Investigating Supercritical CO2-Brine Relative Permeability and Hysteresis for Clastic Reservoirs Through Pore-Scale Imaging and Field-Scale Modelling

Abstract Intermittent injection of CO2 in geological storage formations may result in repeated drainage and imbibition cycles in the subsurface. In addition, Ostwald ripening allows the redistribution of CO2 mediated by transport through the aqueous phase. Both processes are investigated using a combination of high-resolution three-dimensional X-ray imaging, analysis and modelling. Samples from sandstone fields with different rock characteristics were plugged into cores of diameter 6-12 mm and a few cm in length for the cyclic flooding experiments. Equilibrated brine and supercritical CO2 were co-injected at various fractional flows to mimic drainage and imbibition processes under reservoir conditions, i.e. up to temperatures of 50 °C and pressure of 10 MPa. X-ray images of the saturated samples were obtained through micro computed tomography during injection, while the pressure drop across the cores was measured. These X-ray images were analysed, interpreted and transformed into inputs for the subsequent pore network modelling. Both relative permeability and capillary pressure can be obtained simultaneously from the experiments, while the models can explore a wide range of different displacement processes. The measured relative permeabilities show significant trapping of CO2 consistent with the restricted connectivity of the pore space of the reservoir samples, combined with the presence of large pores. Both water and CO2 relative permeabilities are low over a wide saturation range. The experimental results suggest a simple model of relative permeability and capillary pressure hysteresis is applicable for CO2 storage, enabling multiphase flow properties to be determined for multiple cycles of imbibition and drainage from only a single displacement experiment. The measured relative permeabilities are input into field-scale modelling and the results infer that the cumulative CO2 storage volume from 20 years of continuous injection could be overestimated as much as 25% if the hysteresis and supercritical CO2 flow behaviours within the sandstone rocks are not taken into consideration. This is because the flow experiments imply a low CO2 mobility that impacts the well injectivity leading to constraints on the injection rate.