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

Abstract

Water 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.

Study on the influence of water injection pressure on water absorption process and water injection effect of coal

“…According to the comparison of water pressure and water absorption distribution under different water injection times and pressure, the water pressure distribution and water absorption distribution form in the coal body around the water injection hole are different 29 . This is because water pressure has an impact on the maximum water absorption rate and water absorption rate of coal, and the water absorption rate shows a Langmuir change trend with time, resulting in a rapid rise in the water absorption rate in the low water pressure propagation zone, while the water absorption rate in the high‐pressure water propagation zone slows down.…”

Experiment, Model, and Mechanism of Multiscale Dynamic Diffusion–Permeability in Coal under Different Fluids

“…It is generally accepted that there are two kinds of water flow in coal seams, which are Darcy flow driven by hydraulic pressure in fractures and imbibition flow driven by capillary pressure in the coal matrix. , The permeability of water in the fractures of coal has been studied by the steady-state method. It is commonly believed that the higher the hydraulic pressure is, the higher the permeability. , However, the flow behavior of water in the coal matrix is poorly understood. As mentioned earlier, coal is a multiscale pore medium.…”

“…It is commonly believed that the higher the hydraulic pressure is, the higher the permeability. 45 , 46 However, the flow behavior of water in the coal matrix is poorly understood. As mentioned earlier, coal is a multiscale pore medium.…”