Effect of pore and confining pressure on the supercritical CO2 permeability of sandstone: Implications for the effective pressure law

The liquid permeability of rock with distilled water or brine is different from that obtained using gas by variation in the confining pressure P-c and pore pressure P-p. In this study, as part of the research on CO2 geological storage, the permeability of sandstone was measured using supercritical CO2, and the effect of P-c and P-p on this permeability was analyzed. For applying the effective pressure law to the analysis, an effective pressure coefficient for permeability was derived experimentally. In order to utilize supercritical CO2, a non-Darcy flow test with a high flow rate was conducted, and the permeability was estimated through the Forchheimer equation. We contoured iso-permeability lines with confining and pore pressure conditions that have identical permeability, and the effective pressure coefficient,chi, was derived from the gradient of the lines following the definition of the effective pressure law. It was identified that the coefficient could be different depending on the pressure conditions. To clarify the variation of the coefficient, we derived the coefficient of chi(P-c, P-p) as a function of pore and confining pressure. The coefficient increased nonlinearly as the difference between P-c and P-p decreased, with a maximum of 1.36 being observed. The correlation between the effective pressure and the permeability were examined by applying empirical models. It was determined that the power law model was appropriate to estimate the change in supercritical CO2 permeability. Especially, it was deduced that the effective pressure with the derived coefficient would be more valid than the Terzaghi effective pressure.