Evaluating the effects of CO2 Injection in Faulted Saline Aquifers

Abstract

Abstract If carbon capture and storage (CCS) is to be an effective option for decreasing greenhouse-gas emissions, commercial-scale storage operations will require large storage capacities for carbon dioxide (CO 2 ). Saline aquifers have been reported to have very large storage capacities. It is important for research to be conducted in this area, since the possibilities for storage in depleted oil reservoirs are reduced by the fact that they still contain large quantities of hydrocarbon, reducing the amount of pore spaces that could have potentially be available. Disposal of CO 2 into deep saline aquifers involves CO 2 being injected as a supercritical fluid that is less dense and less viscous than the formation water. Due to density differences, the CO 2 becomes buoyant in the water and has a high tendency to migrate from the storage location once the possible pathways exist. This may lead to the critical problem of seepage from the storage area. In Trinidad and Tobago (T&T) many potential reservoirs are highly faulted. Some faults form an integral part of the structural traps whilst others are leaky and provide migration pathways for the injected CO 2 to return to surface. A simulation study was conducted using the commercial compositional simulator CMG-GEM. The model described within this paper seeks to optimize the injection of CO 2 into saline aquifers in the presence of faults Sensitivities for fault transmissibility, fault throw, fault permeability and bottomhole pressure were modelled to see their effects on the amount of CO 2 stored with time. The model included the effects capillary entry pressure of the fault zone. We studied two fault types, i.e. normal faulting and reverse faulting. It was observed that fault throw and transmissibility of the fault had the most impact on the amount of CO 2 that can be injected into a reservoir and ultimately stored. Solubility trapping played an essential role in securing CO 2 in deep saline aquifers. For transmissibility values less than 0.01 all the CO 2 injected into the reservoir remains stored. However the quantities that were stored are relatively small. Most of the stored CO 2 remains in the supercritical phase whilst approximately 10% remains dissolved in the aqueous phase. CO 2 stored due to hysteresis was relatively small i.e.