Coreflood Measurements of CO₂ Trapping

We present a coreflood experiment that measures three properties that control the long-term fate of CO₂ injected into the subsurface during carbon capture and storage (CCS): the residual saturation of CO₂ after brine flooding as a function of initial saturation; the amount of CO₂ that can dissolve in the brine; and the primary drainage capillary pressure. We employ a porous plate method to establish initial saturation, a stirred reactor ensures that the injected brine is pre-equilibrated with CO2 and we use isothermal de-pressurization to determine CO₂ saturation. The fluid pressure was 9 MPa and the temperature varied between 33 and 70°C - the CO₂ is in a super-critical (sc) phase at a temperature and pressure typical of likely storage aquifers with a density between 211 and 705 kgm⁻³. We find that significant quantities of the CO₂ can be trapped, with residual saturations up to 35%; the variation of trapped saturation with initial saturation is accurately matched using the Spiteri et al. model (a quadratic function). We compare the results with experiments performed at similar conditions using decane as the non-wetting phase. More decane is trapped than CO₂, suggesting that the CO₂-brine systems are not completely water-wet.

We show that temperature (density) variation has no effect on the saturation of scCO₂ that is residually trapped. The measured dissolution constant lies between 0.84 and 0.97 moles CO₂/kg brine. The primary drainage capillary pressure is consistent with a strongly water-wet system and the same - to within experimental error - as that measured on an analogue decane-brine system.