Forecast of Reactive CO2 Injection into a Carbonate Formation, Middle East

Abstract

This paper describes the results of reservoir and reactive transport modelling using MoReS-PHREEQC to study the long-term fate of CO2 injection in a carbonate aquifer. The heterogeneous model is based on a geological model built in Petrel, contains faults, various rock types, and various porosity-permeability relationships. The mineralogy of the target formation is dominated by calcite, but also contains dolomite. The results show that CO2 predominantly migrates along the high permeability zones at the top and the bottom of the target formation where CO2 is injected into. Impermeable layers (aquitards) on top of the target formation effectively block upwards migration. However, the plume reaches a conductive fault at the end of the injection (after 30 years), causing upward migration of CO2. Migration of CO2 into the fault appeared to be sensitive to the injection well location and different realizations of the geological model. Accurate mapping of faults and choice of injection well locations are therefore important for controlling CO2 conformance with time. Dissolution and dissociation of CO2 lowers the pH, leading to buffering by calcite and dolomite dissolution, as well as precipitation of anhydrite and celestine. The mineral amounts that dissolved or precipitated are relatively small and lead to porosity increase up to 0.03% within the impacted zone after 1,000 years. The geochemical impact of CO2 injection into the carbonate aquifer is therefore relatively low.