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

Abstract

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 characterize 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.