The Effect of Reservoir Conditions on Wetting and Multiphase Flow Properties in CO2-Brine-Rock System

Abstract

Abstract The wettability of CO2-brine-rock systems will have a major impact on the management of carbon sequestration in subsurface geological formations. Recent contact angle measurement studies have reported sensitivity in wetting behaviour of this system to pressure, temperature and brine salinity. We report results of an investigation into the impact of reservoir conditions on capillarity and multiphase flow through effects of wetting. The semi-dynamic capillary pressure core flooding technique was used with in situ saturation monitoring. The observations were made using a reservoir condition core-flooding laboratory that included high precision pumps, temperature control, the ability to recirculate fluids for weeks at a time and an x-ray CT scanner. The wetted parts of the flow-loop are made of anti-corrosive material that can handle co-circulation of CO2 and brine at reservoir conditions. Eight reservoir condition capillary pressure characteristic curves were measured using CO2 and brine in single fired Berea sandstone at pressures (5 to 20 MPa), temperatures (25 to 50°C) and ionic strengths (0 to 5 M kg-1 NaCl) representative of subsurface reservoirs. A ninth measurement using an N2-water system provided a benchmark for capillarity with a strongly water wet system. In all cases, the capillarity of the system, scaled by the interfacial tension, were equivalent to the N2-water system within measurement uncertainty. Thus reservoir conditions did not have a significant impact on the capillary strength of the CO2-brine system through a variation in wetting. In this work we report the results of the first study looking systematically at the impacts of reservoir conditions on the effective wettability in the CO2-brine-sandstone system. A new method is presented to quantify shifts in effective wetting properties with changing reservoir conditions. We find no impact within the range of reservoir and flow conditions relevant to CO2 storage, consistent with traditional multiphase flow theory but despite observations by others suggesting that wetting properties and multiphase flow in this system are sensitive to pressure, temperature and brine salinity. This provides definitive confirmation that the CO2- brine system performs as a strongly water-wet system in sandstone rocks and the use of analogue fluids for this characterisation may be useful when the full reservoir conditions cannot be replicated in the laboratory. The spatial saturations were also investigated using x-ray computed tomography and were found to be invariant with different reservoir conditions in homogeneous samples. The findings confirm the role of residual trapping in capacity estimates and provide a comprehensive dataset for flow modelling in water wet reservoirs.