Capillary pressure characteristics of CO2-brine-sandstone systems

In carbon geo-sequestration (CGS), CO2 is collected from large industrial emitters and injected deep into the subsurface. In this context it is vital to know how much CO2 can be pressed into the storage rock matrix. The metric which measures this is the primary capillary pressure. Furthermore, once the CO2 resides in the pore space of the rock it can be trapped by capillary forces so that it cannot leak back to the surface. Such residual trapping also depends on the rock matrix and thermo-physical conditions. However, data is sparse for these key metrics, which presents a significant risk for CGS projects. We thus measured these parameters in two rock matrices as a function of various thermo-physical variables. The capillary pressure (P-c) measured ranged from 0.1 to 10 kPa for Bentheimer sandstone and from 10 to 100 kPa for Fontainebleau sandstone; this difference is due to different pore morphologies in the rocks. However, when scaled with contact angle, fluid and rock properties, the curves collapse onto a single master curve. Moreover, P-c during secondary drainage was higher than during primary drainage. 16-21% residual CO2 saturation (S-CO2,S-r) was achieved with supercritical CO2; while higher S-CO2,S-r (25-29%) was achieved with liquid CO2, which is attributed to a) a better sweep efficiency of liquid CO2 due to its higher viscosity, and b) to a change in wettability. We conclude that careful site selection is an important criterion for optimizing CGS projects as rock matrix properties and thermo-physical conditions play a key role.