Experimental Evaluation Method For Aqueous Phase Trapping Damage In Shale Gas Reservoir

Horizontal well and staged fracturing are the main technical means for the development of shale gas reservoirs.However,the capillary force of the nanopore and microfractures often lead to difficult flowback of the fracturing fluid,which caused serious potential damage of mass transfer process.Objectively,comprehensively and accurately evaluating the damage of aqueous phase trapping damage in shale gas reservoirs are the basis for effective prevention and control.Shale gas reservoirs have various forms of gas storage and multiscale mass transfer characteristics.It is necessary to form an experimental evaluation method for aqueous phase trapping damage in shale gas reservoirs.Taking the typical shale of the Longmaxi Formation in Southeast of Chongqing as research object,the paper clarified the main controlling factors of aqueous phase trapping damage.The clay mineral content of shale was 28%,the wetting angle of aqueous phase was between 20° to 40°,the porosity was distributed in 1%-9%,and the permeability was distributed in 0.0001-2.0mD.Gas transfusion mainly occurs in micro-pores and microfractures.Nanopores provided the main site for the generation of adsorbed gas.Sensitive minerals,wettability,pore size distribution,gas generation patterns,initial water saturation,scale and time of fracturing were the main controlling factors for the aqueous phase trapping damage of shale.An experimental evaluation method for the aqueous phase trapping damage based on shale gas production characteristics was explored,and the experimental procedure for damage evaluation was formulated.The main process of the method was:basic experiments(measurement of basic parameters,analytical water salinity of shale,analytical shale sensitivity,measured permeability in back pressure),the evaluation experiments of spontaneous imbibition and retention of aqueous phase which caused the damage of gas transfusion,adsoption,desorption,and diffusion capacity.According to the basic experimental results,5.5%KCl solution was selected as simulated formation water,1.0MPa was selected as the flowback pressure and the back pressure.Based on spontaneous imbibition and flowback experiments,the transfusion damage of shale was evaluated.After 48 hours of spontaneous imbibition,the water saturation of shale sample was greater than 60%,and the permeability damage rate was greater than 47%.The permeability damage rate of shale after spontaneous imbibition was 100%.The flowback effect of shale was positively correlated with the permeability.After flowback for 24 hours,the flowback rate of shale sample was ranged from 1.97%-28.81%.The 10MPa flowback pressure can only overcome the capillary force of aqueous phase caused by macropores.The aqueous phase in most nanopores cannot be returned.Based on the volumetric method and the pressure decay method,the shale desorption and diffusion damage caused by aqueous phase retention have been evaluated.Adsorption and desorption experiments showed that the adsorption and desorption curve of shale conformed to the Langmuir model,the maximum adsorption and desorption capacity of the sample were 1.66-2.60cm3/g and 0.97-1.56 cm3/g.After the damage,the adsorption and desorption capacity of the shale sample were weakened,and the damage degree ranged from moderately weak to moderately strong.Diffusion experiments showed that the methane diffusion coefficient was between 4.15×10-12 to 5.04×10-12cm2/s in stage ?,and between 4.60×10-17to 6.61×10-17cm2/s in stage ?.After the damage,the diffusion coefficient decresed,and the damage degree ranged from moderately weak to moderately strong.The multiscale evaluation of aqueous phase trapping damage for shale gas reservoirs was realized.Based on the multiscale mass transfer rate,the damage of aqueous phase trapping was moderately strong.Based on the Analytic Hierarchy Process(AHP),a evaluation model for aqueous phase trapping damage of shale was constructed.The evaluation index was determined,and the relevant weights were calculated from the discriminant matrix to comprehensively to evaluate the aqueous phase trapping damage of shale.The results showed that,the damage degree of the aqueous phase trapping damage of matrix and fracture were extremely strong and strong.Interface modification,heat treatment,and oxidation treatment have positive effects on the prevention and control of aqueous phase trapping damage in shale.