Abstract:
The present invention relates to a method of breaking an aqueous based viscosified fluid containing a breaker comprising an alkali metal chlorite or hypochlorite through use of a breaker activator comprising thioglycolate ion or a compound capable of producing thioglycolate ion in the aqueous fluid. The method is particularly effective in providing a controlled break in a treatment fluid introduced into a subterranean formation having a static temperature in the range of from about 100° F. to about 200° F.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to compositions and methods for treating subterranean formations. Specifically, the invention is directed to compositions used to break fracturing fluids utilized in the stimulation of subterranean formations. 
     2. Description of the Prior Art 
     It is common practice to treat subterranean formations to increase the gross permeability or conductivity of such formations by procedures which are identified generally as fracturing processes. For example, it is a conventional practice to hydraulically fracture a well in order to produce one or more cracks or &#34;fractures&#34; in the surrounding formation by mechanical breakdown of the formation. Fracturing may be carried out in wells which are completed in subterranean formations for virtually any purpose. The usual candidates for fracturing, or other stimulation procedures, are production wells completed in oil and/or gas containing formations. However, injection wells used in secondary or tertiary recovery operations, for example, for the injection of water or gas, may also be fractured in order to facilitate the injection of fluids into such subterranean formations. 
     Hydraulic fracturing is accomplished by injecting a hydraulic fracturing fluid into the well and applying sufficient pressure on the fracturing fluid to cause the formation to break down with the attendant production of one or more fractures. The fracture or fractures may be horizontal or vertical, with the latter usually predominating, and with the tendency toward vertical fracture orientation increasing with the depth of the formation being fractured. Usually a gel, an emulsion or a foam, having a proppant such as sand or other particulate material suspended therein is introduced into the fracture. The proppant is deposited in the fracture and functions to hold the fracture open after the pressure is released and the fracturing fluid flows back into the well. The fracturing fluid has a sufficiently high viscosity to retain the proppant in suspension or at least to reduce the tendency of the proppant to settle out of the fracturing fluid as the fracturing fluid flows along the created fracture. Generally, a viscosifying agent and/or an emulsifier is used to gel or emulsify the fracturing fluid to provide the high viscosity needed to realize the maximum benefits from the fracturing process. 
     After the high viscosity fracturing fluid has been pumped into the formation and fracturing of the formation occurred, it is desirable to remove the fluid from the formation to allow hydrocarbon production through the new fractures. Generally, the removal of the highly viscous fracturing fluid is realized by &#34;breaking&#34; the gel or emulsion or, in other words, by converting the fracturing fluid into a low viscosity fluid. Breaking the gelled or emulsified fracturing fluid has commonly been accomplished by adding a &#34;breaker,&#34; that is, a viscosity-reducing agent, to the fracturing fluid prior to pumping into the subterranean formation. However, this technique can be unreliable and sometimes results in incomplete breaking of the fluid, particularly when gel stabilizers are present and/or premature breaking of the fluid before the fracturing process is complete. 
     U.S. Pat. No. 5,413,178 discloses a means for breaking an aqueous based fracturing fluid or treatment fluid at a temperature above 175° F. through use of an effective amount of an alkali metal chlorite or hypochlorite. The breaker is particularly effective in the temperature range of 225° F. to about 300° F. 
     It has been determined that alkali metal chlorites generally are ineffective as a breaker within a reasonable time period at temperatures below 175°-200° F. 
     It would be desirable to provide a means by which alkali metal chlorites or hypochlorites could be utilized to provide a controlled break at temperatures below 200° F. and particularly in the range of from about 100° F. to about 200° F. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of controllably breaking an aqueous based fracturing fluid or treatment fluid which has been viscosified with polysaccharides at temperatures in the range of from about 100° to about 200° F. The method is accomplished by the incorporation of an effective amount of a thioglycolate salt comprising thioglycolic acid or its salts which are capable of producing thioglycolate ion in solution in combination with an effective amount of an alkali metal chlorite or hypochlorite whereby the breaker is activated to provide a controlled break. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The viscosifying or gelling agent employed in the present invention includes natural and derivatized polysaccharides which are soluble, dispersible, or swellable in an aqueous liquid to yield viscosity to the fluid. One group, for example, of polysaccharides which are suitable for use in the present invention include gum arabic, gum ghatti, gum karaya, tamarind gum, tragacanth gum, guar gum, locust bean gum and the like. Modified gums such as carboxyalkyl derivatives, like carboxymethyl guar, and hydroxyalkyl derivatives, like hydroxypropyl guar also can be employed. Doubly derivatized gums such as carboxymethylhydroxypropyl guar (CMHPG) can also be used. Modified celluloses and derivatives thereof also can be employed. There are thus literally thousands of such materials which have varying properties that can be employed in the practice of the present invention, for example, cellulose ethers, esters and the like. 
     In general, any of the water-soluble cellulose ethers can be used in the practice of the invention. Those cellulose ethers which can be used include, among others, the various carboxyalkyl cellulose ethers, such as carboxyethyl cellulose and carboxymethyl cellulose (CMC); mixed ethers such as carboxyalkyl ethers, such as carboxymethyl hydroxyethyl cellulose (CMHEC); hydroxyalkyl celluloses such as hydroxyethyl cellulose (HEC) and hydroxypropyl cellulose; alkyhydroxyalkyl celluloses such as methylhydroxypropyl cellulose; alkyl celluloses such as methyl cellulose, ethyl cellulose and propyl cellulose; alkylcarboxyalkyl celluloses such as ethylcarboxymethyl cellulose; and alkylalkyl celluloses such as methylethyl cellulose; and hydroxyalkylalkyl celluloses such as hydroxypropylmethyl cellulose; and the like. A preferred derivatized cellulose is a hydroxyethyl cellulose grafted with vinyl phosphonic acid such as disclosed in U.S. Pat. No. 5,067,565. The entire disclosure of which is incorporated herein by reference. Most preferred polysaccharides are the galactomanans, modified or derivative galactomanans, and cellulose derivatives, examples of which are given above. 
     The chemistry and materials involved in the preparation of polysaccharide gelled fluids of the type described above is well understood in the art. As is well understood, the amount of gelling or viscosifying agent employed in the aqueous gel depends upon the desired viscosity of the solution. The gelling agent generally is present in an amount of from about 10 to about 120 pounds per 1000 gallons of fluid. The preferred concentration is in the range of from about 20 to about 60 pounds per 1000 gallons of fluid. 
     The fluid may include a crosslinking agent to further enhance the development of viscosity by crosslinking the gelling agent in the fluid. The crosslinking agent can comprise a borate releasing compound or any of the well known transition metal ions which are capable of creating a crosslinked structure with the particular gelling agent utilized. Examples of such crosslinking agents sources include a borate releasing compound such as sodium tetraborate, aluminum, zirconium or titanium chelates, antimony compounds and the like. The crosslinking agent can be controlled to permit introduction of the fluid into a wellbore before the viscosity significantly increases. 
     The fluid also may include any of the other conventional additives such as proppants, pH control agents, bactericides, clay stabilizers, surfactants and the like which do not adversely react with the other constituents to inhibit performance of the desired treatment upon a subterranean formation. 
     The breaker system for the gelled fluid comprises an effective amount of at least one compound selected from the group consisting of calcium hypochlorite and alkali metal chlorites or hypochlorites in at least partially water soluble form and an effective amount of a breaker activator comprising a thioglycolate salt or acid compound capable of producing thioglycolate ion in an aqueous fluid. 
     The amount of breaker employed is that amount required to reduce the viscosity of the stabilized gelled fluid at a temperature above about 100° F. to a preselected lower viscosity or to a complete break as most desired within a desired period of time. The optimum or effective amount of breaker employed in the present invention depends on factors such as the injection period desired, the particular gelling agent and its concentration, the particular breaker and the formation temperature as well as other factors. Typically, however, from about 0.1 to about 30 pounds of the chlorite breaker is employed per 1000 gallons of fluid. Most preferably, to achieve a desired break in from about 6 to 24 hours in the fluid, from about 1 to about 10 pounds of chlorite is employed per 1000 gallons of fluid. 
     The amount of breaker activator employed is that amount required to activate the chlorite breaker in the temperature range of from about 100° to about 200° F. to yield a lower viscosity within a desired period of time. The optimum or effective amount of activator depends on factors such as the injection period desired, the particular gelling agent and its concentration, the particular breaker and the formation temperature as well as other factors. Typically, however, from about 0.1 to about 40 pounds of thioglycolate ion is employed per 1000 gallons of fluid. Most preferably, to achieve desired breaks in from about 6 to 24 hours in the fluid from about 0.2 to about 20 pounds of thioglycolate ion is employed. Compounds capable of producing thioglycolate ions in solution include methyl thioglycolate and ammonium, calcium, potassium, sodium salts of thioglycolate and the like. When such compounds are utilized, a sufficient quantity is used to provide the equivalent designated quantity of thioglycolate ion. 
     To further illustrate the present invention, but not by way of limitation, the following examples are provided. 
    
    
     EXAMPLE I 
     Static break tests were performed to evaluate break capability in fluid systems with the breaker activator. The gel was prepared by adding 30 pounds guar/1000 gallon of fluid, 2 gallon &#34;CLAYFIX II&#34; clay stabilizer/1000 gallon fluid, 0.1 gallon &#34;BA-20&#34; buffer/1000 gallon of fluid, 0.5 gallon/1000 gallon of a 50% caustic solution and 0.9 gallon of a borate crosslinker/1000 gallon of fluid. The gelling agent was permitted to hydrate for approximately 20 minutes prior to addition of the crosslinker. The breaker comprised sodium chlorite in the quantities identified in the Table and the activator comprised thioglycolic acid neutralized with sodium hydroxide in the quantities indicated in the Table. The break test was performed at a temperature of 125° F. The initial fluid pH was 10.95 and initial viscosity was 23 cps. before addition of the borate crosslinker. 
     
                       TABLE I______________________________________             Viscosity With Time,             cps.Sample Breaker    Activator  1   2   3   4    6   24No.   lbs/1000 gallon            lbs/1000 gallon                       Hr  Hr  Hr  Hr   Hr  Hr______________________________________1      0         0          C.sup.1                           C   C   C    C   C2      5         2          C.sup.                           C   C   C    C   63      5         4          C.sup.                           C   C   C    C   64     10         1          C.sup.                           C   C   C    --  75     10         2          C.sup.                           C   C   C    27  46     10         4          C.sup.                           C   C   C    15  37     15         0          C.sup.                           C   C   C    C   C8     15         1          C.sup.                           C   C   C    --  69     15         2          C.sup.                           C   C   C    28  410    15         4          C.sup.                           C   C   .sup. WC.sup.2                                         7  311    20         2          --.sup.                           --  --  --    3  712    20         4          --.sup.                           --  --  --    3  5______________________________________ .sup.1 C: Crosslinked .sup.2 WC: Weak Crosslink 
    
     EXAMPLE II 
     The tests of Example I are repeating using 0.75 gallon/1000 of caustic solution to provide an initial pH of 11.33. The initial viscosity was 23 cps. and the test temperature was 150° F. The tests are each run in duplicate. 
     The results are set forth in Table II. 
     
                       TABLE II______________________________________                 Viscosity With Time,Breaker     Activator cps.Sample lbs/      lbs/      1   2    3   4    6    24No.   1000 gallon           1000 gallon                     Hr  Hr   Hr  Hr   Hr   Hr______________________________________1      0        0         C.sup.1                         C    C   C    C    C2      2          1/2     C.sup.                         C    C   .sup. WC.sup.2                                       WC   53      5        1         C.sup.                         WC   15  5    --   34     10          1.5     C.sup.                         WC   20  6    --   35     10        2         C.sup.                         WC   32  6    --   36     10        0         C.sup.                         C    C   C    C    C7     10        1         C.sup.                         WC   7   4    --   38     10        2         C.sup.                         8    4   --   --   29     10        4         C.sup.                         6    4   --   --   2______________________________________ .sup.1 C: Crosslinked .sup.2 WC: Weak Crosslink 
    
     EXAMPLE III 
     The tests of Example I are repeated using 40 pounds guar/1000 gallon of fluid, 1 gallon caustic solution/1000 gallon of fluid and 1.2 gallon/1000 borate crosslinker. The initial pH was 11, initial viscosity was 33 cps. and test temperature was 175° F. 
     The results are set forth in Table III. 
     
                       TABLE II______________________________________                 Viscosity With Time,Breaker     Activator cPsSample lbs/      lbs/      1    2    4    6    24No.   1000 gallon           1000 gallon                     Hr   Hr   Hr   Hr   Hr______________________________________1     0         0         .sup. C.sup.1                          C    C    C    C2     0         2         C    C    C    C    C3     5         0         C    C    C    C    C4     10        0         C    C    C    C    C5     15        0         C    C    C    C    C6     1         1/2       C    C    C    C    277     1         1         C    C    C    C    .sup. WC.sup.28     1         2         C    C    C    C    C9     2         1/2       C    C    C    C    2510    2         1         C    C    25    9    511    2         2         C    C    WC   29   2612    5         1/2       C    WC   30   27    613    5         1         WC   7     5   --   --14    5         2         30   4    --   --   --______________________________________ .sup.1 C: Crosslinked .sup.2 WC: Weak Crosslink 
    
     EXAMPLE IV 
     The tests of Example I are repeated using 40 pounds guar/1000 gallon, 0.25 gallon &#34;BA-20&#34; buffer/1000 gallon, 0.8 gallon zirconium crosslinker/1000 gallon. The initial pH was 6.45, initial viscosity was 34 cps. and test temperature was 175° F. 
     The results are set forth in Table IV. 
     
                       TABLE IV______________________________________              Viscosity With Time,              cps.Sample Breaker    Activator  1   2    3   4    24No.    lbs/1000 gallon             lbs/1000 gallon                        Hr  Hr   Hr  Hr   Hr______________________________________1      5          0          C.sup.1                            C    C   C    C2      5          1/2        C.sup.                            C    C   C    23      5          1          C.sup.                            C    C   5    24      5          1          C.sup.                            C    C   6    2______________________________________ .sup.1 C: Crosslinked 
    
     EXAMPLE V 
     The tests of Example IV are repeated using different reducing agents to attempt to activate the sodium chlorite breaker which is present in an amount of 5 pounds/1000 gallon. Test temperature was 175° F., initial pH of 6.45 and initial viscosity of 34 cps. The various compounds tested are identified in the Table V along with the results of the tests. 
     
                       TABLE V______________________________________               Viscosity With Time,               cps.Sample Activator &amp; Concentration,                         1    2   4   6   24No.   lbs/1000 gallon         Hr   Hr  Hr  Hr  Hr______________________________________1     Sodium sulfite 1.2      .sup. C.sup.1                              C   C   C   C2     Sodium sulfite 2.4      C    C   C   C   C3     Thiourea       .75      C    C   C   C   C4     Thiourea       1.5      C    C   C   C   C5     Hydroxylamine  .75      C    C   C   C   C hydrochloride6     Hydroxylamine  1.5      C    C   C   C   C hydrochloride7     Triethanol amine                1.5 gal  C    C   C   C   C8     Triethanol amine                  3 gal  .sup. WC.sup.2                              C   C   C   C9     Dithiodiglycolic acid                  1      C    C   C   C   C10    Dithiodiglycolic acid                  2      C    C   C   C   C______________________________________ .sup.1 C: Crosslinked .sup.2 WC: Weak Crosslink 
    
     The foregoing examples clearly illustrate the effect of the breaker activator upon the alkali metal chlorite breaker. The data of Table V illustrates how other reducing agents fail to activate the breaker even at elevated temperatures where smaller quantities of thioglycolic effectively activate the breaker in the gelled fluid. 
     While that which is considered to comprise the preferred embodiments of the present invention has been described herein, it is to be understood that various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the spirit or scope of the present invention as set forth in the appended claims.