The combined effect of fractures and mineral content on coal hydromechanical response

Abstract

The hydromechanical behaviour of fractured coal is a complex function of interaction between coal bulk and fracture deformation driven by fluid pressure and external stress. Despite the research studies conducted to date, the combined effect of mineral content and fracture structure on hydromechanical behaviour of sorptive fractured coal remains unexplored. To study this combined effect, we performed a series of X-ray computed tomography (XRCT) imaging on a range of coal specimens with non-sorbing (helium) and sorbing (CO2) gases at different effective stress paths using a newly developed X-ray transparent triaxial system. The compressibility of system components was obtained from processed 3D XRCT images which were used to interpret the results. The results of this study show that coal matrix/solid compressibility has a positive nonlinear relation with mineral content irrespective of mineral type. Effective stress coefficient is also a strong function of both mineral content and fracture porosity. Furthermore, the increase in mineral content leads to less fracture opening by an increase in helium pressure. Interestingly, the effect of mineral content on the bulk strength of coal is more significant than the effect of fracture porosity. Finally, coal with more open fractures shows less bulk swelling by gas adsorption under external stress due to damping effect of fracture volume on developed internal volumetric swelling strain.