Measurement of adsorptive-mechanical properties of fractured coal cores

The growth of natural gas recovery from unmineable coal seams has motivated the investigation of the gas adsorption and swelling properties of coal, since those properties play a critical role on the evolution of seam permeability. Indeed, while advective fluid flow occurs through coal fractures at the seam scale, adsorption in nano-sized coal matrix pores induces swelling. Hence, fracture permeability depends on both confining stresses and adsorption. We developed an experimental device to measure the adsorptive-mechanical properties of coal cores, such as the specimen stiffness, the macroscale Biot coefficient, and the tangent Biot coefficient which quantifies adsorption. The device consists in a triaxial cell controlled by electromechanical pumps to manage stresses and CO₂ injection and flow, and permits specimen characterization at in-situ relevant stresses, fluid pressure, and temperature. The device fits specimens 38 mm diameter and slenderness 2:1. We show the results of a preliminary test on naturally fractured coal cores from Forzando mine in South Africa. Together, the experimental characterization and theoretical framework let us understand better the geomechanics of coal seams in views of Coal Bed Methane recovery.