Feasibility Study On Bio-enzyme Used For Gel Breaking Of Fracturing Fluid

Hydraulic fracture refers to the major measures of increasing production of oil-gas ifeldand injection of water injection well. When performing fractuirng work under low temperature（20-50°C）, rhcology, shear resistance and sand suspension performance are all easily satisfied,therefore the key of obtaining favorable fractuirng effects lies in solving gel-breaking problemof fractuirng fluid under low temperature. Conventional chemical gel breaker featuresadvantages such as seirous chemical pollution, limited gel-breaking degree, etc. under lowtemperature. To improve gel-breaking effect of fracturing lfuid，this thesis select, evaluate andcompound the biological enzyme by taking the commonly-used ammonium persulfatc breaker（chcmical breaker） as reference, so as to seek gel breaker applicable to water based fractuirngfluid under low temperature.Firstly, optimize the fractuirng lfuid formula which is used to select and evaluategel-breaker by referring to Petroleum Industry Standard SY/T5107-2005RecommendedPractices on Measuring the Properties of Water-based Fracturing Fluid and SY/T6376-2008General Technical Speciifcations of Fracturing Fluids as well as on-site construction formula.Formulation:0.4%HPG+0.45%。Na₂CO₃+1%clay stabilizer+0.3%cleanup additive+0.3%gel-breaker+0.05%bactericide （base lfuid）;0.8%borax （cross-linking lfuid），withcross-linking ratio of100:8.Evaluation results of ammonium persulfatc gcl-brcakcr through the aforementionedfractuirng formula under low temperature are as follows: when temperature is20°C,ammonium persulfatc still has no b1reaking efefct in nOOmg-L^-1; when temperature is30°C-40°C, the concentration of persulfate should be catalyzed to over450m-g-L1; and whentemperature reaches50°C, breaking requirements can only be achieved as the concentration ofammonium persulfate is over800mg·L^-1，Persulfate requires long gel-breaking time, about6.5-9h. Atfer gel-breaking by persulfate, residue content of fractuirng fluid is580-730mg·L^-1’with core damage rate exceeding36%.It can be reflected by the results of selecting and evaluating vairous biological enzymesthat ifve kinds of biological enzyme possess gel-brcaking cffcct. Through performanceevaluation, the breaking effect of such ifve kinds can be sequenced as:DBT-4>JN-2>RST-3>KBT-l>LFC-5. Among them, evaluation result of the best effect （DBT-4） is as follows: gel-breaking concentration is less than10mg·L^-1and residue content afterbreaking is less than258mg·L^-1, with core damage rate of less than23.26%and breaking timeof less than3h. Moreover, selective compound is performed on the four biological enzymes,and three of them own better formula effect through performance evaluation:40℃,DBT-4:KBT-1=1:1=5mg·L^-1;40℃,JN-2:KBT^-1=1:1=5mg·L^-1;and30℃,DBT-4:JN-2=1:1=10mg·L^-1. By comparing evaluation result with that before compound, both residue content andcore damage rate are lowered while shear resistance is improved.Comprehensive analysis and comparison of gel-breaking effects between biologicalenzyme and ammonium persulfate show that: gel-breaking concentration of biological enzymeis under1/50of ammonium persulfate, residue content is1/2of ammonium persulfate, coredamage rate is30%lower than ammonium persulfate and breaking time is1/2of ammoniumpersulfate, all of which indicate that biological enzyme is feasible for gel-breaking offracturing fluid under low temperature, featuring low cost, good effect, zero pollution andother advantages.