Capillary trapping has been identified as a key storage process that leads to the immobilisation of CO2 as a non-wetting droplets surrounded by brine in the water-wet porous rocks of saline aquifers limiting the extent of CO2 plume migration and enhancing the storage security. On the other hand, CO2 injection into commercial oil fields have several advantages and account for the majority of the current portfolio of CO2 storage sites and likely to remain the dominate storage options for the initial phase storage projects. Oil fields are well characterized and have the infrastructure that can be repurposed for CO2 injection for enhanced oil recovery (EOR) and carbon storage. In contrast, oil reservoirs, most of which in carbonate rocks, are characterised by a mixed-wet state in which the capillary trapping of nonpolar fluids have been observed to be significantly reduced relative to trapping in water-wet rocks typical of saline aquifers unaltered by the presence of hydrocarbons. There are, however, no observations characterising the extent of capillary trapping that will take place with CO2 in mixed-wet rocks. We use X-ray computed tomography (CT) at the core scale and microtomography (μCT), at high voxel resolutions upto 2 ^m, to investigate the pore-scale arrangement of supercritical CO2 droplets and measure the contact angles in situ comparing water-wet and mixed-wet carbonates at temperatures and pressures representative of subsurface oil reservoirs and saline aquifers. The measurements were made while maintaining chemical equilibrium between the fluids (CO2 and brine) and rock phases to prevent reaction with the core sample and replicate conditions far away from the injection site. Initial-residual (IR) CO2 characteristic curves were measured first on the sample at original water-wet state, then were measured again after altering the wetting properties to a mixed-wet system. In particular, CO2 trapping was characterized before and after wetting alteration so that the impact of the wetting state of the rock is directly observed. The measurements were also compared with trapping of N2. Here we show that residual CO2 trapping of supercritical CO2 in a limestone altered to a mixed-wet state with oil is significantly less than trapping in water-wet systems characteristic of saline aquifers. We anticipate this work to highlight a controversial issue for the early deployment of carbon storage - that those sites which are economically most appealing as initial project opportunities are the very locations in which the contribution of capillary trapping to storage security will be minimised. This should serve as a starting point for modelling studies to incorporate the reduced impact of capillary trapping on CO2 injection projects using hydrocarbon reservoirs.

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