Monte Carlo simulation of the maximum migration distance of CO[formula omitted] in a sloping aquifer

Abstract

Carbon capture and storage in saline aquifers is broadly considered a viable technology for reducing anthropogenic impact on the environment. However, a secure CO2 storage can be compromised by the depositional environments of target reservoirs, which can influence the CO2 migration in subsurface. The disposal of CO2 in a sloping aquifer can result in a substantial gas migration in the updip direction from the injection well until all gas becomes trapped. It thus can reach far more than desired regions from the well where the potential leakage paths to the surface can exist. This study aims at understanding the principal parameters that the maximum migration distance in the updip direction depends on, which can be important for estimating the risks of CO2 leakage through old abandoned wells in the regions of intensive subsurface exploration. Using the Monte Carlo method, an extended parametric study is performed by running 3-D reservoir simulations of CO2 storage in random aquifers characterized by various petrophysical and thermobaric parameters, saturation functions and brine salinity, and for the wells operated under different target rates. Herewith, the modeling accounts for such relevant physical phenomena as the phase transitions, the gravity and capillary forces, the hysteresis behavior of the relative permeability, etc. It is shown that the maximum migration distance at the post-injection stage is characterized by just two similarity criteria and the critical gas saturation for the drainage and imbibition processes. It is found that the CO2 plume shape at the moment of the well shut-in influences the subsequent gas transport. A new simple relation parameterizing the revealed dependence is proposed and validated against the sampled maximum migration distances. The relation is useful for an instant estimating the size of the contaminated zone around a projected CO2 well and for ranking the potential storage sites.