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

Abstract

The liquid permeability of rock with distilled water or brine is different from that obtained using gas by variation in the confining pressure Pc and pore pressure Pp. 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 Pc and Pp 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, χ, 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 χ(Pc, Pp) as a function of pore and confining pressure. The coefficient increased nonlinearly as the difference between Pc and Pp 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.

Estimation of the Non‐Darcy Coefficient Using Supercritical CO2 and Various Sandstones

“…Previous work in this laboratory has also conducted non-Darcy flooding tests using supercritical CO 2 (hereafter scCO 2 ), confirming the permeability dependence of both the fluid and the rock properties (Choi et al, 2017). Experimental studies of core flooding using CO 2 have been undertaken to aid efforts to develop geological storage of CO 2 , although these were not non-Darcy flow tests (Al-Zaidi et al, 2018;Krevor et al, 2012;Valle et al, 2018;Wang et al, 2014).…”

“…The methodology for deriving equation (2) and its experimental application have been reported previously (Grigg et al, 2004;Takhanov, 2011;Zeng & Grigg, 2006). Previous work in this laboratory has also demonstrated the application of the modified Forchheimer equation to scCO 2 permeability testing (Choi et al, 2017).…”

“…The above Forchheimer equation (1) can be integrated and modified to derive the following equation for compressible fluids such as scCO 2 (Choi et al, 2017;Tek et al, 1962;Tiss & Evans, 1989):…”

“…Journal of Geophysical Research: Solid Earth configuration could obtain data almost without noise because it allows mathematical compensation of the fluid temperature and the nonlinearity of the transmitters. More details of each experimental system are given by Choi et al (2017). This experimental system allowed non-Darcy flow tests for scCO 2 to be conducted under a range of flow rates and a steady flow condition to be maintained in the specimen.…”

Experimental Bedford limestone permeability dependence on confining stress and pore pressure. A comparative study with previous works

“…However, a significant difference with the hydrostatic mode is that A 2 is negative, what means that permeability reduces by increasing pore pressure. This behavior has been reported by Choi et al (2017) but discarded by augmenting insufficient data points or experimental errors. In this work however, there are multiple data series that seem to support the unexpected trend, even though some experimental noise is present (see Figure 9).…”