Competitive methane desorption by supercritical CO2 injection in coal

A large diameter (?70 mm) dry coal sample was used to study the competitive displacement of CH4 by injection of supercritical CO2, and CO2–CH4 counter-diffusion in coal matrix. During the test, a staged loading procedure, which allows the calibration of the key reservoir modelling parameters in a sequential and progressive manner, was employed. The core-flooding test was history matched using an Enhanced Coalbed Methane (ECBM) simulator, in which Fick’s Law for mixed gas diffusion and the extended Langmuir equations are implemented. The system pressure rise during the two loading stages and the CO2 breakthrough time in the final production stage were matched by using the pair of constant sorption times (9 and 3.2 days) for CH4 and CO2, respectively. The corresponding diffusion coefficients for CH4 and CO2 were estimated to be 1.6 × 10-12 and 4.6 × 10-12 m2/s, respectively. Comparison was made with published gas diffusion coefficients for dry ground samples (ranging from < 0.063 to ?3 mm) of the same coal at relatively low pressures (< 4 MPa). The CO2/CH4 gas diffusion coefficient ratio was well within the reported range (2–3), whereas the CH4 diffusion coefficient obtained from history matching of the core-flooding test is approximately 15 times smaller than that arrived by curve-fitting the measured sorption uptake rate using a unipore diffusion model. The calibrated model prediction of the effluent gas composition was in good agreement with the test data for CO2 mole fraction of up to 20%.