Abstract:
An aqueous viscoelastic fluid for enhancing the productivity of a subterranean formation contains, in addition to an anionic surfactant and a cationic surfactant, hydroxyethyl cellulose (HEC). The fluid may be pumped neat into the formation, foamed with nitrogen to qualities exceeding 90% or emulsified with liquid carbon dioxide to emulsion quality approaching 90%.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to viscoelastic fluids useful in the fracturing of subterranean formations and includes foamed and emulsified fluids which exhibit high viscosity. 
       BACKGROUND OF THE INVENTION 
       [0002]    Subterranean formations of oil and gas wells are often treated by hydraulically fracturing the formations to increase the production of oil or gas. Fracturing of the formations is accomplished by pumping fluids into the bore holes of the oil or gas wells under high pressure so that cracks or fissures are opened into the surrounding formation. The fluids preferably are loaded with proppants in order to wedge open the cracks or fissures in the formation, and thereby permanently increase permeability and the flow of hydrocarbons from the oil or gas well. The viscosity of fracturing fluids is preferably high such that effective volumes of proppant may be carried in the fluid. The fracturing fluid&#39;s viscosity is proportionally related to the created fracture geometry and fracture width so that more viscous fluids will produce longer and wider fractures. 
         [0003]    The fracturing fluid is typically an aqueous gel or a hydrocarbon gel which contains polymer. Such fluids are typically formed by gelling one or more crosslinkable polymers in the aqueous or hydrocarbon fluid. While such fluids are often effective, their use normally requires extensive well clean up. For instance, polymers often leave residues that form cakes which clog the well. In addition, such polymers often leave undesirable traces in the recoverable hydrocarbons that may be expensive to remove. 
         [0004]    More recently, interest has shifted to polymer free fluids. Such fluids are much less damaging and clean up faster than fracturing fluids which contain polymers. For instance, U.S. Pat. Nos. 6,468,945 and 6,410,489 disclose viscoelastic fluids containing a cationic surfactant and an anionic surfactant which may be foamed or emulsified. Such fluids exhibit low dynamic surface tension, are tolerant of recycled treatment water and further are fully compatible with KCl treatment fluids. In addition, such fluids form viscoelastic gels quickly and may be mixed batch-wise in advance or on-the-fly. In addition, such fluids are effective at low surfactant loadings and, at such low surfactant loadings, are easily foamed or emulsified using 50%-96% by volume of gas or liquid CO 2 . 
         [0005]    While such fluids are suitable for use in conventional oil and gas reservoirs, improved performance of such fluids, especially in non-conventional shale gas and oil, coal bed methane is desired. Such alternatives are particularly desired in shallow, brittle shales where water cost, availability and disposal has become prohibitively expensive and environmental unacceptable. 
       SUMMARY OF THE INVENTION 
       [0006]    The flow of recoverable hydrocarbons from a subterranean formation may be increased by introducing into the formation during stimulation an aqueous fluid containing an anionic surfactant and a cationic surfactant and hydroxyethyl cellulose (HEC) to create a fracture. The aqueous fluids described herein have particular applicability in the treatment of low permeability reservoirs including shallow brittle shales, coal bed methane wells as well as conventional reservoirs. 
         [0007]    The anionic surfactant of the fluid is preferably sodium xylene sulfonate and the cationic surfactant is preferably a C 10  to C 24  alkyl trialkyl (“the alkylated moiety”) quaternary ammonium chloride salt. In a most preferred embodiment, the alkyl group forming the alkylated moiety is a C 18  alkyl. 
         [0008]    The aqueous fluid may be mixed with from 5% to 96% or more of gaseous foaming agent or liquid CO 2  to form a foam, emulsified or energized fluid. Suitable foaming agents include nitrogen and carbon dioxide. The gelled fluid exhibits good foaming characteristics at low loadings, such as about 2 L/m 3 , for each of the anionic and cationic surfactants. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    In order to more fully understand the drawings referred to in the detailed description of the present invention, a brief description of each drawing is presented, in which: 
           [0010]      FIG. 1  shows the effect of increased viscosity at 30° C. of a viscoelastic fluid described herein which contains cationic and anionic surfactants and hydroxyethyl cellulose (HEC) in contrast to a similar viscoelastic fluid which does not contain HEC.  FIG. 1  also demonstrates that a synergy exists between HEC and the viscoelastic fluid and that viscosity is adversely affected when guar is used (instead of HEC). 
           [0011]      FIG. 2  shows the viscosity profile of a nitrogen foamed viscoelastic surfactant containing cationic and anionic surfactants with and without HEC at 30° C.; and 
           [0012]      FIG. 3  shows the viscosity profile at 30° C. of a CO 2  emulsified viscoelastic surfactant which contains cationic and anionic surfactants with and without HEC. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0013]    The addition of hydroxyethyl cellulose (HEC) to an aqueous fracturing fluid containing an anionic surfactant and a cationic surfactant improves the viscosity of the fluid over a wide range of temperatures. The HEC is preferably of high molecular weight. Generally the average molecular weight of the HEC polymer ranges from about 0.75×10 6  to about 2×10 6 , preferably from about 1.4×10 6  to about 1.6×10 6 , most preferably about 1.5×10 6 . The amount of HEC polymer added to the aqueous fracturing fluid is between from about 0.25 to about 5.0 kg/m 3 . The resulting fluid is viscoelastic and micellar in nature. 
         [0014]    The anionic surfactant is preferably sodium xylene sulfonate. The cationic surfactant described herein is typically a C 10  to C 24  alkyl trialkyl quaternary ammonium chloride salt. The alkyl group forming the alkylated moiety can be a C 10  to C 24  alkyl group, preferably a C 12  to a C 20  alkyl. In a most preferred embodiment, the alkyl group forming the alkylated moiety is a C 18  alkyl. 
         [0015]    Typically, aqueous solutions of the anionic surfactant and the cationic surfactant are used. The volume ratios of cationic surfactant solution to anionic surfactant solution are dependent on the amount of surfactant in solution. Typically, the volume ratio of cationic surfactant solution to anionic surfactant solution is typically between from 0.5:1 to 10:1, preferably from 1:1 to 2:1 (based on a 50 vol. % solution of cationic surfactant and a 40 vol. % solution of anionic surfactant). 
         [0016]    The surfactants are typically mixed with water and blended together “on the fly” during a fracturing treatment. The water acts as the external phase of the fluid when emulsified or foamed. 
         [0017]    At total surfactant loadings of up to about 6%, but usually less than 1%, the fluid exhibits the requisite viscosity for fracturing and effective transport of proppant at temperatures between from about 10° C. to about 100° C. 
         [0018]    In one embodiment, the cationic surfactant solution is added to the water in a quantity of from about 2 L/m 3  to about 30 L/m 3  and the anionic surfactant solution is added to the water in quantities of about 2.0 L/m 3  to about 30 L/m 3  (based on a 50 vol. % solution of cationic surfactant and a 40 vol. % solution of anionic surfactant). 
         [0019]    The resulting viscoelastic gel is capable of transport of conventional proppants such as sand, resin coated sand and sintered bauxite as well as ultra lightweight proppants having an apparent specific gravity of less than about 2.45, such as ground or crushed shells of nuts like walnut, coconut, pecan, almond, ivory nut, brazil nut, etc.; ground and crushed seed shells (including fruit pits) of seeds of fruits such as plum, olive, peach, cherry, apricot, etc.; ground and crushed seed shells of other plants such as maize (e.g., corn cobs or corn kernels), etc.; processed wood materials such as those derived from woods such as oak, hickory, walnut, poplar, mahogany, etc., including such woods that have been processed by grinding, chipping, or other form of particalization, processing, etc. Further the proppant may include porous ceramics or organic polymeric particulates. The porous particulate material may be treated with a non-porous penetrating material, coating layer or glazing layer. For instance, the porous particulate material may be a treated particulate material, as defined in U.S. Patent Publication No. 20050028979 wherein (a) the ASG of the treated porous material is less than the ASG of the porous particulate material; (b) the permeability of the treated material is less than the permeability of the porous particulate material; or (c) the porosity of the treated material is less than the porosity of the porous particulate material. When present, the amount of proppant in the well treatment fluid is typically between from about 0.5 to about 20.0, preferably between from about 1 to about 12, pounds of proppant per gallon of the viscoelastic fluid. 
         [0020]    When treating a subterranean formation which is sensitive to water (such as dry coal beds), it is often necessary to minimize the amount of water in the well treatment fluid by energizing with either nitrogen or liquid carbon dioxide (CO 2 ), foaming with a gas, such as nitrogen or emulsifying the fluid with liquid CO 2 . Typically, the word “energized” refers to a fluid containing two phases whereby less than 63 volume percent of the internal phase is either a gas or a liquid (e.g. nitrogen or liquid CO 2 ). Typically, the term “foamed” refers to a fluid that contains more than 63 volume percent of the internal phase is either a gas or a liquid. Suitable foamed fluids typically have a foam quality greater than or equal to 70%, preferably up to 95% when nitrogen is used and greater than or equal to 70%, preferably up to 85% when liquid CO 2  is used. Foaming a fluid increases the viscosity of the fracturing fluid and allows for a reduction in the amount of water without loss of treatment fluid volume. Further, it is believed that the presence of the HEC in the fluid modifies the surfactant film on the bubbles or dispersed droplets in the foam and emulsion to provide increased foam and emulsion viscosity. While nitrogen and liquid CO 2  are described as examples of a suitable gas or liquid that can be used for the internal phase of the foamed fluid it is to be appreciated that in accordance with the invention, any other gases or fluids known in the art may be utilized. 
         [0021]    In a preferred embodiment (based on a 50 vol. % solution of cationic surfactant and a 40 vol. % solution of anionic surfactant), the loadings of each of the anionic and cationic surfactants solutions in the gelled foamed, emulsified or energized fluid are about 3 L/m 3 . At such loadings the viscosity of the foamed fluid is at least 25 cP at room temperature. Typically, the ratio of cationic surfactant to anionic surfactant in the foamed or emulsified fluid is in the range from about 1:1 to about 2:1, by volume. 
         [0022]    The fluid may optionally contain an inorganic salt as a clay stabilizer, such as potassium chloride, choline chloride or ammonium chloride. However, clay stabilizers, foaming agents, biocides and other additives are generally unnecessary in most cases. 
         [0023]    The viscoelastic fluid described herein may be batch-mixed or mixed on-the-fly. 
         [0024]    The fluid exhibits a fairly low surface tension, in the area of about 30 dynes/cm 2 . The low surface tension of the fluid aids in rapid flow-back for cleaning up the well after fracturing. 
         [0025]    The fluids described herein are effective as fracturing fluids in any type of well, including high pressure and low pressure wells. In a preferred embodiment, the fluid is used to treat low pressure wells. An example of low pressure wells suitable for using the viscoelastic fluid defined herein are those low pressure wells (of between about 1 to about 100 psi) such as coal bed methane wells. Such wells may or may not produce water. These types of wells are perforated in many coal seams with varying productivity in each zone. 
         [0026]    In addition to coal, the viscoelastic fluids described herein find particular applicability in the treatment of low permeability subterranean formations, such as those comprised primarily of sandstone, limestone, dolomite, shale, siltstone, and diatomite, as well as coal. In particular, the fluids provide a robust alternative in shallow, brittle shales where water cost, availability and disposal has become increasingly expensive and environmentally unacceptable. 
         [0027]    The following examples are illustrative of some of the embodiments of the present invention. Other embodiments within the scope of the claims herein will be apparent to one skilled in the art from consideration of the description set forth herein. It is intended that the specification, together with the examples, be considered exemplary only, with the scope and spirit of the invention being indicated by the claims which follow. 
         [0028]    All percentages set forth in the Examples are given in terms of weight units except as may otherwise be indicated. 
       EXAMPLES 
     Examples 1-4 
       [0029]    A premixed surfactant FAC-3W was prepared by mixing a 1:1 ratio of C 18  trimethyl quaternary ammonium chloride solution (40%, also known as FAC-1W) to sodium xylene sulfonate solution (50%), at room temperature for five minutes. To  11  L/m 3  FAC-3W was added an additional 1.5 L/m 3  FAC-1W. To the resulting system was then added 1 kg/m 3  HEC (Example 3,  FIG. 1 ) and 1 kg/m 3  underivatized guar (Example 4,  FIG. 1 .) The aqueous system was then evaluated using a flow-loop rheometer which was equipped with a constant volume circulating pump and an independent air driven pump. The flow-loop was further fitted with a 10,000 psi site glass for observation. The temperature was increased to 30° C. with the heating jacket on the loop. The pressure was 1,000 psi to more closely simulate well conditions. The viscosity measurements were carried out at a nominal shear rate of 100 s −1 . The results are illustrated in  FIG. 1  wherein Comparative Example 2 shows the results of 1 kg/m 3  HEC by itself.  FIG. 1  demonstrates the synergy between the fluid containing the cationic surfactant/anionic surfactant (Example 1) and the same fluid containing HEC (Example 3). No synergy was seen with the fluid and guar (Example 4). 
       Examples 5-6 
       [0030]    The fluid containing FAC-3W and FAC-1W (Example 1) and the fluid containing the HEC (Example 3) were then foamed with 70-85% nitrogen or 70-80% liquid CO 2 . The rheology of the fluid was then evaluated using the flow-loop rheometer at 30° C. described in the Examples above. In order to get a 70 quality foam, 95 mls of fluid was loaded into the flow-loop which had a 312 ml capacity. The balance was then filled with 70 vol. % N 2  gas or 70 vol. % liquid CO 2 . After the N 2  or CO 2  was added, the fluid in the circulation loop was foamed/emulsified by shearing it with the aid of the nozzle and the temperature increased with the heating jacket on the loop. The pressure was 1,000 psi to more closely simulate well conditions. The viscosity measurements were carried out at a nominal shear rate of 100 s −1 . The results are set forth in  FIG. 2  for nitrogen and  FIG. 3  for liquid CO 2 .  FIG. 2  shows that addition of 1 kg/m 3  HEC increases foam viscosity 40% to 67% depending on the nitrogen ratio.  FIG. 3  shows that addition of 1 kg/m 3  HEC increases emulsion viscosity from 30% to 77% depending on the CO 2  ratio. Further, the addition of 1 kg/m 3  HEC increased the base gel viscosity by 21%. 
         [0031]    From the foregoing, it will be observed that numerous variations and modifications may be effected without departing from the true spirit and scope of the novel concepts of the invention.