Stress and permeability modelling in depleted coal seams during CO2 storage

Abstract

Depleted coal seam gas reservoirs are considered as one of the best sites for underground CO2 sequestration which can contribute to climate change mitigation. Due to coal's high internal surface area, a large amount of CO2 can be stored in the coal matrix. Despite the high CO2 storage capacity, the swelling of coal near the injection wells remains a major challenge. Most field trials have shown a significant decline in injection rate and permeability in the vicinity of vertical injection wells, while horizontal wells showed no such decline. It is not well studied how matrix swelling and well trajectory affect permeability in horizontal carbon dioxide injection wells. In this study, a mathematical model is developed to determine stress distribution and permeability around vertical and horizontal CO2 injection wells in depleted coal seams. The model incorporates injection pressure, matrix swelling effect, in situ stresses, and wellbore trajectory. The results indicate that during CO2 injection, matrix swelling significantly impacts the stress path and increases both horizontal stresses. Stress distribution and permeability around the well are affected significantly by wellbore trajectory and swelling. As CO2 injection continues in vertical wells, radial, tangential, and mean effective stresses increase near the well and within the reservoir. As a result, permeability declines during injection in a vertical well. In horizontal wells, however, tangential and mean horizontal stress near the wellbore decrease with injection. Hence, the model confirms no decline in permeability near a horizontal injection well which is in good agreement with the field trial observations.