Effects of Supercritical CO₂ Injection on the Shale Pore Structures and Mass Transport Rates

Characterizing the pore structures and transport properties of low-permeability shales is critical for evaluating these formations as potential seals or storage sites for geological CO2 sequestration. Here, we use low-pressure gas adsorption in conjunction with nuclear magnetic resonance (NMR) to character-ize the pore-size distribution of shales before and after injection of supercritical CO2. Nitrogen gas was used as the detecting phase for the adsorption experiments and pentane liquid was used for the NMR experiments. We also performed time-resolved NMR and gravimetric microbalance measurements to observe mass transport during desaturation. We use these data to estimate the self-diffusion coefficient of pentane and changes in the saturation state of the pore network. We analyzed samples with a range of compositions from the Wolfcamp shale before and after exposure to supercritical CO2 for 3 days. Integrating the gas adsorption and NMR data shows how supercritical CO2 injection alters the pore-size distribution for pore sizes <1 nm to 1 mm. Our results provide insights on how the pore structure and mass transport properties of different shale lithologies may evolve during storage of supercritical CO2.