A fast and reliable methodology to evaluate maximum CO2 storage capacity of depleted coal seams: A case study

Evaluating CO2 storage capacity of formation before implementing CO2 injection is of great importance for CO2 geological storage. Coal seams are promising CO2 geological storage sites due to their high surface area and wide geographical spread. In this study, a fast and reliable methodology is proposed to assess the CO2 storage capacity of coal seams. The methodology provides a model considering gas flow within multiple radial hydraulic fractures (MRHF) connected to depleted well, gas transport in natural fractures, diffusion in coal seam matrix, and adsorption onto matrix surface. Using Stehfest numerical inversion technology and Gauss elimination method, the semi-analytical solution for bottom-hole pressure (BHP) is obtained based on the derived continuous line-source function in coal seams. Following that, a case study regarding the CO2 storage capacity of a coal seam in Wyoming is conducted, which shows that CO2 storage capacity in the coal seam reaches 4.8 × 108 m3. In addition, the sensitive analyses of the reservoir and engineering parameters on CO2 storage capacity are also investigated. The findings confirm that, on the one hand, higher constrained injection pressure (CIP) results in larger CO2 storage capacity. On the other hand, among these key parameters, hydraulic fracture length exhibits the most apparent effects on CO2 storage capacity whereas the impacts of skin factor and stress sensitivity are insignificant. To be more specific, CO2 storage capacity increases almost 7.5 times when the hydraulic fracture length increases 3 times when CIP = 3.5 MPa whereas CO2 storage capacity slightly increases by approximately 4.5% from permeability modulus = 0.0125 to 0.05 for the identical CIP. The methodology provided in this study lays the foundation of highly efficient CO2 geological sequestration in coal seams.