Experimental study and modelling of coal stress induced by gas adsorption

In practice, adsorption stress significantly influences the effective stress of a coalbed and thus the permeability during coalbed methane (CBM) recovery. However, in current coalbed permeability models, this stress is simply calculated from the adsorption strain and is rarely determined by experimental methods. In this paper, a tri-axial rock test system was used to measure this stress. Based on the experimental results, a Langmuir-like equation is able to accurately describe the changes in adsorption stress with gas pressure, and the adsorption stress with respect to adsorption amount is well described using a linear relationship. In addition, an effective horizontal stress model under uniaxial strain conditions is established. These models are directly established based on the experimental results and do not contain any uncertain parameters, and they are able to truly reflect the coal stress induced by gas adsorption and gas pressure. Finally, the evolution of coal adsorption stress with gas pressures and adsorption amount were analyzed, and the coal seam anisotropy was discussed. The models proposed in this paper shed light on the development of coalbed permeability models.