Measurement and modeling of adsorptive–poromechanical properties of bituminous coal cores exposed to CO2: Adsorption, swelling strains, swelling stresses and impact on fracture permeability

International audienceThe reservoir response of unmineable coal seams to primary and enhanced natural gas recovery is strongly affected by gas sorption and the swelling properties of the coal reservoir rock. In-depth understanding of the process of gas sorption/desorption in the coal matrix, induced deformation and measurement of relevant physical parameters are critical for predictive reservoir management. Models used in industry practice are based on swelling strains measured in “free” swelling coal or on empirical correlations between strain and adsorption, and predict permeability changes based on changes of porosity or stress calculated assuming an analogy with thermoelasticity. However, not only coal seams are subjected to in-situ stresses and geometrical boundary conditions but also sorption and strain are strongly coupled. Representative experiments and a truly coupled model for coal seams are needed in challenging applications. We present a set of triaxial testing measurements on 38 mm diameter fractured sub-bituminous/bituminous coal cores exposed to CO2. Testing includes the measurement of fluid uptake, adsorption-induced strains and stresses, and the impact on simultaneously measured permeability. Noteworthy, we measured increases in effective stress of up to 29 MPa when injecting CO2 at 5 MPa and preventing the coal core to swell. The results are analyzed with a poromechanical model in which coal matrix microporosity and adsorption-induced phenomena are embedded into a fractured reservoir rock with transverse isotropic properties. The adsorptive–mechanical coupling in the coal matrix is integrated through an adsorption stress function and fractured coal permeability is estimated as a function of Terzaghi's effective stresses (parallel and perpendicular to the bedding plane). The experimental results and model predictions help identify the characteristic response of coal microporosity and cleat macroporosity on the poromechanical response of coal cores, and suggest that order of magnitude changes of reservoir permeability observed in the field are linked to sorption-induced change on Terzaghi's effective horizontal stress under laterally constrained displacement condition. Together, the modeling and experimental characterization offer unprecedented insights into the mechanics of coal