Experimental Study on Fluid Transport Processes in the Cleat and Matrix Systems of Coal

Abstract

The fluid transport phenomena were studied on a cleat-plug (with cleats) and a matrix-plug (cleat-free) of a Chinese coal under controlled confining stresses (10−40 MPa). The single-phase fluid flow tests with argon (Ar) and water were performed under the steady-state. The gas breakthrough tests (two-phase fluid flow tests) with helium (He), argon (Ar), methane (CH4), and carbon dioxide (CO2) were conducted on the water-wetted plugs under the nonsteady state by monitoring the gas pressure changes with time in two closed compartments separated by the plug. It was found that the permeability of both the matrix- and cleat-plugs for Ar and water are measurable under the test confining stresses and the steady-state. The cleats in the cleat-plug are not easily closed by the high confining stress, evidenced by the fact that the permeability of the cleat-plug under the 40 MPa confining stress is still higher than that of the matrix-plug under the 20 MPa confining stress. The permeability coefficient of a confined coal with respect to gas varies mainly with the mean gas pressure and the effective stress. A mathematical equation combining these two factors is proposed to model the observed permeability data with respect to Ar for the cleat-plug under the confining stresses of 10−40 MPa. The associated coefficients of determination (R2) with the regressions are in the range of 0.88−0.98. It is encouraging that the prediction made by this model may be extended to the other conditions. During the gas breakthrough tests, a residual pressure difference between the up- and downstream pressures is observed for the water-wetted cleat-plug, which is indicative of the capillary forces in the gas/water/coal system. However, the up- and downstream pressure profiles for the water-wetted matrix-plug vary continuously over time and no capillary breakthrough is observed. It represents a mixed gas transport process including diffusion and imbibition. The different flow patterns for gas passing through the water-wetted cleat- and matrix-plugs are due to the differences in their capillary threshold pressures, which in turn depend on the throats radii of the largest interconnected flow paths of the plugs and the types of the fluid.