Impact of rock microstructures on the supercritical CO₂ enhanced gas recovery

Supercritical CO2 can be used as a driving gas to displace the native gas CH4 for methane production enhancement. A preferential flow path may result in an earlier CO2 breakthrough. This early breakthrough will in turn reduce the overall sweeping efficiency. In this study, numerical simulations for the supercritical CO2 enhanced gas recovery were conducted on a virtual rock core which contains an inclined fracture for potential preferential flow paths. This paper contains four parts. First, a 3D virtual core is expressed by local porosity. X-ray CT scanning captures a series of 2D digital images of the rock sample. Each 2D CT image is then processed to ascribe a map of pixel values. This map is related with local porosity through threshold techniques. These 2D CT images are then used to build a 3D virtual rock core within which the porosity is defined at each voxel. Second, other physical properties such as diffusivity and permeability within the core are introduced through their empirical relationships with the porosity. Third, a convection and diffusion equation is formulated based on the CO2 injection-driven flow. In this equation, the heterogeneity of the rock core is introduced via the distribution of initial porosity. Further, its convection velocity is proportional to the concentration gradient of the injected CO2 and the diffusive coefficient is contributed by both molecular and hydrodynamic diffusions. Therefore, a porosity-based finite element CO2-CH4 displacement model is completely formulated. Finally, the effects of fracture-induced heterogeneity on mass transfer and local diffusion mechanisms are numerically studied. The dramatic impact of the preferential pathway on the earlier CO2 breakthrough as well as the overall gas recovery is observed in the numerical simulations.