Pore Structure Alteration And Permeability Enhancement Mechanism Of Shale Under Cyclic Thermal Impact

Cyclic thermal impact technology can effectively remove water trap damage,improve reservoir fracture network,increase permeability,reduce development costs,enhance heat injection safety,and improve shale gas production capacity by rapid heating and repeated injection heat treatment.Therefore,in this study,the outburst black shale of the Lower Silurian Longmaxi Formation in Changning,Sichuan Province is taken as the research object,and the thermal impact test is carried out.The change of shale pore structure before and after different thermal impact effects were analyzed by using specific surface and porosity analyzer,nuclear magnetic resonance and scanning electron microscope.The permeability of shale before and after different thermal impact times was tested by the pulse porosity tester.The numerical simulation test of thermal impact process of shale was carried out.From the above tests,the following conclusions were drawn:(1)According to the nitrogen adsorption and desorption test results,adsorption pores and slippage flow pores are widely developed in the shale samples selected in this paper.The shale pore morphology is mainly composed of parallel plate pores,accompanied by some ink bottle pores.(2)Under the action of cyclic thermal impact,the number of pores in shale increases obviously,especially the number of slippage flow pores.The roughness of shale surface continues to deteriorate,some pores are interconnected,and the difference of pore size expands.The cumulative porosity of shale increases with the number of thermal impact,and the effect of multiple thermal impact on the pore structure of shale is exponentially improved compared with that of single thermal impact at a certain number of times.(3)Under the action of cyclic thermal impact,the relationship between shale permeability and the number of thermal impact increases as a logarithmic function.Compared with a single thermal impact,the improvement of shale permeability is increased exponentially by multiple thermal impact treatments under a certain number of times.There is a critical temperature in the range of 300?350? for the increase effect of thermal impact on shale permeability.(4)Through numerical simulation,the change process of internal and external thermal damage,acoustic emission and isotherm of the shale model under thermal impact is visually reproduced with thermal impact time.