Study On Mechanism Of Microbial Removal Of Fracturing Fluid Damage

Hydraulic Fracturing is an important technology for the effective development of low permeability and ultra-low permeability reservoirs.However,in the process of fracturing,Incomplete flow back will result in oil formation damage,which will reduce the efficiency of fracturing.Some bacterial strains can grow with guar gum as carbon source therefore,it would be feasible to removal the fracturing fuild damage of oil formation after guar–based hydraulic fracturing.Firstly,the biodegradability of guar gum was tested by biological experiments.Based on the physical and chemical properties of guar gum,guar gum degrading strains were screened by dilution plate method,viscosity change and so on.Ultimately,one of dominant bacterial strains was isolated and then some researches were carried out using this bacterial stain(CGS)Throughthe change of viscosity and molecular weight of guar gum solution during microbial degradation,the mechanism of microbial degradation offracturing fluid including guar gum residue,filter cake and insoluble residue,was studied.The effect of microbes degradation on filter cake was studied by methods of qualitative macroscopic observation and quantitative chemical absorption.The particle size of insoluble residue in the process of degradation was studied and analyzed by laser granularity method.The quality of residue in the degradation process was studied and analyzed by gravimetric method.Core flow experiments were carried out to simulate then damage and removal of formation after hydraulic fracturing.Bacterial strains was used to test the biodegradation of fracturing fluid residue,filter cake and insoluble residue in the cores.The results showed that the screened strain CGS was suitable for the formation environment and was able to degrade guar gum.Strain CGS was facultative aerobic with strong motility.The acid-base tolerance range was pH 6-10 with the highest salinity tolerance(6.76×10~4 mg/L)and the suitable growth temperature(40?-55?).The strain CGS can significantly reduce the viscosity of guar gum solution.After the biodegradation of 60h,the viscosity will be reduced to 2.8mPa·s,which means the guar gum was broken absolutely.Meanwhile,the average molecular weight of guar gum was also decreased.The molecular weight decreased by 85%at 60h degradation.Strain CGS also has good ability to degrade insoluble residues through the measurement of the median particle size and total mass.At 48h,the median particle size decreases to 66%of the initial median size,and the degradation rate of residue is 36.73%.In addition with the degradation of the strain CGS,the average plugging rate of the uncompletely broken zone was 29.74%.The core removal rate was up to 93.47%after36h was used by the strain CGS of the residue damaged core.The research conclusions and knowledge are of great significance to the development of microbiological gel breaking technology and fracturing fluid damage removal technology in fractured oil formation.