High-pressure X-ray imaging to interpret coal permeability

Abstract

Coal seam gas (CSG) is an unconventional resource drawing attention globally and is considered to play a crucial role in the future natural gas market. In CSG reservoirs, the characterisation of fracture aperture sizes is critical since coal permeability and wellbore performance are highly dependent on this parameter. To optimise gas recovery and predict gas production rate, it is important to analyse reservoir properties at in situ conditions. By using a high-pressure flow cell and X-ray microcomputed tomography (micro-CT) imaging, we visualise how the internal structure of coal changes under prevailing reservoir conditions. Important differences in the size of fractures can be seen from images of the coal sample obtained at different pressure. As anticipated, aperture size was found to decrease with increasing confining stress from atmospheric to reservoir pressures and it was more sensitive to the change of initial pressure. Similar conclusions were drawn for both gas and Klinkenberg-corrected permeability using helium and methane gases. In the permeability experiments, methane gas is used to mimick in situ conditions whereas helium is used for comparison as a non-adsorbing gas. There was less change in fracture aperture size if the ends of a coal cleat are sealed by minerals. We successfully applied micro-CT imaging on coal samples under high confining stress. The coal sample studied has a relatively large diameter compared to the common core sizes used for high-pressure imaging. Overall, this paper demonstrates a quantitative analysis of coal permeability at reservoir conditions, which provides a better understanding of deformation and further insights into enhanced CSG exploration and development techniques.