Liquid CO2 injection for geological storage in deep saline aquifers

Abstract

CO2 will remain in supercritical (SC) state (i.e. p>7.382MPa and T>31.04°C) under the pressure (p) and temperature (T) conditions appropriate for geological storage. Thus, it is usually assumed that CO2 will reach the aquifer in SC conditions. However, inflowing CO2 does not need to be in thermal equilibrium with the aquifer. In fact, surface operations are simpler for liquid than for SC CO2, because CO2 is transported in liquid state. Yet, problems might arise because of thermal stresses induced by cold CO2 injection and because of phase changes in the injection tubing or in the formation. Here, we propose liquid CO2 injection and analyze its evolution and the thermo-hydro-mechanical response of the formation and the caprock. We find that injecting CO2 in liquid state is energetically more efficient than in SC state because liquid CO2 is denser than SC CO2, leading to a lower overpressure not only at the wellhead, but also in the reservoir because a smaller fluid volume is displaced. Cold CO2 injection cools down the formation around the injection well. Further away, CO2 equilibrates thermally with the medium in an abrupt front. The liquid CO2 region close to the injection well advances far behind the SC CO2 interface. While the SC CO2 region is dominated by gravity override, the liquid CO2 region displays a steeper front because viscous forces dominate (liquid CO2 is not only denser, but also more viscous than SC CO2). The temperature decrease close to the injection well induces a stress reduction due to thermal contraction of the media. This can lead to shear slip of pre-existing fractures in the aquifer for large temperature contrasts in stiff rocks, which could enhance injectivity. In contrast, the mechanical stability of the caprock is improved in stress regimes where the maximum principal stress is the vertical.