Abstract:
The invention provides a method for treating tight gas sand and shale subterranean formations, the method comprising: forming a solvent-surfactant blend by combining a solvent, a surfactant and a co-surfactant; adding a diluent to the solvent-surfactant blend to form a micro emulsion; wherein the wettability of the formation altered from water-wet to gas-wet, and the amount of water imbibed into the formation is reduced.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/228736, filed Jul. 27, 2009, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to methods for treating a well penetrating a subterranean formation. More specifically, the invention relates to a microemulsion to improve shale gas production by controlling water imbibition. 
       BACKGROUND 
       [0003]    Some statements may merely provide background information related to the present disclosure and may not constitute prior art. 
         [0004]    Hydraulic fracturing is commonly used to stimulate shale gas reservoirs. In mid nineties, most of the shale gas reservoirs were fractured utilizing the crosslinked polymer fluids. In an effort to reduce costs, slickwater fracturing has emerged as the method of choice. The success of slickwater fracturing has been attributed to its ability to contact a larger surface of the reservoir with minimum fracturing fluid damage at the fracture face and within the proppant pack. In a typical treatment several million gallons of water is pumped at an average rate 65 bpm with proppant ranging in concentration from 0.25 to 1.0 ppg. Several chemicals are added during the treatment. The common additives include scale inhibitor, friction reducers, biocides, clay stabilizers, oxygen scavengers, surfactants, and the like. 
         [0005]    One of the continuing challenges in slickwater fracturing of shale gas reservoirs is the post treatment fluid recovery. Published data show 60 to 90% of the injected fluids stay in the reservoir. It must be assumed that these large quantities of water are trapped in the area surrounding the fracture and within the fracture itself. The trapped fluid has a detrimental effect on the relative permeability and effective flow area and without question impairs well productivity. The water trapping could be due to interfacial tension between the injected slickwater and the reservoir shale, or capillary end effect on and around the vicinity of the face fractured shale. To minimize phase trapping, commonly available surfactants are added to slick water to reduce surface tension between the treating fluid (i.e., slickwater) and gas and thereby recovering more of the treating fluid and restoring the relative permeability to gas. 
         [0006]    For strongly water-wet shale gas reservoirs, capillary forces promote the retention of injected in pore spaces; which in-turn curtails the flow of gas into the fracture. Wardlaw and McKeller (1998) found that gas reservoir with residual oil drain water more efficiently than oil-free reservoirs. Apparently, the presence of residual oil alters the reservoir rock wettability to less water-wet. Likewise, if surfactants or other surface-coating chemicals are injected into the shale-gas reservoir, the capillary pressure is reduced by decreasing the gas-water surface tension, and/or rock surface wettability is altered to less water-wet. Penny et al. (1983) changed the wettability of the rock surface from water-wet to oil-wet by using surfactant dissolved in methanol and successfully mitigate the water-block effect. Li and Firoozabadi (2000) noted that fluorochemical surfactants could alter the water-wet sandstone and chalk surfaces to intermediate-wet or more gas-wet. 
         [0007]    There is a need to provide a microemulsion additive that is highly effective in altering the wettability of gas shale reservoir from water-wet to more gas-wet. 
       SUMMARY 
       [0008]    In a first aspect a method for treating tight gas sand and shale subterranean formations is disclosed. The method comprises forming a solvent-surfactant blend by combining a solvent, a surfactant and a co-surfactant; adding a diluent to the solvent-surfactant blend to form a microemulsion; wherein the wettability of the formation altered from water-wet to gas-wet, and the amount of water imbibed into the formation is reduced. 
         [0009]    In a second aspect, a method for treating a subterranean formation comprising at least in part tight gas sand and shale rocks is disclosed. The method comprises forming a solvent-surfactant blend by combining a solvent, a surfactant and a co-surfactant; adding water to the solvent-surfactant blend to form a microemulsion; wherein the wettability of the formation altered from water-wet to gas-wet, and the amount of water imbibed into the formation is reduced. 
     
    
     DETAILED DESCRIPTION 
       [0010]    At the outset, it should be noted that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developer&#39;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. The description and examples are presented solely for the purpose of illustrating the embodiments of the invention and should not be construed as a limitation to the scope and applicability of the invention. While the compositions of the invention are described herein as comprising certain materials, it should be understood that the composition could optionally comprise two or more chemically different materials. In addition, the composition can also comprise some components other than the ones already cited. 
         [0011]    In the summary of the invention and this 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 of the invention and this detailed description, it should be understood that a concentration range listed or described as being useful, suitable, or the like, is intended that any and every concentration 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 and every possible number along the continuum between about 1 and about 10. Thus, even if specific data points within the range, or even no data points within the range, are explicitly identified or refer to only a few specific data points, it is to be understood that inventors appreciate and understand that any and all data points within the range are to be considered to have been specified, and that inventors have disclosed and enabled the entire range and all points within the range. 
         [0012]    The present invention relates to a microemulsion-based system that is able to alter the wettability of a shale gas reservoir from water-wet to intermediate-wet or more gas-wet. This will result in minimizing the amount of water imbibed to the shale formation during a fracturing treatment. The microemulsion system is formed by the combination of solvent surfactant blends with an appropriate oil-based or water-based carrier fluid. 
         [0013]    The solvent-surfactant blend generally includes a solvent and surfactant package. In one embodiment, the solvent is selected from the group of dipolar aprotic solvents including N-methyl-2-pyrrolidone, dimethylformamide, dimethylacetamide and dimethyl sulfoxide. One particularly useful solvent is N-methyl-2-pyrrolidone. The N-methyl-2-pyrrolidone is effective due to its solvency, low-toxicity and biodegradability. In an alternate embodiment, the dipolar aprotic solvent may be replaced with refined vegetable oil, such as canola oil. It will also be understood that combinations of oil and different solvents, such as canola oil and N-metyl-2-pyrrolidone, are also encompassed within the scope of some embodiments. 
         [0014]    The surfactant of the solvent-surfactant blend is capable of forming an oil-in-water microemulsion upon combination with an appropriate quantity of water. Some examples of suitable surfactants include one or more of the following: lauryl alcohol ethoxylates, linear fatty alcohol ethoxylates (C9-C11), linear fatty alcohol ethoxylates (C12-C13), linear fatty alcohol ethoxylates (C12-C15) and polyorganosiloxanes. One surfactant mixture includes linear fatty alcohol ethoxylates (C11), dicocodimethylquaternary (Tomadry N-4), decyl-dimethyl amine oxide and polyorganosiloxane. 
         [0015]    Some exemplary surfactants include, but are not limited to, cationic, anionic, zwitterionic, or nonionic. In other embodiments, the surfactant is a viscoelastic surfactant (VES) fluid system. VES fluid system is a fluid viscosified with a viscoelastic surfactant and any additional materials, such as but not limited to salts, co-surfactants, rheology enhancers, stabilizers and shear recovery enhancers that improve or modify the performance of the viscoelastic surfactant. 
         [0016]    The useful VES&#39;s include cationic, anionic, nonionic, mixed, zwitterionic and amphoteric surfactants, especially betaine zwitterionic viscoelastic surfactant fluid systems or amidoamine oxide viscoelastic surfactant fluid systems. Examples of suitable VES systems include those described in U.S. Pat. Nos. 5,551,516; 5,964,295; 5,979,555; 5,979,557; 6,140,277; 6,258,859 and 6,509,301, which are all hereby incorporated by reference. The system of the invention is also useful when used with several types of zwitterionic surfactants. In general, suitable zwitterionic surfactants have the formula: 
         [0000]      RCONH—(CH 2 ) a (CH 2 CH 2 O) m (CH 2 ) b —N + (CH 3 ) 2 —(CH 2 ) a′ (CH 2 CH 2 O) m′ (CH 2 ) b′ COO − 
 
         [0000]    in which R is an alkyl group that contains from about 14 to about 23 carbon atoms which may be branched or straight chained and which may be saturated or unsaturated; a, b, a′, and b′ are each from 0 to 10 and m and m′ are each from 0 to 13; a and b are each 1 or 2 if m is not 0 and (a+b) is from 2 to about 10 if m is 0; a′ and b′ are each 1 or 2 when m′ is not 0 and (a′+b′) is from 1 to about 5 if m is 0; (m+m′) is from 0 to about 14; and the 0 in either or both CH2CH2O groups or chains, if present, may be located on the end towards or away from the quaternary nitrogen. Preferred surfactants are betaines. 
         [0017]    The surfactant package can further include a co-surfactant, as e.g. glycol ether. Glycol ether component may be used with the solvent-surfactant blend as a coupling agent between the solvent and the surfactant, thereby stabilizing the microemulsion. Although propylene glycol ether is one effective co-surfactant, alternative suitable glycol ethers include, but are not limited to, propylene glycol methyl ether, propylene glycol methyl ether acetate, propylene glycol n-butyl ether, ethylene glycol monobutyl ether, diethylene glycol monobutyl ether acetate, diethylene glycol monobutyl, and the like. 
         [0018]    The solvent-surfactant blend optionally includes a salt. The addition of a salt to the solvent-surfactant blend reduces the amount of water needed as a carrier fluid and also lowers the freezing point of the well treatment microemulsion. Among the salts that may be added for stability and co-solvent substitution, NaCl, KCl, CaCl2, and MgCl are presently preferred. Others suitable salts can be formed from K, Na, Br, Cr, Cs and Bi families 
         [0019]    In another embodiment of the present invention, the viscosity of the microemulsion is increased with Claytone (organophillic clay). 
         [0020]    After blending the solvents, surfactants and glycol ethers, a diluent may be added to form a microemulsion. In some cases, the diluent is water; however, any suitable diluent may be used. 
         [0021]    In one illustrative example, the microemulsion contains from about 15% to about 20% by volume of the surfactant mixture (linear fatty alcohol ethoxylates (C-11), dicocodimethylquaternary and polyorganosiloxane), from about 10% to about 20% by volume N-methyl-2-pyrrolidone, from about 5% to about 15% propylene glycol ether, and the balance is water. 
         [0022]    The microemulsions may be used in slickwater fracturing fluids. When used in slickwater application the microemulsion may be added in any suitable amount to the carrier fluid. In some cases, from about 0.2% to about 2% by volume microemulsion is added to the carrier fluid. Slickwater fracturing fluids typically contain water a friction reducer, biocide, scale inhibitor and clay stabilizer, in addition to other suitable components. 
         [0023]    In addition to slickwater hydraulic fracturing fluids, the microemulsion may be used in viscosified fracturing fluids, gravel packs, water conformance control, acid fracturing, waterflood, drilling fluids, wellbore cleanout fluids, fluid loss control fluids, kill fluids, spacers, flushes, pushers, and carriers for materials such as scale, paraffin, and asphaltene inhibitors, and the like. When used, viscosification systems can include polymers, including crosslinked polymers, viscoelastic surfactant systems (VES), fiber viscosification systems, mixed fiber-polymer and fiber-VES systems, slickwater (low viscosity) systems, and so on. 
         [0024]    In addition to those applications described above, the microemulsions may also be useful for such operations acidizing operations, drilling operations and hydrogen sulfide mitigation applications. Also, it will be understood that the well treatment microemulsions can be used in additional, alternative applications. For example, it is contemplated that the well treatment microemulsion could also be used to clean surface equipment and downhole equipment. 
         [0025]    The well treatment microemulsions can also be used to deliver acids during acidizing operations. Acids commonly used include hydrochloric, acetic, formic, and hydrochloric-hydrofluoric acids. In a presently preferred embodiment, the selected solvent-surfactant blend (dilute or concentrate) is combined with an acidified carrier fluid to prepare a microemulsion suitable for acidizing operations. Preferably, the microemulsion includes about 0.2%-5% by volume of the solvent-surfactant blend and about 3%-28% by volume of acid. In a particularly preferred embodiment, the microemulsion includes about 0.2%-5% of the solvent-surfactant blend and about 15% by volume of hydrochloric acid. The concentration of the well treatment microemulsion in gelled fluids lowers the friction created by contact with conduits, thereby facilitating the injection and withdrawal of the well treatment microemulsion. 
         [0026]    As described in U.S. patent application Ser. No. 12/156,201, incorporated in its entirety by reference hereto, using appropriate hydrogen sulfide scavengers, microemulsions can also be used for hydrogen sulfide mitigation. In such cases, the well treatment microemulsions are injected into the wellbore so that escaping hydrogen sulfide gas is “stripped” through the well treatment microemulsions. The microemulsion is periodically injected into problem wells to mitigate hydrogen sulfide production. Alternatively, the microemulsion can be injected downhole via capillary tubing on a continuous basis. In yet another alternate embodiment, the well treatment microemulsion can be placed in a container that is placed in fluid communication with the hydrogen sulfide. 
         [0027]    As described in U.S. patent application Ser. No. 7,392,844, incorporated in its entirety by reference hereto, microemulsions may be useful for removal of pipe dope and hydrocarbons, oil based, and synthetic oil based drilling muds and the dispersement of paraffins and asphaltenes. They may further be used as a displacement spacer system, either as a single spacer or as a multiple spacer in conjunction with a second fluid, for the removal of oil/synthetic oil based mud cake and hydrocarbons prior to cementing or prior to introduction of a completion brine. 
         [0028]    Any additives normally used in such well treatment fluids can be included, again provided that they are compatible with the other components and the desired results of the treatment. Such additives can include, but are not limited to breakers, anti-oxidants, crosslinkers, corrosion inhibitors, delay agents, fibers, particles, proppant, gravel, buffers, fluid loss additives, pH control agents, solid acids, solid acid precursors, etc. The wellbores treated can be vertical, deviated or horizontal. 
         [0029]    The foregoing disclosure and description of the invention is illustrative and explanatory thereof and it can be readily appreciated by those skilled in the art that various changes in the size, shape and materials, as well as in the details of the illustrated construction or combinations of the elements described herein can be made without departing from the spirit of the invention.