Experimental study of gas and water transport processes in the inter-cleat (matrix) system of coal: Anthracite from Qinshui Basin, China

The operation and numerical simulation of CO2-ECBM processes requires a thorough understanding of the fluid conductivity properties of the coal matrix. Therefore, single phase (water) and two-phase (water and gas) fluid flow tests on a cylindrical anthracite coal matrix plug of 28.5 mm in diameter and about 20 mm in length were conducted in a triaxial flow cell at a confining pressure and axial load of 20 MPa. The absolute permeability coefficient, determined by single phase steady-state water flow tests, was about 2.0 ∙ 10− 20 m2 (20 nDarcy). Two-phase flow tests ("gas breakthrough tests") were performed by imposing high differential gas pressures (up to 7 MPa) on the water-saturated sample and monitoring the pressure changes over time in closed upstream and downstream reservoirs. Argon (Ar), methane (CH4) and carbon dioxide (CO2) showed significant differences in their pressure transient curves during gas breakthrough tests, which indicated different controlling processes. The inert gas Ar exhibited capillary pressure-controlled breakthrough behavior. The maximum effective permeability, measured after gas breakthrough at maximum gas saturation, was 1.8 ∙ 10− 21 m2 (1.8 nDarcy); a residual pressure difference referred to as the “capillary threshold pressure” was 0.9 MPa for the Ar–water–coal system. The sorbing gas CH4 exhibited solely diffusion-controlled transport behavior, and no indications of capillary pressure effects. The reactive and sorbing gas CO2 showed initially capillary pressure-controlled and subsequently diffusion-controlled breakthrough behavior. The mass balance of the gas breakthrough tests indicated that the bulk of CO2 and CH4 were taken up by the coal sample. Sorption accounted for the strong uptake of CH4 by the coal sample, whereas both sorption and, to a lesser extent, CO2–water–mineral reactions were inferred for CO2.