Enzyme Breaker Technologies: A Decade of Improved Well Stimulation

Abstract

Abstract Fracturing fluids are most commonly aqueous systems comprising various polymers and crosslinking agents added to facilitate effective proppant transport and placement. A goal of fracturing practitioners has been to apply breaker technologies effective to eliminate or minimize the residual gel damage left behind by such systems in order to optimize the well stimulation. As a result of R&D efforts to that end, polymer-linkage-specific (polymer-specific) enzyme breakers were first introduced for fracturing applications in 1992. To date, these enzyme breakers have been employed in thousands of fracturing treatments around the world. Recent case studies evaluating long-term cumulative production have shown that wells in which polymer specific enzyme breakers have been applied demonstrate extraordinary performance when compared to production from offsets using other technologies. The long-term success of polymer-specific enzymes in fracturing operations for improving long-term production has been predicated upon a number of characteristics unique to enzymes, including their specificity to target polymer linkages and their functionality as catalysts which are not "spent" in the reactions they initiate. Additionally, contrary to popular misinformation, enzymes are effective at extremes of pH and temperature when properly applied. These unique properties are thought to make polymer-specific enzymes the polymer-degradation additive, which would produce the most effective proppant pack, thereby maximizing long-term productivity. The numerous case histories and offset comparisons documented in previous studies were, as a rule, relatively short-term in the prospective life of a well. The current endeavor seeks to provide detailed follow-up analyses of truly long-term production data from 170 fracture stimulated wells in the Canyon Sand, Penn, Lobo-Wilcox, Redfork, Hosston, and Grayburg/SanAndres formations and, the deep McKittrick field. Production histories over periods of up to 8 years were evaluated on wells treated with the polymer-specific enzymes and compared to offsets completed using conventional breaker technologies. The results clearly demonstrate the benefits of application of polymer-specific enzyme breakers on long-term well productivity. Introduction The enzymes historically used as breakers are non-specific mixtures that randomly hydrolyze polymers. These "conventional" enzymes are predominately mixtures of hemicellulase, cellulase, amylase and pectinase in unspecified ratios. Such enzymes are specific to react with guar, cellulose, starch and pectin polymers, respectively. All of these enzymes are hydrolases and, as such, are capable of binding with any of the aforementioned polymers. Since each of these enzymes is reactive with the linkages found in only one specific type of polymer, only the enzyme specific to that particular polymer will promote a cleavage. The other enzymes, once bound, can neither react with nor release from the polymer, effectively blocking the "right enzyme" from cleaving the polymer. This phenomena, known as competitive inhibition, results in the creation of polymeric fragments — generally the molecular weight of the polymer strand to which it is attached, plus the enzyme itself. In the case of crosslinked fluids, the "combined molecular weight" could be many times higher than the original molecular weight of the linear polymer due to crosslinking of the residual fragments. The result of this is a partial degradation of the polymer into predominately short- to medium-chain length polysaccharides, which are relatively insoluble and therefore may cause significant permeability damage.17–19 Polymer-specific enzymes were first introduced by Tjon-Joe-Pin and Brannon in 1992 for low-temperature, high-pH fracturing applications (60 - 140°F, pH 3–11) to affect improved cleanup of borate crosslinked fluids1. Since then more than 10,000 fracture stimulations have been performed on reservoirs with bottom-hole static temperatures (BHSTs) from 60°F to more than 300°F using guar-linkage-specific enzymes (GLSE). The GLSE complex consists of two isolated enzymes specific towards the linkages available between the sugar units of the guar polymer. Numerous research and laboratory studies have indicated this approach for degrading the guar polymer will inherently out-perform the conventionally used oxidative breakers (typically persulfate salts) and conventional non-specific enzyme mixtures.1–19