state, from a hydrophilic state to a less hydrophilic state, or from a hydrophobic state to a more hydrophobic state. As a result, a high rate of water production and, subsequently, a high rate of methane production may be obtained.

[26] The anionic viscoelastic surfactant may be made from a reaction of a fatty acid with a base and/or salt. The fatty acid may be an alkyl carboxylic acid and the alkyl group in the fatty acid may contain about 14 to about 24 carbon atoms, about 16 to about 22 carbon atoms, or about 18 to about 20 carbon atoms. Specific examples of the number of carbon atoms include 12, 14, 16, 18, 20, 22, and 24 carbon atoms. The alkyl group may be branched or straight and may be saturated or unsaturated.

[27] For example, the fatty acid may be oleic acid, stearic acid, palmitic acid, linoleic acid, linolenic acid, γ-linolenic acid, erucic acid, arachidic acid, arachidonic acid, myristic acid, palmitic acid, pentadecanoic acid, margaric acid, nonadecylic acid, heneicosylic acid, tricosylic acid, stearic acid, behenic acid, lignoceric acid, cerotic acid, myristoleic acid, palmitoleic acid, sapienic acid, elaidic acid, vaccenic acid, linoelaidic acid, a-linolenic acid, stearidonic acid, eicosapentaenoci acid, dihomo-y-linolenic acid, paullinic acid, gondoic acid, nervonic acid, or docosahexaenoic acid.

[28] When the above carboxylic acids contain an aliphatic chain of sufficient length, generally of at least about 16 or about 18 carbon atoms, they are able to act as viscoelastic surfactants when the pH is above their pK_a values so that the surfactants are in ionized form. To obtain viscoelastic behavior, the solution may also contain some added salts such as potassium chloride (KC1). Incorporating such carboxylic acids, when in the form of viscoelastic surfactants at pH above their pK_a values and/or in the presence of a salt will have the effect of thickening the composition.

[29] The fatty acid is reacted with a base and/or salt to product the anionic viscoelastic surfactant. The base may be potassium hydroxide or sodium hydroxide.

Additionally, the salt may include potassium, sodium, and ammonium salts, such as potassium chloride, ammonium chloride, and tetramethyl ammonium chloride. In one embodiment, the fatty acid may be, for example, oleic acid and the base may be sodium hydroxide or potassium hydroxide.

[30] The concentration of the anionic viscoelastic surfactant in the composition may be about 0.5% to about 10%, about 0.5% to about 8%, or about 0.5% to about 6% by weight of the fluid.

[31] The acid-generating material may be any degradable or hydrolysable material. Hydrolysis is a chemical reaction in which water reacts with another compound to form two or more substances. For example, the acid-generating material may be an ester, polylactic acid, polyester, polylactone, or mixtures thereof. The acid-generating material may include any acid having a molecular weight of about 100,000 to about 1 ,000,000 Da. In one embodiment, the acid-generating material may be solid. However, the acid-generating material may be in the form of a fiber, a particulate, or a liquid. The acid-generating material may hydrolyze into solid or non-solid compounds. For example, polylactic acid undergoes hydrolysis to form a liquid when exposed to a high pH environment. The acid-generating material may be any material that hydrolyzes when exposed to high pH condition (for example, a pH of greater than about 10) for some time. For example, hydrolysis of the acid-generating material may occur after being exposed to high pH conditions for about 1 to about 24 months, about 1 to about 18 months, about 1 to about 12 months, about 6 to about 12 months, or about 12 to about 18 months.

[32] The acid-generating material may include solid cyclic dimers, or solid polymers, of certain organic acids, that hydrolyze under known and controllable conditions of temperature, time, and pH; the degradation products are organic acids. One example of a suitable material is the solid cyclic dimer of lactic acid (known as "lactide"), which has a melting point of about 95 to about 125°C, (depending upon the optical activity). Another is a polymer of lactic acid, (sometimes called a polylactic acid (or "PLA"), or a polylactate, or a polylactide). Another example is the solid cyclic dimer of glycolic acid (known as "glycolide"), which has a melting point of about 86°C. Yet another example is a polymer of glycolic acid (hydroxyacetic acid), also known as polyglycolic acid ("PGA"), or polyglycolide. Another example is a copolymer of lactic acid and glycolic acid. These polymers and copolymers are polyesters.

[33] Nature Works LLC, Minnetonka, Minn USA, owned by Cargill Inc.,

Minneapolis, Minn. USA, produces a solid cyclic lactic acid dimer called "lactide" and from it produces lactic acid polymers, or polylactates, with varying molecular weights and degrees of crystallinity, under the generic trade name NatureWorks'^M PLA. The PLA polymers are solids at room temperature and are hydrolyzed by water to form lactic acid. Those available from Nature Works™ typically have crystalline melt temperatures of from about 120 to about 170°C, but others are obtainable. Poly(D,L-lactide) is available from Bio-Invigor, Beijing and Taiwan, with molecular weights of up to about 500,000. Bio- Invigor also supplies polyglycolic acid (also known as polyglycolide) and various copolymers of lactic acid and glycolic acid, often called "polyglactin" or poly(lactide-co- glycolide). The rates of the hydrolysis reactions of these materials are governed, among other factors, by the molecular weight, the crystallinity (the ratio of crystalline to amorphous material), and the physical form (size and shape of the solid). Amorphous regions are more susceptible to hydrolysis than crystalline regions. Lower molecular weight, less crystallinity and greater surface-to-mass ratio result in faster hydrolysis.

Hydrolysis is accelerated by increasing the temperature, by adding acid or base, or by adding a material that reacts with the hydrolysis product(s).

[34] The acid generated by the acid-generating material may include

carboxylic acids. For example, the acid may be citric acid, acetic acid, formic acid, hydrochloric acid, oxalic acid, or benzoic acid. The acid concentration may be that which is sufficient to reduce the pH to a level below about 8. In one embodiment, the acid concentration may be that which is sufficient to reduce the pH to a pH in a range of from about 6 to about 7.

[35J If in the form of a fiber, the acid-generating material may have a fiber- length range of about 2 mm to about 25 mm, about 3 mm to about 18 mm, or about 5 mm to about 7 mm. The fiber-diameter may be about 1 μπι to about 200 μπι, about 1.5 μπι to about 60 μη , or about 10 μπι to about 20 μη . One of the advantages offered by the aforementioned fibers is that the fibers will degrade through hydrolysis in the presence of traces of water and heat. With time, they may dissolve and be carried away by the produced hydrocarbon fluid, eliminating any potential damage to well production.

[36] Mixtures of fibers may also be used. For example, the fibers may be a blend of long fibers and short fibers. The long fibers may be rigid and the short fibers may be flexible. The fibers may not be of uniform size and may include a mixture of different sizes of or compositions of fibers. The fibers may be loaded into the base fluid in concentrations of about 2 to about 72 g/L, about 12 to about 36 g/L, or about 18 g/L.

[37] The blend of fibers may contain fibers having different aspect ratios and different flexibilities. The blend may be a blend of two fibers, or a blend of three fibers but blends of more fibers may be used. The fibers may be a blend of different lengths of fibers. At least one fiber type may be rigid and the rest of the fibers may be flexible. The rigid fibers may be longer than the flexible fibers. A "flexible" fiber may be defined as having a Young's Modulus of less than about 20 GPa (kN/mm.sup.2) and a rigid fiber may be defined as having a Young's Modulus of greater than about 20 GPa. Note that the fiber lengths specified are not intended to be precise; fibers as received, or as cut to length, inevitably are a mixture of lengths distributed around the intended length. [38] The length ratio of long fibers to short fibers may be from about 1 : 1 to about 3:1. At least a portion of the fibers may be acid-soluble. At least a portion of the fibers may be biodegradable. The length of the long fibers may be between about 8 and about 15 mm. The long fibers may be organic. The long fibers may include water- insoluble polyvinyl alcohol. The short fibers may include water-soluble polyvinyl alcohol. In other embodiments, the short fibers may include a mixture of fibers of two different lengths, for example one type of fiber having an average length of from about 1 to about 2 mm and a second group of fibers having an average length of from about 3 to about 8 mm. The short fibers may include a mixture of two different lengths of polyvinyl alcohol fibers; as examples, the two different lengths may be in a weight ratio of from about 90:1 to about 1 :90, the two different lengths may be in a length ratio of from about 2 to about 6 or about 2.5 to about 7. The short fibers may include a mixture of multiple lengths of polyaramid fibers. The short fibers may include inorganic fibers (for example made from calcium oxide and silica).

[39] If the acid-generating material is in the form of particles, the average size may be of from about 5 to about 1000 μπι, about 10 to about 300 μπι, or about 15 to about 150 μηι. The particle loading range may be the same as the fiber loading range. Particles can be made of, for example, polylactic acid or any polyesters. The particles may be a mixture of coarse, medium, and optionally also fine particles.

[40] The fine particles may have an average particle size of from about 5 to about 15 microns. As an example, about 10 weight percent of the fine particles may be smaller than about 1 micron and about 10 weight percent of the fine particles may be larger than about 30 microns. The fine particles may have an average particle size of from about 5 to about 10 microns. About 10 weight percent of the medium particles may be smaller than about 20 microns and about 10 weight percent of the fine particles may be larger than about 150 microns. The medium particles may have an average particle size of from about 20 to about 150 microns. About 10 weight percent of the coarse particles may be smaller than about 5 microns and about 10 weight percent of the coarse particles may be larger than about 1500 microns. The coarse particles may have an average particle size of from about 300 to about 1200 microns. The mixture of particles may contain from about 0 to about 15 weight percent fine particles, about 20 to about 40 weight percent medium particles, and about 40 to about 60 weight percent coarse particles. [41] The acid-generating material may be in the form of an encapsulated acid to provide delayed acid generation (for example, to slow hydrolysis). Those skilled in the art will recognize that encapsulation refers to methods by which a material is isolated from the continuous phase of a fluid. Such isolation may be provided by (but would not be limited to) a shell coating or an emulsion. Suitable coatings include polycaprolate (a copolymer of glycolide and epsilon-caprolactone), and calcium stearate, both of which are hydrophobic. Polycaprolate itself slowly hydrolyzes. Generating a hydrophobic layer on the surface of the acid by any means may delay the hydrolysis. Note that coating here may refer to encapsulation or simply to changing the surface by chemical reaction or by forming or adding a thin film of another material, for example an oil.

[42] Mechanisms by which the encapsulated acid may be released include time, hydrolysis, temperature, shear (for example, through a drill bit), pH change, vibration, mechanical force (i.e., fracture closure), or irradiation and combinations thereof.

[43] The acid-generating material may be in any shape. For example, chips, fibers, beads, ribbons, platelets, films, rods, strips, spheroids, toroids, pellets, tablets, capsules, or shavings. Additionally, the acid-generating material may have any round cross-sectional shape, oval cross-sectional shape, trilobal shape, star shape, flat shape, rectangular shape, cubic, bar shaped, flake, cylindrical shape, filament, thread, or mixtures thereof. If the acid-generating material is a solid material, it may be either amorphous or/and crystalline in nature.

[44] For optimal cleanup after the treatment, the acid-generating material is a solid-particle or fiber of polylactic acid, polyglycolic acid, or polyester. The solid-particle average size range is about 5 μιη to about 1000 μιη, about 10 μ^ηι to about 300 μπι, or about 15 μιη to 150 μπι. The solid-particle concentration range may be about 6 g/L to about 72 g/L, about 12 g/L to about 36 g/L, or about 15 g/L to about 20 g/L.

[45] In one embodiment, the acid-generating material may be used when the formation temperature of the well exceeds about 50°C, about 60°C, about70°C, about 80°C, about 90°C, about 100°C, about 110°C, or about 120°C. The acid-generating material may not substantially degrade below a temperature of about 60°C, about 70°C, about 80°C, about 90°C, about 100°C, about 1 10°C, or about 120°C during duration of the treatment.

[46] The composition of the anionic viscoelastic surfactant and the acid- generating material may be a water-based fluid. Water may be present in the composition in an amount by weight greater than or equal to about 50% by weight of the composition. The water may be from any source so long as the source contains no contaminants that are incompatible with the other components of the composition (for example, by causing undesirable precipitation). The composition may optionally contain a gas such as air, nitrogen, or carbon dioxide to provide an energized fluid or a foam.

[47] To prepare the aqueous composition, the fatty may be added to an aqueous solution in which the base and/or salt has been dissolved, followed by addition of the acid-generating material. Standard mixing procedures known in the art may be employed.

[48] The method may also include injecting a proppant that is substantially insoluble in the fluids of the formation. Proppant particles carried by the composition remain in the fracture created, thus propping open the fracture when the fracturing pressure is released. Any proppant can be used, provided that it is compatible with the formation, the composition including the anionic viscoelastic surfactant and the acid-generating material, and the desired results of the treatment. Such proppants may be natural or synthetic, coated, or contain chemicals. For example, the proppant may be glass beads, ceramic beads, sand, walnut shells, gravel, and bauxite. Mixtures of suitable proppants may be used. The proppant may have an average particle size of about 0.15 mm to about 2.39 mm (about 8 to about 100 U.S. mesh), about 0.25 to about 0.43 mm (about 40 to about 60 U.S. mesh), about 0.43 to about 0.84 mm (about 20 to about 40 mesh), about 0.84 to about 1.19 mm (about 16 to about 20 U.S. mesh), about 0.84 to about 1.68 mm (about 12 to about 20 U.S. mesh), or about 0.84 to about 2.39 mm (about 8 to about 20 U.S. mesh). The concentration of the proppant in the composition may be about 0.05 to about 3 kg/L.

[49] The composition injected into the subterranean methane formation may further contain one or more additives such as co-surfactants, breaker aids, salts, anti-foam agents, scale inhibitors, corrosion inhibitors, fluid-loss additives, proppant flowback inhibitors, pH control agents, and bactericides.

[50] Delivery of the composition including the anionic viscoelastic surfactant and the acid-generating material downhole may be performed by injecting the fluid or fluids into a well: (1) through drilling pipe; (2) through coiled tubing including for example, a microcoil with a diameter of about 1.25 cm (one-half inch) or less; (3) through the annulus space between any tubular strings positioned in the wellbore; (4) by using bailers or downhole containers; through any tubular strings positioned in the wellbore; (5) pumping downhole through casing; or (6) any combination of the foregoing methods.

[51] The method may include pumping the composition containing the acid- generating material and the anionic viscoelastic surfactant down the wellbore as a slurry or mixture of suspended solids and liquids. The slurry may be prepared at or near the site of its intended use (for example, a wellbore) or to reduce the expense associated with the transport of equipment and materials, and the expertise to prepare a slurry on site, the slurry may be prepared at a remote location and shipped to the site of its intended use. The slurry may be easily pumpable and pourable, and where it is prepared offsite, remain stable for long periods of time, for example, about 30 days or more, exhibiting minimum separation of liquid and solids and no packing of the solid particles or fibers.

[52] An acid pre-flush may be pumped down the wellbore before pumping the composition or slurry of the anionic viscoelastic surfactant and the acid-generating material and an acid post-flush may be pumped down the wellbore after pumping the composition. The acid in the pre- and post-flushes may be the same or different. The acid post-flush may change the wettability of the surface of the coalbed methane formation immediately.

[53] To hydraulically fracture a coalbed methane formation or treat a subterranean coalbed methane formation, a composition including the anionic viscoelastic surfactant and the acid-generating material may be injected into the coalbed methane formation, for example, in a wellbore. Over time, the acid-generating material will degrade or hydrolyze to release an acid that reacts with the anionic viscoelastic surfactant to produce an oily liquid. The wettability of the surface of the coalbed methane formation may be changed from hydrophilic to hydrophobic due to the reaction of the anionic surfactant with the acid. As a result, the coalbed surface may return to a hydrophobic or oil-wet state, which may initially enable a high rate of water production and may, subsequently, enable a high rate of methane production.

EXAMPLE

[54] Core flow tests were carried out to test the effects of exposure to an anionic surfactant, followed by exposure to an acid on the permeability of a coal pack. A sequence of fluids was injected through a coal pack. Initially potassium chloride was injected through the coal pack to measure initial and regained permeability. Then, the anionic viscoelastic surfactant, J533, was injected through the coal pack, followed by a post-flush with acetic acid. As shown in Table 1, the initial permeability of the coal pack was reduced by 80% after the anionic viscoelastic surfactant was injected. However, the permeability of the coal pack was fully recovered after acetic acid injection indicating that the coal surface returned to oil-wet.

Table 1

[55] The above results demonstrate that subsequent hydrolysis of an acid- generating material to produce an acid after exposure to an anionic viscoelastic surfactant, may improve the permeability of a coalbed methane formation. As a result, water may be removed, which will reduce the hydrostatic pressure in the subsurface of the coalbed methane formation so that methane can desorb from the coal.

[56] Although only a few example embodiments have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the example embodiments without materially departing from the method for treating a coalbed methane formation. Accordingly, all such modifications are intended to be included within the scope of this disclosure as defined in the following claims. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents, but also equivalent structures. Thus, although a nail and a screw may not be structural equivalents in that a nail employs a cylindrical surface to secure wooden parts together, whereas a screw employs a helical surface, in the environment of fastening wooden parts, a nail and a screw may be equivalent structures. It is the express intention of the applicant not to invoke a means plus function format for any limitations of any of the claims herein, except for those in which the claim expressly uses the words 'means for' together with an associated function. [57] Although the preceding description has been described herein with reference to particular means, materials, and embodiments, it is not intended to be limited to the particulars disclosed herein; rather, it extends to all functionally equivalent structures, methods, and uses, such as are within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS

1. A method of treating a subterranean coalbed methane formation

comprising:

injecting into the subterranean coalbed methane formation a composition comprising an anionic viscoelastic surfactant and an acid-generating material; and

degrading the acid-generating material in the subterranean coalbed methane formation to release an acid; and

changing a wettability of a surface of the subterranean coalbed methane formation due to a reaction of the acid with the anionic viscoelastic surfactant.

2. The method according to claim 1 , wherein the wettability of the surface of the subterranean coalbed methane formation is hydrophobic prior to injecting the composition.

3. The method according to claim 1 or 2, further comprising changing the wettability of the surface of the subterranean coalbed methane formation from hydrophobic to hydrophilic due to the presence of the anionic viscoelastic surfactant in the subterranean coalbed methane formation before degrading the acid-generating material.

4. The method according to claim 3, wherein the wettability of the surface of the subterranean coalbed methane formation changes from hydrophilic to hydrophobic due to the reaction of the acid with the anionic viscoelastic surfactant.

5. The method according to any one of the preceding claims, wherein the reaction of the acid with the anionic viscoelastic surfactant produces an oily liquid.

6. The method according to any one of the preceding claims, wherein the acid- generating material hydrolyzes to release the acid.

7. The method according to claim 6, wherein the hydrolysis occurs about 1 to about 24 months after injecting the composition into the subterranean coalbed methane formation.

8. The method according to any one of the preceding claims, wherein the acid released by the acid-generating material changes a pH of the surface of the coalbed methane formation from a pH in a range of from about 1 1 to about 12 to a pH in a range of from about 6 to about 7.

9. The method according to any one of the preceding claims, wherein the anionic viscoelastic surfactant is produced from a reaction of a fatty acid with a base.

10. The method according to claim 9, wherein the fatty acid is oleic acid and the base is sodium hydroxide or potassium hydroxide.

11. The method according to any one of the preceding claims, wherein the acid- generating material is a polyester selected from the group consisting of polylactic acid and polyglycolic acid.

12. The method according to any one of the preceding claims, wherein the acid- generating material is selected from the group consisting of a fiber, particulate, and liquid.

13. The method according to any one of the preceding claims, wherein the composition further comprises nitrogen gas and is a foam.

14. The method according to any one of the preceding claims, wherein the composition is injected into a wellbore in the coalbed methane formation.

15. The method according to any one of the preceding claims, further comprising injecting at least one of an acid pre-flush and an acid post-flush, the pre-flush being injected into the subterranean coalbed methane formation prior to injecting the composition and the post-flush being injected into the subterranean coalbed methane formation after injecting the composition.