LAUSR

Abstract Pulse gas fracturing (PGF) is a method of injecting high-pressure gas into coal cyclically to stimulate the coalbed methane reservoir. However, the evolution of coal pore structure during pulse gas fracturing is still unclear, especially when the injection pressure is lower than the breakdown pressure of coal. In this paper, 11 groups of PGF experiments with the injection pressure amplitude no more than 8 MPa were carried out on coal. Furthermore, the pore size distribution and the crack morphology of coal samples before and after PGF were observed. The experimental results show that pulse gas fracturing with relatively lower injection pressure increases the total pore volume and the permeability exponentially. Moreover, with the increase of pulsation frequency or of injection pressure amplitude, the volume of macropores and its proportion in total pore volume rise significantly, and the increasing rate gradually grows. However, the mesopore volume and its proportion in the total pore volume go up with the increase of pulsation frequency, but remain basically unchanged with the rise of injection pressure amplitude. Besides, the volume of micropores and transition pores slightly fluctuates. In addition, PGF hardly generate wide main fractures in the coal body, and instead mainly causes the tip of pores to propagate and induces substantial micro-cracks. The accumulation of irreversible deformation such as plastic deformation of pores and propagation of micro-cracks is the fundamental cause why the total pore volume and the permeability of coal grow continuously. The research results lay a foundation for understanding the fracturing effect of PGF.