Pore network modelling of CO₂-shale interaction for carbon storage: Swelling effect and fracture permeability

Underground CO₂ storage is a key strategy to achieving net-zero targets by 2050, which requires gigatonne-scale containment of gaseous CO₂ in geological formations. Shale rocks play a role in both trapping CO₂ and preventing its escape, thus ensuring containment security. However, shale integrity can be compromised upon interaction with CO₂, which should be carefully evaluated. This study explores the dynamic behaviour of CO₂-shale interaction at the pore scale, focusing on the physiochemical interactions between CO₂ and shale, including the impact of shale swelling, where CO₂ adsorption causes matrix deformation and alters fracture sizes. Here, we utilise image-based analyses to develop a triple-porosity pore network model (PNM), reflecting the complex nano- to micro-scale structure of shale, to examine CO₂ injection into methane-saturated environments. The study particularly focuses on the impact of matrix deformation caused by gas sorption (swelling), competing with mechanical stress effects. Findings indicate that CO₂ injection leads to a reduction in fracture permeability by up to 17 % and 10 % in low- and high-density fractured shales, respectively, under high confining pressure (50 MPa), and by 15.5 % and 8 % under lower confining pressure (25 MPa). Although fracture permeability versus CO₂ injection pressure reduces monotonically at the lower confining pressure, that of the higher confining pressure is non-monotonic, where the fracture permeability shows an increase due to effective stress change. Additionally, the average fracture aperture size decreases by 50 nm in low-density and 25 nm in high-density fractured shales, highlighting the critical balance between swelling effects and mechanical stresses in the geological sequestration of CO₂.