Influence of Matrix Shrinkage and Compressibility on Gas Production From Coalbed Methane Reservoirs'

Abstract

Abstract The effect of methane desorption on gas flow through coal was investigated. Adsorption/desorption isotherms were established for coal samples. Permeability of cylindrical samples to different gases was measured at simulated in-situ conditions in a triaxial stress field. Gas pressure-permeability-desorption relationship was established for helium, methane and carbon dioxide. Related experiments measured coal compressibility, and volumetric changes of coal matrix with methane adsorption and desorption. The hypothesis that desorption of methane shrinks coal matrix, and results in increased permeability was confirmed. The increase in matrix permeability with reduction in pressure from 1000 psi to atmospheric was highest for carbon dioxide (the most adsorptive gas), on the order of a factor of five for methane, and non-existent for helium. The corresponding volumetric shrinkage of coal matrix on desorption of methane was 0.6%. When measured values of compressibility and shrinkage were used as input parameters into a reservoir simulation model, 60% more gas was produced over a five year period. Introduction Simulation of coalbed methane reservoirs and predicting long-term gas production is a complex process. The number of variables involved is large, and information concerning these parameters and variation in their values over the life of a producing reservoir is not well quantified. Knowledge of conventional gas reservoirs is not very useful due to the fundamental differences between sandstone reservoirs and coalbed methane reservoirs. In conventional reservoirs, primary storage is in the form of compressed gas in pore spaces. In coalbed reservoirs, the major quantity of gas is stored in adsorbed form, and the gas available for flow is the desorbing gas. The second major difference is the presence of the natural fracture (cleat) system in presence of the natural fracture (cleat) system in coalbeds. Thus there are two main phenomena involved: desorption followed by diffusion of gas towards the cleats, and Darcian flow of gas and water in the cleats. Flow of methane in solid coal is subjected to two distinct processes: desorption of gas, and reduction in pressure. processes: desorption of gas, and reduction in pressure. Both these factors affect permeability of coal, and hence production, of gas. production, of gas. An experimental study - preliminary in nature - was carried out to investigate the effect of these two factors - desorption and pressure - flow characteristics of coal. A brief description of the background that led to this study, experimental work, a discussion of the results and their significance, are presented in the following sections. Background Decline in Pressure Once production from a coalbed methane reservoir starts, the reservoir pressure starts declining. This decrease in pressure - although it takes place at a very slow rate - increases the effective stress. This increase in effective stress results in lower permeability. Stress dependency of permeability has been studied by several researchers in the past. In general, compressibility declines with increase in stress although there is insufficient data to explicitly define the relationships. With large compressibility values such as those measured at Rock Creek site in Alabama, a pressure drawdown of only 300 psi - an increase in effective stress of 300 psi can reduce the permeability by half or more. psi can reduce the permeability by half or more. P. 171