Mechanisms and Influencing Factors of Nonlinear Gas Seepage during Coal Seam Gas Injection

Addressing the challenge of gas extraction from low-permeability coal seams in China, the low-permeability characteristics of coal seams can be effectively improved through gas-injection flooding-enhanced drainage technology, by which the permeability is significantly enhanced. In this study, the nonlinear seepage behavior of single-component gases (N2, CO2, and CH4) through coal cores with varying characteristic lengths (100, 200, and 300 mm) was experimentally investigated. By employing gas physical properties and the quasi-static method, parameters such as the Reynolds number and threshold pressure gradient were introduced to characterize the seepage dynamics. The influence of characteristic length on gas flow patterns was systematically examined, and the key factors affecting the seepage characteristics of single-component gases were identified. The results showed that the adsorption of gas by the coal matrix significantly affects coal core permeability and the gas permeability coefficient. Coal core permeability is characterized by kN2 > kCO2 > kCH4, and the gas permeability coefficient is characterized by κN2 ≈ κCO2 > κCH4. In the process of gas injection to displace coalbed methane (CH4), N2 should be preferentially injected if the goal is to improve CH4 drainage rate; CO2 should be selected if the goal is to improve CH4 drainage efficiency and realize geological storage. Influenced by the dynamic viscosity of the gas, the pore characteristics of the coal core, and adsorption, the threshold pressure gradient of the gas sequence followed λCO2 > λCH4 ≈ λN2 . There is a critical pressure (Pab=sfi) when the gas migrates in the coal core. When the injection pressure is lower than the critical pressure, the gas seepage is affected by the adsorption and slip effects, showing nonlinear characteristics. When the injection pressure exceeds the critical pressure, the seepage characteristic tends to stabilize, gradually approaching the linear flow; the coal core is at 300 K, and its critical pressure is Pab=sfCO2(6.8 MPa) > Pab=sfCH4(6.2 MPa) > Pab=sfN2(2.8 MPa). The results of this study can provide theoretical support for the CO2/N2-driven flooding of coalbed methane.