A reservoir simulation model is usually required to represent the combined effects of gas transport, water flow, and coal swelling/shrinking on the extraction of coalbed methane (CBM). Although a number of models have been developed in recent years, most of them are based on coal permeability models under the assumptions of uniaxial strain and constant overburden load. Under these assumptions, coal permeability is defined as a function of pore pressure only. This allows the gas transport and water flow to be decoupled from coal deformation. In this study, these two assumptions are relaxed: a general porosity and effective permeability model is developed to represent the behavior of both the primary medium (coal matrix) and the secondary medium (fractures) under conditions of variable stress. The effective permeability model is defined as a function of effective stress, and can be applied to the full spectrum of mechanical conditions spanning prescribed in-situ stresses through constrained displacement. Moreover the effect of matrix moisture is often ignored in previous simulation studies, but some experimental results showed that the presence of moisture in the coal matrix had profound influences on the gas storage and transport in coal seams by reducing the gas adsorption capacity of the coal, decreasing the gas effective diffusivity through the coal matrix, and varying the coal swelling strain. In order to investigate the magnitude and interplay the moisture effects on CBM production, approximate relationships have been built to quantify the moisture effect on the gas adsorption capacity, the gas effective diffusivity and the coal swelling strain. These relationships have been calibrated by comparison with a selection of literature data. Both the permeability model and the moisture effect correlations are implemented into a fully coupled model of coal deformation, gas transport in the matrix system, and gas-water flow in the fracture system.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 2012|