Abstract:
The allosteric effectors, sulfate and phosphate, can be used with mannanohydrolase enzymes to better control the break of gelled fracturing fluids in wellbore operations. The fracturing fluid contains a hydratable polymer of guar, a guar derivative, a cellulose derivative, a water soluble biopolymer or a combination thereof and a crosslinking agent. The mannanohydrolase enzyme catalyzes the hydrolysis of β-(1,4) mannosidic linkages to break the polymeric backbone.

Description:
FIELD OF THE INVENTION 
     The invention relates to gelled fracturing fluids used in wellbore operations. More specifically, the present invention relates to methods of hydrolyzing gelled fracturing fluids using enzymes, whereby the enzymatic activity is regulated using allosteric effectors. 
     BACKGROUND OF THE INVENTION 
     Hydraulic fracturing is used to create subterranean fractures that extend from the borehole into the rock formation to improve the rate of production. Generally, a high viscosity fracturing fluid is pumped into the well at sufficient pressure to fracture the subterranean formation. In order to maintain the increased exposure of the formation, a solid proppant may be added to the fracturing fluid, which is carried into the fracture by the high pressure applied to the fluid. 
     Some conventional fracturing fluids include guar gum (galactomannans) or guar gum derivatives, such as hydroxypropyl guar (HPG), carboxymethyl guar (CMG), or carboxymethylhydroxypropyl guar (CMHPG). These polymers can be crosslinked in order to increase their viscosities and increase their capabilities of proppant transport. 
     Once the formation is adequately fractured and the proppant is in place, the fracturing fluid is recovered typically through the use of breakers. Breakers generally reduce the fluid&#39;s viscosity to a low enough value allowing the proppant to settle into the fracture, thereby exposing the formation to the well. Breakers work by severing the bonds of the polymer. This “breaks” the polymer reducing its molecular weight. The fracture then becomes a high permeability conduit for fluids and gas to be produced back to the well. 
     Breakers control the timing of the break for the fracturing fluid. Gels that break prematurely can cause suspended proppant material to settle out of the gel before being introduced a sufficient distance into the produced fracture. Also, premature breaking can result in a premature reduction in the fluid viscosity resulting in a less than desirable fracture length in the fracture being created. For purposes of the present application, premature breaking will be understood to mean that the gel viscosity becomes diminished to an undesirable extent before all of the fluid is introduced into the formation to be fractured. 
     On the other hand, gelled fluids that break too slowly can cause slow recovery of the fracturing fluid and a delay in resuming the production of formation fluids. Additional problems can result, such as the tendency of proppant to become dislodged from the fracture, resulting in an undesirable closing and decreased efficiency of the fracturing operation. 
     Optimally, the fracturing gel will begin to break when the pumping operations are concluded. For practical purposes, the gel should be completely broken within a specific period of time after completion of the fracturing period. At higher temperatures, for example, about 24 hours is sufficient. A completely broken gel will be taken to mean one that can be flushed from the formation by the flowing formation fluids or that can be recovered by a swabbing operation. 
     By comparison, certain gels, such as those based upon guar polymers, undergo a natural break without the intervention of chemical additives; however, the break time can be excessively long. Accordingly, to decrease the break time of gels used in fracturing, chemical agents are incorporated into the gel and become a part of the gel itself. Typically, these agents are either oxidants or enzymes that operate to degrade the polymeric gel structure. 
     Still, obtaining controlled breaks using various chemical agents, such as oxidants or enzymes, is challenging. Oxidants are ineffective at low temperature ranges from ambient temperature to 130° F. They require either higher temperatures to function as breakers or a coreactant to initiate cleavage of the viscosifying polymer. Oxidants do not necessarily break the polysaccharide backbone into monosaccharide units. The breaks are nonspecific, creating a mixture of macromolecules. Further, common oxidants are difficult to control because they not only attack the polymer, but they also react with any other molecule that is prone to oxidation. For example, oxidants can react with the tubing and the linings in the well, or the resins on resin-coated proppants. 
     In contrast, enzymes, are catalytic and substrate specific, hydrolyzing distinct bonds of the polymer. As a catalyst, they will hydrolyze many polymeric bonds before they eventually degrade. Enzymes can avoid the high temperatures associated with the chemical oxidants. However, they often operate under narrow pH and temperature ranges. 
     Conventional enzymes used to degrade galactomannans have maximum catalytic activity under mildly acidic to neutral conditions (pH 5 to 7). Activity profiles indicate the enzymes retain little to no activity at higher pH values. Their activity rapidly declines above pH 8, and they begin to denature above pH 9. This can pose a problem, for example, in borate-crosslinked guar gels, because these gels are also pH dependant, generally needing a pH in excess of 8 to initiate the gellation. As the pH increases, the resulting gel becomes stronger. Normally, when enzymes are used with borate-crosslinked gels, the gels are buffered to maintain a pH range of 8.2 to 8.5 to ensure gellation, but to inhibit enzyme degradation. This technique requires high concentrations of both borate and enzyme. Unfortunately, while ensuring good breaks, the initial gel stability and proppant transport capability is weakened. The determination of the optimum enzyme concentration is a compromise between initial gel stability and an adequate break. 
     Moreover, these enzymes work well at ambient to moderate temperatures (75° F. to 150° F.). At elevated temperatures, (&gt;150° F.) they quickly denature and lose activity. The galactomannans used in conventional enzyme formulations have a temperature maximum of approximately 150° F. Activity profiles indicate that the enzyme retains little to no activity past this point, while many downhole fracturing operations are conducted at temperatures in excess of 150° F. 
     It would be an important advantage for the well operator to have greater control over the enzyme activity and, hence, the rate of hydrolysis of the polymer, to allow for a more timely and adequate break. Control over the enzyme activity could also allow the well operator to better work within the constraints of pH and temperature, and even allowing for less enzyme to be used fracturing fluid formulations. 
     SUMMARY OF THE INVENTION 
     The present invention relates to the use of allosteric effectors to better regulate the activity of the enzyme breakers. It has been discovered that the mannanohydrolase enzymes disclosed in U.S. Pat. No. 8,058,212 and U.S. Pat. No. 8,096,360, hereby incorporated by reference, possess allosteric binding sites. The present invention uses sulfates and phosphates as effectors to better control the break of the fracturing gel containing mannanohydrolase enzymes. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  (SEQ ID NO:1) represents the nucleotide sequence that codes for a mannanohydrolase used in an aspect of the invention. 
         FIG. 1B  (SEQ ID NO:2) represents the amino acid sequence of a mannanohydrolase used in an aspect of the invention. 
         FIG. 2  (SEQ ID NO:3) is the sequence of the gene that is codes for an enzyme breaker made in accordance with embodiments of the present invention. 
         FIG. 3  represents the reduction in the viscosity of crosslinked guar polymer by the action of an enzyme breaker. 
         FIG. 4  represents the comparison of the effects of SO 4   2−  and PO 4   3−  on mannanohydrolase activity. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Illustrative embodiments of the invention are described below as they might be employed in the operation and in the treatment of oilfield applications. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, 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. Further aspects and advantages of the various embodiments of the invention will become apparent from consideration of the following description. 
     As an embodiment of the present invention, a method of fracturing a subterranean formation that surrounds a wellbore is provided. In this embodiment, a crosslinked polymer gel is provided that includes an aqueous fluid, a hydratable polymer, a crosslinking agent capable of crosslinking the hydratable polymer, a sulfate or a phosphate, and an enzyme breaker comprising a mannanohydrolase. The crosslinked polymer is then injected to a desired location within the wellbore and into contact with the formation under sufficient pressure to fracture the surrounding subterranean formation. The polymer may be injected after it is fully crosslinked or while the crosslinking is underway. Once the fracturing is complete, the enzyme breaker is allowed to degrade the crosslinked polymer gel so that it can be recovered or removed from the subterranean formation. 
     Besides the method embodiments, compositions are also provided as embodiments of the present invention. As another embodiment of the present invention, a fracturing fluid composition is provided. The fracturing fluid comprises an aqueous fluid, a hydratable polymer, a crosslinking agent capable of crosslinking the hydratable polymer, a sulfate or a phosphate, and an enzyme breaker comprising a mannanohydrolase. 
     The enzyme breakers of the present invention preferably comprise mannanohydrolases. In an aspect, the mannanohydrolase enzyme originates from the thermophilic and anaerobic  Caldocellum saccharolyticum . Isolation of the gene encoding for the mannanohydrolase enzyme is described in E. Luthi et al, “Cloning, Sequence Analysis, and Expression in  Escherichia coli  of a Gene Coding for a β-Mannanase From the Extremely Thermophilic Bacterium ‘ Caldocellum saccharolyticum’, Applied and Environmental Microbiology , March 1991, pp. 694-700, herein incorporated by reference. 
     The gene for this mannanohydrolase was codon optimized to increase the efficiency of its expression in  E. coli , as disclosed in U.S. Pat. No. 8,058,212. The nucleotide sequence of the htβ gene is set forth in  FIG. 1A  (SEQ ID NO:1), while its corresponding amino acid sequence is set forth in  FIG. 1B  (SEQ ID NO:2). The nucleotide sequence includes the coding sequence for the mannanohydrolase and the leader sequence on the N-terminus. 
     In another aspect, the enzyme breaker is derived from a gene from the alkaliphilic  Bacillus  sp. N16-5. In an aspect, the enzyme breaker is a member of the glycosyl hydrolase family 5 and is an active β-mannanase. This enzyme breaker can be prepared in accordance with the methods described in U.S. Pat. No. 8,096,360. The nucleotide sequence for the enzyme is shown in  FIG. 2  (SEQ ID NO:3). In an aspect, the gene is codon optimized for expression in  E. coli.    
     The enzyme breakers of the present invention catalyze the hydrolysis of β-(1,4) mannosidic linkages and can be used to break the polymer backbone of galactomannan polymers. 
     Generally, reactions take place in the enzyme&#39;s active site, which is specifically structured for a particular substrate and chemical reaction. In some cases, other binding sites exist within the enzyme or on its surface. In a process called allosteric regulation an effector molecule can bind to these sites and affect the enzyme activity. Allosteric activators are those molecules that bind to the enzyme and enhance the enzyme&#39;s activity against a particular substrate, while allosteric inhibitors decrease the enzyme&#39;s activity. It is thought that enzymes with allosteric binding sites exist in two different conformations. One of these conformers will have a lower activation energy for the catalyzed reaction. The binding of an allosteric activator molecule can shift the equilibrium from one conformational state to the other. 
     It has been discovered that both of the aforementioned mannanohydrolase enzymes possess allosteric activator binding sites for sulfates and phosphates, although other 1,4-β-mannanases are contemplated and within the scope of the present invention. The preferred sulfate or phosphate compounds as the allosteric effectors are those soluble in water and brines. Using such effectors enhances the enzymatic breakdown of the polymer so that the reaction occurs faster, is more efficient and complete, resulting in no rebound of viscosity upon cooling of the breaker fluid. An advantage of the present invention is the ability to control the rate of hydrolysis of the guar polymer. This would give the well operator more control over the rate at which the fracturing fluid broke. Another possible advantage of the present invention is that less enzyme product is needed to achieve a complete break in the viscosity of the polymer. A reduction in the amount of enzyme breaker needed results in a cost savings in terms of enzyme production, shipping, and storage. 
     The enzyme breaker and the effectors, phosphate and sulfate, may be diluted in various concentrations that are effective and convenient for use in fracturing jobs. In an aspect, the enzyme breaker of the present invention may be diluted to a concentration of about 1:24 and is present in the crosslinked polymer gel in a range of about 0.25 gpt to about 4 gpt of 1/100 dilution: alternatively, in a range of about 0.5 gpt to about 2.5 gpt; alternatively, in a range of about 0.5 gpt to about 1 gpt; or alternatively, in a range of about 1 gpt to about 2 gpt. In an aspect, the total protein concentration of the stock enzyme breaker from which the dilutions are made is greater than 1 mg/mL. Preferably, the effector will be used in about a 1:1 stoichiometric ratio with the enzyme breaker. In an aspect, a 1 gpt of a 20 wt. % aqueous solution of Na 2 SO 4  is used. But, an aqueous solution of 0.005 wt. % of sulfate or phosphate and greater may also be used. Other suitable dilution concentrations of enzyme breaker and allosteric effector will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     The methods and compositions described herein can be used with a variety of hydratable polymers. In an aspect, the hydratable polymer has repeating units of mannose linked by β-(1,4) mannosidic linkages. In another aspect, the hydratable polymer is a polysaccharide and comprises guar, guar derivatives, cellulose derivatives, water soluble biopolymers, or combinations thereof. GW-3LDF is a particularly suitable guar polymer that is commercially available from Baker Hughes Inc. In an aspect, the intrinsic viscosity of the hydratable polymer in the aqueous fracturing fluid is greater than about 14 dl/g. In another aspect, the intrinsic viscosity of the hydratable polymer in the aqueous fracturing fluid is greater than about 16 dl/g. Other suitable types of hydratable polymers that can be used in the methods and compositions described herein will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     Because some enzyme breakers of the present invention have a maximum activity under alkaline pH ranges, they can be combined with other breakers that operate in different pH ranges to allow for better control of hydrolysis of fracturing fluids over a much greater pH range. In an aspect, the crosslinked polymer gel can further include a second enzyme breaker that is catalytically active and temperature stable in a pH range of about 4 to about 10. Suitable enzymes that can be used include those described in U.S. Pat. No. 5,201,370, which is hereby incorporated by reference. 
     Divalent cations can affect the activity of the enzyme breaker of the present invention. In an aspect, the crosslinked polymer gel can further include a divalent cation. Suitable divalent cations can include Mg 2+ , Ca 2+ , Cd 2+ , Ni 2+ , Co 2+ , or Mn 2+ . Other suitable divalent cations that can be used in the present invention will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     The methods and compositions described herein can be used with a wide variety of crosslinking agents. A suitable crosslinking agent can be any compound that increases the viscosity of the hydratable polymer by chemical crosslinking, physical crosslinking, or any other mechanisms. For example, the gellation of the hydratable polymer can be achieved by crosslinking the hydratable polymer with metal ions including borate compounds, zirconium compounds, titanium compounds, aluminum compounds, antimony compounds, chromium compounds, iron compounds, copper compounds, zinc compounds, or mixtures thereof. One class of suitable crosslinking agents is zirconium-based crosslinking agents. Suitable crosslinking agents can include zirconium oxychloride, zirconium acetate, zirconium lactate, zirconium malate, zirconium glycolate, zirconium lactate triethanolamine, zirconium citrate, a zirconate-based compound, zirconium triethanolamine, an organozirconate, or combinations thereof. XLW-14 is a particularly suitable zirconate-based crosslinking agent that is commercially available from Baker Hughes Inc. Suitable borate-containing crosslinking agents can include, for example, alkaline earth metal borates, alkali metal borates, borate esters, probertite, ulexite, nobleite, frolovite, colemanite, calcined colemanite, priceite, pateroniate, hydroboractie, kaliborite, or combinations thereof. XLW-32 is a particularly suitable boron-based crosslinking agent that is commercially available from Baker Hughes Inc. Suitable titanium-containing crosslinking agents can include, for example, titanium lactate, titanium malate, titanium citrate, titanium ammonium lactate, titanium triethanolamine, titanium acetylacetonate, or combinations thereof. Suitable aluminum-containing crosslinking agents can include, for example, aluminum lactate, aluminum citrate, or combinations thereof. Other suitable crosslinking agents that are compatible with the compositions and methods described herein will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     The amount of crosslinking agent and other additives used in the present invention can vary depending upon the desired effect of the additives. For example, the crosslinking agent can be present in the crosslinked polymer gel in an amount sufficient to provide the desired degree of crosslinking between molecules within the hydratable polymer. The amounts of additives that can be used in the present invention will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     Besides the polymers, crosslinking agents, effectors, and enzyme breakers, various additives can be useful in the present invention. Additives used in the oil and gas industry and known in the art, including but not limited to, corrosion inhibitors, non-emulsifiers, iron control agents, delay additives, silt suspenders, flowback additives, proppants, and gel breakers, can also be used in embodiments of the present invention. Other suitable additives useful in the present invention will be apparent to those of skill in the art and are to be considered within the scope of the present invention. 
     EXAMPLES 
     The following examples describe the preferred embodiments of the present invention. Other embodiments within the scope of the claims will be apparent to one skilled in the art from the consideration of the specification or practice of the invention disclosed herein. 
     Example 1 
     A solution was made comprising: 17 ppt of guar polymer, GW-3LDF; 1 gpt of clay stabilizer, Claytreat-3C; 1.5 gpt of buffer, BF-7L; 1 gpt of crosslinking agent, XLW-32; 1 gpt mannanohydrolase, GBW-22C, diluted 1/100 with distilled water; and 1 gpt of a 1% wt. solution of Na 2 SO 4  (all components available from Baker Hughes Inc.). The solution was mixed with an overhead stirrer. 52 mL of crosslinked fluid was loaded on to a Chandler HTHP 5500 Viscometer and run at 500 psi, 150° F., and 100 s −1  for an hour. As shown in  FIG. 3 , the addition of a small amount of Na 2 SO 4  drastically improves the activity of the mannanohydrolase enzyme against the guar polymer. Moreover, there does not appear to be a difference between the activity of the enzyme in high concentrations of the effector (3 g/L) versus smaller concentrations of the effector (1 gpt 20% wt. Na 2 SO 4  or 2 mg/L). 
     Example 2 
     To compare the effects of phosphate and sulfate a solution was made comprising: 17 ppt of guar polymer, GW-3LDF; 1 gpt of clay stabilizer, Claytreat-3C; 1.5 gpt of buffer, BF-7L; 1 gpt of crosslinking agent, XLW-32; 1 gpt mannanohydrolase, GBW-22C, diluted 1/100 with distilled water; and 1 gpt of a 1% wt. solution of K 2 HPO 4  (all components available from Baker Hughes Inc.). As can be seen from  FIG. 4 , both SO 4   2−  and PO 4   3−  provide similar effects on mannanohydrolase enzyme activity. 
     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.