Permeability enhancement of the Utica shale due to the adsorption of supercritical carbon dioxide

Abstract:

Supercritical carbon dioxide (CO₂) has potential advantages to enhance the hydraulic fracturing in unconventional shales, such as low viscosity and reducing flow blockage. The adsorption of supercritical CO₂ could also affect shale permeability, but the mechanisms involved are poorly understood. We performed pressure pulse-decay experiments on intact shale samples from the Utica formation to evaluate the permeability evolution. The experiments were conducted on horizontally oriented shale samples with a temperature of 41.1 °C (± 0.1 °C) and under a constant confining pressure. We measured permeability at high effective stress (13.9 MPa) using argon before and after permeating the samples with argon or supercritical CO₂ at low effective stress (2.1 MPa).

Our results show that after treatment with argon for three days, the shale permeability slightly changes. After treatment with supercritical CO₂ for three days, the permeability with argon significantly decreases. From the observation of shale microstructure before and after the treatment, we found that the permeability decrease is due to pore size reduction. For samples with high initial permeability (e.g., 800 nD), the change of supercritical CO₂ permeability is minor. However, for samples with low initial permeability (e.g., 4 nD), we observed a net increase of the permeability with supercritical CO₂ owing to enlarged nanopores connected for fluid flow. The increase of nanopore size may be due to the adsorption of supercritical CO₂. The net increase in the supercritical CO₂ permeability is found in samples with high total organic content (TOC). We concluded that the adsorption of supercritical CO₂ induces the permeability enhancement of horizontally oriented shale samples with low initial permeability and high TOC.