A field-scale investigation of residual and dissolution trapping of CO2 in a saline formation in Western Australia

• The paper presents designs that optimise residual and dissolution trapping of CO2.
• Horizontal wells enhance residual trapping the most.
• Water alternating gas (WAG) enhances dissolution trapping the most.
• WAG has the most potential for both residual and dissolution trapping.
• However, vertical injection wells are the most economically viable option.

In a typical carbon capture and storage project, residual and dissolution trapping is expected to play a significant role in minimising the risk associated with the containment. Various methods of maximising each trapping mechanism individually have been proposed in the literature. Yet, in actual projects, it is expected that these trapping mechanisms occur interactively. This area still remains unexplored.This paper presents the results of a numerical investigation of technically feasible engineering designs to co-optimise both trapping mechanisms. It integrates a geological model developed for an actual saline formation in Australia, along with other available reservoir engineering data using a compositional numerical reservoir simulator. The scenario involves injecting supercritical CO2 at a commercial rate into a thick, deep, layered, heterogeneous, high porosity, high-permeability sandstone formation that initially contains only water of high salinity. The design objective is to inject more than 200 million tonnes of CO2 over 40 years. Compositional interactions between formation water and injected CO2 are modelled using the Peng–Robinson equation of state. We examine different injection schemes to find out the scenario that maximises dissolution and residual trapping. These schemes include vertical wells, horizontal wells, water alternate gas injection (WAG) and simultaneous water and gas injection (SWAG).The results show that heterogeneity has a significant effect on the location of perforations. The middle region of the Wonnerup Member appears to be the optimum location for CO2 injection where desired amount of CO2 can be injected while a maximum benefit of residual trapping can be achieved. Horizontal wells appear to enhance residual trapping the most, whereas WAG enhances dissolution trapping the most. Furthermore, WAG scenarios result in the least mobile CO2, and therefore, show the highest cumulative potential for both residual and dissolution trapping. However, in terms of relative economics, vertical injection wells appear to be the most viable option.