On the estimation of CO2 capillary entry pressure: Implications on geological CO2 storage

The CO2 entry pressure for a specific pore space decreases with increasing storage site depth because the interfacial tension is reduced and the system becomes less water-wet with increasing depth. This is based on the assumption that the pore throat size and shape remain the same, i.e., they are not stress-dependent. The dependency of capillary entry pressure with depth in geological CO2 storage has been reported by only few quantitative investigations, which, however, did not account for the interfacial tension and wettability effects of the brine/CO2/solid system.

In this work, a workflow and methodology are proposed to quantify the dependency of capillary entry pressure with depth in subsurface geological CO2 storage. The cap-rock pore spaces are treated as straight capillary tubes whose cross-sections are obtained directly from 2D SEM rock images, and the CO2 capillary entry pressure invading these pore spaces is simulated, under arbitrary wetting conditions, with the use of an in-house novel semi-analytical model. In this model, the brine/CO2 interfacial tension is obtained as a function of the two phases (CO2 and brine) density difference, and the contact angle is evaluated based on the Frumkin–Derjaguin equation, and the disjoining pressures isotherm curves is computed from DLVO theory under various CO2 storage pressure, temperature and brine ionic strength conditions. The dependencies of brine/CO2 interfacial tension, contact angle and capillary entry pressure on CO2 storage depth and brine ionic strength are also investigated.

This newly developed model can be used to compute CO2 entry pressure and associated fluid configurations in realistic cap-rock pore spaces which can be extracted from 2D rock images from core samples located at various storage depths and under the examined storage site’s pressure, temperature and brine ionic strength conditions. The proposed workflow for capillary entry pressure estimation and its relationship with depth can enhance our understanding and improve the design and safe storage of CO2 in geological formations.