The variation in produced gas composition from mixed gas coal seam reservoirs

Abstract

This paper investigates predictive modelling of gas production from coal seam reservoirs that contain mixtures of CO2 and CH4. A series of laboratory experiments are presented that are analogues of the gas production process where gas is drained at a controlled rate from a coal core sample with a known initial gas content and composition. The experiments were performed at two initial gas compositions and for two outflow rates. There was excellent agreement in the mass balance of the experiments with the difference between the initial gas volume and cumulative outflow ranging between 0.8 and 1.8% of the initial volume. During the early stages of gas desorption the CO2 concentration of the outflow gas was significantly lower than that in the adsorbed gas but increased significantly at low pore pressures. Thus, the CH4 was preferentially desorbed relative to CO2. For example, in an experiment in which the initial gas content of the coal was ~30% CO2, the CO2 concentration of the produced gas was only ~13% and did not increase significantly until the pore pressure fell below 0.8 MPa, ultimately increasing to ~95%. A reservoir simulator was used to match the desorption observations, and the results indicate, in a finding consistent with previous work, that the extended Langmuir adsorption model based on pure gas isotherms does not provide accurate predictions. A modified extended Langmuir approach was developed, based on using measurements of the initial gas content and its composition, provided a more accurate fit to the experimental observations of gas desorption. A fully predictive procedure using the 2D Equation of State adsorption model with the pure gas adsorption isotherms provided good agreement with gas desorption observations. A series of hypothetical reservoir simulation case studies are presented that illustrate changes of CO2 concentration with time for a variety of initial compositions.