Influence of Rock Wettability on CO2 Migration and Storage Capacity in Deep Saline Aquifers

Abstract

CO2 geo-storage (CGS) is a key element of anthropogenic greenhouse gas management. Technically, in CGS, CO2 is captured from large point-source emitters and injected deep underground into geological formations. In this context, it is important to accurately predict the CO2 storage capacity of the target formation, as well as the associated migration of the CO2 plume versus time. This estimation heavily relies on reservoir scale simulations, which require mesoscale physical input parameters. One parameter, which critically affects these mesoscale input variables is the CO2-wettability of the rock; however, CO2-wettability has received little attention despite its importance; specifically, CO2-wettability influences residual fluid saturations, capillary pressures, CO2 cluster morphologies, CO2-brine interfacial areas, and relative permeabilities. The objective of this study is thus to investigate, using high performance simulations, the impact of wettability on CO2 storage capacity and on CO2 migration patterns for various types of reservoirs.We therefore performed 3D reservoir-scale CO2 injection simulations into a typical storage reservoir at 1200 m depth using the multiphase, multicomponent, non-isothermal numerical simulator TOUGH2 combined with the fluid properties module ECO2M for the system CO2-NaCl-H2O. The homogeneous model consisted of 87500 cells and had a porosity of 0.15, a horizontal permeability of 1000 mD, and a vertical to horizontal permeability ratio of 0.1. CO2 was injected for one year at a rate of 0.1 Mt/yrs and the CO2 plume behaviour was simulated for the next 10 years. Five different wettability scenarios were evaluated, namely strongly water-wet, weakly water- wet, intermediate-wet, weakly CO2-wet and strongly CO2-wet conditions.We demonstrate that wettability has a highly significant impact on the evolution of the CO2 plume, both in time and in space. Importantly, the results show that the CO2 plume reached the top of the model surface (-800 m) after only 3 years (after the end of injection) in the case of the strongly CO2-wet reservoir, but after 10 years in case of the weakly CO2-wet reservoir. The CO2 was better retained in the less CO2-wet rock: a depth of -869 m in the case of the intermediate-wet reservoir, -913 m in the case of the weakly water-wet reservoir and -990 m in the case of the strongly water-wet reservoir was reached after 10 years. At all times, CO2 migrated furthest in the strongly CO2-wet reservoir, and was most contained in the strongly water-wet reservoir. Furthermore, the shape of the CO2 plume is significantly affected by the rock wettability: the plume has a rain-drop like appearance in case of the strongly water-wet reservoir, but looks candle-like in the CO2-wet case.