CO2 and H2O Leakage Rates From the Injection Zone to Overlying Units as a result of Geologic Sequestration

Carbon sequestration in deep geologic formations is one method of mitigating anthropogenic CO2 emissions. Minimizing CO2 leakage rates is requisite for successful sequestration in order to curtail possible negative impacts to overlying aquifers and human populations. Much work has been done on CO2 movement within the injection zone. However, leakage rates through the caprock and subsequent movement through the overlying formations are still unconstrained. The ability to predict how CO2 will move in the subsurface and the potential leakage of CO2 and H2O from storage formations is integral to the development of quantitative risk assessment models for CO2 storage projects. The multiphase, multi-component flow and transport model PFLOTRAN was used to simulate leakage from a deep storage formation through a fault zone to gain insight into factors that control leakage rates and investigate their significance. In these simulations most of the free-phase CO2 plume dissolves or spreads horizontally at the interface of the caprock and injection formation. However, a small portion of the CO2 flows through the fault zone. Breakthrough of the CO2 and H2O into the overlying aquifer happens almost immediately after CO2 injection begins. Leakage rates through the fault were relatively constant during the 3-year injection period and terminated quickly after injection ceased. As expected, rates of CO2 leakage decrease with CO2 injection rate. In contrast, rates of water leakage through the fault increase with decreased CO2 injection rates. Total CO2 leakage over the 3-year injection period decreases in scenarios where the distance between the injection point and fault zone are greater. However, the same inverse relationship between CO2 and water leakage observed for varying injection rates was also observed for variable injection point - fault distance. This inverse relationship is attributed to the complex relationships between relative permeability, saturation, pressure, and capillarity. These leakage rates were then used in a simple 1-D reactive transport model investigating metal mobility in the fault zone as the CO2 reacts with rocks in the fault zone.