Suitability of the Aquistore CCS Site for a CO2 Circulation Test: Towards CO2-Plume Geothermal (CPG) Power Plant Implementations

Abstract

It is commonly accepted that a drastic decrease in global carbon dioxide (CO2) emissions is necessary in order to reach the climate goals set by the Paris agreement in 2015. A key technology towards achieving that goal is CCUS - Carbon, Capture, Utilisation, and Sequestration. By using supercritical CO2 instead of brine/water as a geothermal working fluid, geothermal energy production can possibly be expanded to regions with lower temperature gradients in subsurface formations, while permanently storing the CO2 underground. This first-order, conceptual study investigates the suitability of the Aquistore CCS site for a CO2 circulation pilot test. For doing so, numerical simulations were performed to learn about the site responses to CO2-circulation, the amount of back-produced CO2 versus brine, and to estimate the flow behaviour in a potential CO2 gas production well. A key requirement for a successful CO2 circulation pilot test is to prevent liquid loading in the CO2 gas production well. Liquid loading occurs when a liquid (here brine or water) accumulates in the production well. It can be avoided by maintaining an annular flow regime in the multi-phase fluid production stream in the production well. The resulting flow regime is mainly controlled by the total fluid mass flow rate in the production well. The flow regime in the production well also depends strongly on the overall transmissivity of the reservoir. Our simulation results suggest that steady-state conditions will likely occur within weeks to a few months after the start of CO2 circulation. Moreover, our results show that the amount of back-produced CO2 is one order of magnitude larger than the amount of back-produced brine. In the majority of cases, we observe that the back-produced fluid production stream will ultimately flow in an annular flow pattern. Further analyses of CO2 circulation results indicate a need for better characterisations of the subsurface multiphase fluid flow behaviour. To this end, we discuss attempts to constrain the uncertainty associated with the Aquistore reservoir characterisation and CO2 plume growth through high-resolution history matching of non-isothermal injection data and time-lapse seismic monitoring surveys.