Reservoir Simulation Study for CO2 Sequestration in Saline Aquifers

Abstract
Sequestration of carbon dioxide in geological formations has been recognized as one of the most promising ways to cope with greenhouse gas emissions, and saline aquifers are one of the potential storage target. This study evaluates feasibility of CO2 storage in saline aquifers under different conditions. Simulation studies are carried out based on a homogeneous aquifer model. Compositional reservoir simulator (CMG-GEM) was used to simulate 10 years of CO2 injection phase and simulate the fate of CO2 post injection from hundreds to thousands of years. Different subsets of sensitive parameters are independently investigated to understand the impact of various parameter son different trapping mechanisms. Simulation results are also compared with other published research studies to provide additional insight of factors affecting CO2 sequestration process.

Keywords: CO2 injection, solubility trapping, residual trapping, mineral trapping

Summary and Conclusion
Understanding the influence of involved parameters in the process CO2 storage independently with their contribution on the given trapping mechanisms is helpful to reduce uncertainties associated with the storage and injectivity capacity. Most importantly, this in turn will minimize the risks associated with the leakage through cap rock which possibly can be caused by loss of the integrity of the cap rock due to pressurization of the formation.

A hypothetical deep saline aquifer is modelled for CO2 injection using compositional reservoir simulation software GEM-CMG. Fate of injected CO2 was investigated under different trapping mechanisms for different conditions. Over range of parameters,the study investigated two main tasks. The first task showed the feasibility of CO2 solubility trapping mechanism on the given aquifers under different reservoir conditions. In geological formations, CO2 solubility during and after injection process potentially play the most important role in deciding the fate of CO2 for stabilizing and securing the storage mechanisms. The feasibility of solubility trapping mechanism is found to be very case dependent (Table 9). The simulation results showed that aquifers dip and permeability seemed to have a significant effect on CO2 propagation and dissolution profiles in the formation. In addition, CO2 storage feasibility under solubility and residual trapping mechanisms were investigated.

The simulation results showed that accounting for hysteresis trapping is essential for predicting CO2 migration and distribution in the formation when the geological CO2 sequestration processes are modelled. The second task showed the importance of aquifer mineralogy on the conversion of CO2 into carbonate minerals in saline aquifer.

The processes of dissolution and precipitation are occurred close to the injection well where the gas phase was dominated. In this study, the rate of mineral dissolution and precipitation processes were sufficient enough to change porosity. Mineral precipitation depends highly on the amount and type of the source minerals. Meanwhile, it could lead to enhance geo-chemical reactions with both positive and negative consequences. Changes in the rock properties occurred around the areas surround the well. A small porosity change was resulted in permeability changes by following similar trend of changes. Overall, mineral trapping enhancement is controlled by the solubility trapping. Increase in residual trapping reduces solubility and mineral trapping mechanisms. As it is  expected, without considering residual trapping, more CO2 in the formation will be available to dissolve and lead to increase in HCO-3 and results in more mineral precipitation. However, more CO2 is accumulated under the seal.That is, consideration of combined trapping mechanism is essential for CO2 sequestration modelling since more accurately captures the behaviour of the CO2 distribution and potentially provides more reasonable insight and support for applications in geological settings especially where structural traps are absent or uncertain.