Permeability evolution in tectonic coal: The roles of moisture and pressurized water‐injection

Abstract

AbstractWater often influences the exploitation of Coalbed methane (CBM) as well as the hydraulically enhanced permeability; it is therefore useful to objectively quantify the permeation properties of a CBM reservoir with different moisture contents. Based on the occurrence of CBM and the laws governing gas flow in CBM reservoirs, gas seepage experiments were carried out on the coal collected from the Baijiao mine in Sichuan, China. Various moisture contents were evaluated using natural coal samples, dry coal samples heated in an oven for various periods, and coal samples wet by soaking them in different pressure water under a constant effective confining stress. Results showed that: (1) The coal moisture decreased gradually with the drying time and increased with the soaking time. When coal is soaked in high pressure water (5–20 MPa), the moisture content in coal will increase from 2.60 to 3.85% with the increasing of water pressure. (2) The permeability of soaked coal decreased with the increasing of gas pressure, while the permeability of natural coal firstly decreased and then increased with the pore pressure decrease. (3) For the coal with higher moisture contents, gas permeability is more sensitive to the reduction of pore pressure, which may be due to the alteration of fracture structure induced by high‐pressure water injection. (4) Moisture content of coal affects the slippage effect in coal pores, and a higher moisture in coal will result in the decrease effective pore throat, incurring a stronger slippage flow and a higher apparent permeability of coal. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.