NUMERICAL SIMULATION OF THE SIMULTANEOUS FLOW OF METHANE AND WATER THROUGH DUAL POROSITY COAL SEAMS DURING THE DEGASIFICATION PROCESS (UNCONVENTIONAL GAS RECOVERY)

Described in this study are the mathematical and numerical developments for a series of finite difference models which simulate the unsteady state behavior of coal seam degasification wells. Included in this series are models capable of predicting the performance of vertical drainage wells and horizontal drainage wells drilled from a shaft bottom. For the vertical wells the models allow for the simulation of post-fracture performance. The hydraulically induced fractures can be specified as either infinite or finite (constant) conductivity.; The natural fracture system of the coal seam, the face and butt cleat, is identified as a porous network and assigned effective permeability and porosity values representative of coal. A time-dependent diffusion/sorption model is included to describe the phenomenon of gas desorbing from the coal matrix and diffusing into the porous network. The coal seam is then modeled as a dual porosity reservoir using a pseudosteady state approach similar to the Warren and Root model. This treatment of gas sorption has been shown to give comparable results to the more sophisticated unsteady state approach with substantial savings in computation time.; Unique to the models developed in this study is the "dual mechanism" transport of gas. In this treatment gas is assumed to be traveling under the influence of two fields: a concentration field and a pressure field. Transport through the concentration field is a Knudsen flow process and is modeled with Fick's Law of diffusion. Transport through the pressure field is a laminar flow process and is modeled with Darcy's Law. The combination of these two flow mechanisms yields a pressure, saturation, and composition dependent slippage factor. This is in contrast to Klinkenberg's theory which predicts a constant slippage factor.; All models described in this study are single well, point distributed models in either rectangular, cylindrical, or elliptical coordinate geometry. The fully implicit, generalized Newton-Raphson procedure has been employed to solve the coupled, nonlinear equations generated by the models.; The models have been used to evaluate the many parameters associated with the methane drainage process, in order to determine those which most significantly affect production. Where possible these parameters, along with their effects, are put into dimensionless form, and type curves for predicting recovery, based on these dimensionless groups, are presented.