Changes in pore structure and permeability of low permeability coal under pulse gas fracturing

Abstract

In order to enhance effectively permeability of coal seams and increase the efficiency of gas extraction, the pulse gas fracturing is proposed as a new stimulation method. In this study, the pulse gas fracturing experiment of low permeability coal on the laboratory scale was executed and sequential 267 pulse times were preformed. The mercury intrusion porosimetry, scanning electron microscope and permeability measurements were conducted to investigate the changes in the pore structure and permeability caused by the pulse gas injection. The results show as follows. (1) The volume of the coal specimen repeats the swelling-shrinkage process during the pulse gas fracturing which contributes to improve the size and shape of pores and generate micro-cracks in the coal. The deformation of the coal sample has a fatigue threshold in the pulse gas fracturing and its value is near 100 pulse times. (2) The pulse gas fracturing promotes the transfer of smaller pores to larger pores and improves the pore size distribution. After the pulse gas fracturing, the average incremental pore volume of transition pores, mesopores and macropores increases 25.85%, 117.86% and 105.07%, respectively. The total pore volume of the coal sample has an increase of 80.65%. The macropores, mesopores and transition pores volumes increase by 165.22%, 438.33% and 27.27%, respectively. The results indicate that pulse gas fracturing can improve the pore space and the pore distribution, and ultimately increase the permeability of the coal. The change that micropores transfer into larger pores also results that the cumulative pore specific surface of transition pores, mesopores and macropores has an increase of 10.18%, contributing to coalbed methane (CBM) sorption/desorption and diffusion. (3) The crossover network cracks are formed in the coal during the pulse gas fracturing. The porosity and permeability of the coal are obviously improved by the pulse gas fracturing, indicating that the pulse gas fracturing can be used to effectively enhance the permeability of CBM reservoirs. It is worth noting that there is a critical pulse time for the increase of the coal permeability and its value is about 100 pulse times under the stress condition and pulse gas injection method of this research.