Rock Permeability Effect on Residual CO2 Trapping Efficiency in Carbonate Formations

Abstract The degree of success of CO2 storage depends on many factors that control the safety and efficiency of such projects. Formation permeability is considered one of the main factors affecting this promising technique. Specifically, it controls the degree of CO2 trapped by residual trapping, one of the main trapping mechanisms. Therefore, the need to provide deeper insights on how the latter can affect CO2 storage is highly desired. The scope of this study is to explore experimentally the effect of rock properties, particularly the permeability on CO2 residual trapping efficiency in carbonate/brine/CO2 geo-systems. Carbonate Limestone samples are selected with different permeability ranges i.e., low (4-5 mD), mid (20-30 mD) and high (>60 mD). Full characterization of the core samples was done first including SEM, NMR, XRD analysis and generation of capillary pressure curves. Later, a series of CO2 core flooding experiments are carried out at supercritical conditions (P= 10 MPa and T= 323 K). Unsteady state method was adopted for this process including: 1) Drainage process to determine the initial saturation of CO2 (Sgi), and 2) Imbibition process to determine residual CO2 saturation (Sgr) at a constant flow rate of 0.5 mL/min for both. Finally, residual trapping efficiency of CO2 is assessed by the material balance approach. The results indicate that CO2 stored by residual trapping is highly dependent on the rock properties and the associated pore network. Notably, carbonate rocks of low, moderate and high permeability significantly influence the residual trapping capacity. As a first observation, it was found that no proper correlation exists between Sgi and Sgr when permeability increases. However, the trapping efficiency decreases with the increase of permeability which implies that low permeability rocks with narrow pore size distribution are better candidates for CO2 residual trapping. After detecting a great discrepancy in literature regarding the effect of permeability in carbonate/CO2/brine system, and due to the scarcity in experiments and data sets generated in this research area, further experiments are needed to contribute to the establishment of well-defined trends of residual CO2 saturation behavior. This study presents a thorough examination on the effect of rock properties on residual trapping efficiency in limestone reservoirs.