Effect of water on methane diffusion in coal under temperature and pressure: A LF-NMR experimental study on successive depressurization desorption

Abstract

Coalbed methane production is a process of desorption, diffusion, and seepage with the successive decrease of reservoir pressure. However, desorption at atmospheric pressure has been mainly used to investigate the gas diffusion of water-bearing coal. The influence of confining pressure and gas pressure on methane diffusion is not considered in this case. So, the question is, how does the variation of the methane diffusion coefficient of water-bearing coal during desorption? How is it different from dry coal? What is the difference between the methane diffusion behavior of anthracite and bituminous coal? To address these problems, we select low-volatile anthracite in the south of Qinshui Basin and high-volatile bituminous coal in the south of Junggar Basin. The methane diffusion of water-bearing and dry coal during the successive depressurization desorption is compared by low field nuclear magnetic resonance. The influence mechanism of water on methane diffusion is explored from the pore structure, wettability, and external stress. The results show that the methane diffusion coefficient of water-bearing anthracite and bituminous coal increases with the decrease of gas pressure. The methane diffusion of water-bearing coal is affected by the combination of the increase of methane desorption volume (positive effect) and the increase of effective stress (negative effect). When the gas pressure decreases from 6 MPa to 2 MPa, the negative effect is dominant, and the effective diffusion coefficient of methane in coal is poor; When the gas pressure is less than 2 MPa, the positive effect is dominant and the effective diffusion coefficient increases significantly. The homogeneity of methane effective diffusion path of water-bearing anthracite is enhanced, resulting in higher methane desorption rate and effective diffusion coefficient than dry coal. The methane effective diffusion pore size of water-bearing bituminous coal decreases, resulting in the effective diffusion coefficient of methane less than that of dry coal. These results are of great significance to reveal the methane diffusion behavior of coalbed methane in the actual production.