Mechanical effects of sorption processes in coal and implications for coalbed methane recovery

Abstract

In this thesis, I present the results of a theoretical and experimental study of the response of dry coal matrix material to pure CH4, pure H2O and CH4-CO2 mixtures under in-situ conditions, with the aim of providing fundamental data that can contribute to assessing future strategies for enhanced coalbed methane (ECBM) recovery. The main conclusions are as follows: 1.Four thermodynamic models were developed to describe the (equilibrium) swelling behaviour of coal matrix material due to adsorption of a binary gas mixture (such as CO2-CH4 mixtures). They covered three possible end-members interactions between gas and sorption sites, plus a generalized case. Model predictions were compared with literature data on the swelling behaviour of Bowen Basin coal (Australia) exposed to CH4-CO2 mixtures, and showed that adsorption by and swelling of this type of coal is determined by the partial pressures of the gas species and by their selective adsorption (preferred affinity for sorption) at the adsorption sites. 2.In-situ CBM content is determined not only by the geological factors generally considered, such as coal rank, coal composition, moisture content and temperature, but also by lithostatic or confining stress, which is usually ignored. The effects of in-situ stress on methane sorption by coal is well captured by a specifically developed thermodynamic model. The model predicted a maximum CH4 concentration of ~0.76 mol/kgcoal for dry high volatile bituminous coal at a burial depth of ~900m, which is ~3% lower than conventional predictions (i.e. without considering the effect of in-situ stress). 3.Theoretical models were developed for time-dependent swelling of coal matrix material upon adsorption of a single gas, taking into account the coupled effects of stress, strain, chemical potential and diffusion. Comparison with experimental data illustrated that time-dependent swelling of medium volatile bituminous coal was controlled by the diffusion of unadsorbed molecules through diffusion paths linking distant adsorption sites; not by the jump frequency of adsorbed molecules between closely spaced adsorption sites or by pore size distribution effects cited in the literature. 4.Swelling strains measured for high volatile bituminous coal (Brzeszcze, Poland) due to adsorption of water vapour, as attained at (apparent) equilibrium, tend to be a factor of up to 1.45 times higher perpendicular to bedding than in the bedding plane. In addition, coal sample size strongly influences swelling kinetics, but not the equilibrium swelling strains, suggesting that the swelling is controlled by diffusion. The volumetric swelling strains attained at equilibrium show a near-linear dependence on relative humidity, reaching 1.37-1.43% at around 95% relative humidity. 5.Swelling strains developed at (apparent) equilibrium in the above coal, due to water adsorption, were reduced by applied stresses. This effect is caused by the combined effects of a) permanent time-dependent (compressive) deformation (creep), b) the thermodynamic effect of a stress-driven reduction in water sorption capacity and c) stress-driven closure of transport paths within the coal matrix. Nonetheless, the effects of stress on the swelling response of (Brzeszcze) high volatile bituminous coal to water are small and can be neglected at typical in-situ stresses of 10-30 MPa.