Abstract: To study the pore structure evolution of shale gas reservoir under cyclic impact load, the shale of Wufeng Formation in Changning County, Sichuan Province, was taken as the study case, and the kinetic response characteristics of shale under cyclic impact load at low, medium and high velocity were worked out based on the Hopkinson bar experimental system and micron CT system, and the shale pore structure before and after impact was analyzed. The study shows that The dynamic impact stress to strain curves show that the dynamic elastic modulus modulus of rock samples under low-velocity cyclic impact increased with the increase of cyclic impact count, and the load-bearing capacity increased gradually; the rock samples were compressed first and then damaged under medium velocity and high velocity cyclic impact, with the increase of cyclic impact load, the surface area, volume and fractal dimension of shale connected fractures were increased and the absolute permeability increased, which indicates that increasing the cyclic impact load can form a complex pore network in the rock samples and enhance the permeability; medium velocity and high velocity cyclic impact could expand and penetrate the pores inside the rock samples, and the porosity of rock samples under medium velocity cyclic impact was doubled compared with the original rock samples, and the spatial dispersion of pore distribution was enhanced. This study can theoretically support the study related to multistage pulse combustion-explosion fracturing of shale gas reservoirs.

Key words: shale, fracturing, pore structure, cyclic impact load, micron CT, multistage pulse combustion-explosion fracturing