A Superior, High-Performance Enzyme for Breaking Borate Crosslinked Fracturing Fluids Under Extreme Well Conditions

摘要

Enzyme breakers have been previously used for hydrolyzing guar gels at temperatures below 150°F. There is an industry-wide demand for enzyme breakers that can function under higher-temperature (200-250°F) and extreme pH (≧10.5) conditions. To meet this demand, efforts have been made to develop an exceptionally thermostable cellulase enzyme, referred to hereafter as mannanase, that was originally discovered in a hydrothermal vent sample. This mannanase exhibits well-differentiated performance under extreme downhole conditions encountered in gas shales and deeper oil/gas wells. This superior mannanase can effectively break linear and borate crosslinked guar under broad ranges of temperature (80°F up to at least 225°F as seen by rheology, and up to 275°F using residual activity analysis) and pH (3.0 up to 10.5). The results of rheological tests show that only a small dose is required (100 ppm or less) to achieve the complete break. The enzymatic reaction can be triggered by the changes of temperature and pH during fracturing operations. This mannanase also exhibits a dose response that allows the operator to generate a desirable viscosity/ time profile by adjusting enzyme dosage. Even in the presence of fluid additives, such as buffers, salts, stabilizers, and crosslinkers, this mannanase is active for effective viscosity reduction. This mannanase breaker belongs to the glucanase family. It reduces gel viscosity by specifically targeting β-1,4 glycosidic bonds between the mannose units in guar. The carbohydrate-profiling tests demonstrate that this enzyme effectively and efficiently breaks the long-guar polymers into small, soluble fragments that will eliminate gel rehealing. The conductivity tests demonstrate extensive cleaving of guar and removal of polymer residues that cause formation damage and reduce fracture conductivity.