Impact of Caprock Type on Geochemical Reactivity and Mineral Trapping Efficiency of CO2

Abstract CO2 capture and storage (CCS) is an important method to mitigate greenhouse gas (GHG) emissions by injecting CO2 into deep geological formations such as depleted hydrocarbon, unminable coal beds, and deep saline aquifers. However, due to the density variations between the supercritical CO2 and formation water, CO2 migrates upwards to the atmosphere. The risk of this CO2 migration can be prevented by different trapping mechanisms (e.g. structural trapping, capillary trapping, dissolution trapping, and mineral trapping). The trapping efficiency of these trapping mechanisms is highly influenced by various factors including CO2 injection scenarios, injection well configuration, reservoir wettability, reservoir heterogeneity, reservoir temperature, and formation water salinity. One of these factors, which has received little attention, is the caprock type. Although caprock wettability has been investigated previously as a factor affecting residual and structural trapping capacities, the effect of caprock type on mineral trapping efficiency has not been addressed yet. Thus, in this paper, we studied the impact of caprock type on geochemical reactivity and mineral trapping capacity by simulating a permeable sand reservoir overlying by three different semi-permeable caprock layers with different mineralogy (i.e. sandstone, siltstone, and shale). The Chemical Composition of these different caprock samples was measured using quantitative X-ray diffraction (XRD) instrument. XRD results indicated that siltstone and sandstone samples consisted mainly of quartz (~50wt %), while shale sample consisted mainly of illite (33)% and quartz (31%), in addition to few other smaller fractions of Illite, Chlorite, Albite, K-feldspar, Hematite, Ankerite, and Calcite. Our simulation results show that caprock type has a significant effect on geochemical reactivity and the associated mineral trapping mechanism. The results clearly indicate that the geochemical reactivity of siltstone caprock is relatively high, compared to shale and sandstone caprock cases. Furthermore, the results show that siltstone caprock scenario has the highest mineral trapping capacity, followed by shale and sandstone caprock scenarios, respectively. Moreover, the results indicate that sandstone caprock has the most increase in reservoir porosity and permeability. Thus, we conclude that the caprock mineral composition plays important roles in the geochemical reactivity and the associated mineral trapping of CO2.