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 {bullet operator} 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 {bullet operator} 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.