Experimental investigation on pore characteristics of heterogenous coal structures reservoir and coalbed methane diffusion/seepage behaviors

Abstract

Coalbed methane (CBM) reservoir mass transfer performances, including adsorption/desorption, diffusion and seepage, are related to the types of intact and cataclastic coal structures. By “cataclastic coal,” we mean that coal produces fractures in different directions caused by tectonic stress and is divided into fragments along the fractures surface, but there is no significant displacement between fragments. This paper sampled coal specimens from a coal mine to investigate the dynamic behaviors of reservoir by using experiments and Lattice Boltzmann Simulation Method (LBM). The results show that Langmuir VL, specific surface areas, and pore volumes occurring at cataclastic coal seam (570–572 m) are higher than in intact coals. The dominant pore size in samples 1, 2, 3, and 4 is distributed between 2 and 10 nm, leading to increased adsorption capacity in cataclastic coal. Cataclastic coals' fractal dimension D1 is higher than in intact coal, but D2 is lower than in intact coal, which means tectonic damage causes the pore surface to become coarser and the pore structures to become homogenous. Because of increased pore volume in cataclastic coal, its dynamic diffusion coefficient is better than in intact coal, meaning the upper coal seam has beneficial gas releasing efficiency. Based on LBM simulation and tri-axial permeability experiments, it is found that cataclastic coal with developed natural fractures is characterized by high velocity of fluids, but its damage ratio of permeability is stronger than in intact coal. Finally, it summarizes the reservoir characters of Gaohe coal field and proposes that it should pay attention on the lower intact coal seam for the stimulation of CBM reservoir.