Experimental Investigation of the Effect of Interfacial Tension on CO2 Geo-Storage Efficiency in Carbonate Saline Aquifers

Abstract Carbon sequestration in saline aquifers has been demonstrated to be a successful and promising technique to combat climate change. The degree of success of carbon capture and storage (CCS) projects depends on several factors including operational variables, reservoir characteristics, and monitoring features. In particular, interfacial properties of the fluids present in the saline aquifer play an important role in this matter. This study focuses on the role of interfacial tension (IFT) of CO2 injected into the saline aquifer and the native brine in CO2 storage efficiency. In fact, two main trapping mechanisms (structural and residual) are notably affected by CO2/brine IFT. The latter directly influences the capillary sealing efficiency which is crucial for CO2 containment, also the storage capacity and CO2 volumetrics. Therefore, it is of great importance to obtain accurate measurements of IFT under realistic and representative conditions of the saline aquifer e.g., temperature, pressure, salinity and salt type. This study provides accurate IFT measurements using a brine solution replicating the formation brine found in the subsurface that contains a mixture of salts to compare it with the monovalent salt (NaCl) used in the majority of the previous studies. The measurements are conducted for three different solutions: fresh water, 21.4 wt% NaCl and 21.4 wt% formation brine at three different temperatures (298 K, 323 K and 343 K) for pressures ranging from 0.1 MPa to 20 MPa. This comparative analysis allows us to detect the deviation of IFT values between the two brine solutions (formation brine and NaCl). Indeed, a deviation of CO2/brine IFT was detected between the two brine solutions, and it can be noticed that at any condition, formation brine/CO2 IFT exceeds the IFT of NaCl/CO2. For instance, a deviation records around 5 mN/m at a pressure of 10 MPa and temperature of 323 K. This study also presents for the first time the direct effect of formation brine/IFT on the CO2 storage capacity. For this matter, two sister cores sharing the same characteristics e.g., porosity, permeability, wettability and mineralogy but saturated with two different brine solutions having the same salinity (NaCl and formation brine) are used to detect the effect of interfacial properties on the CO2 storage capacity. This experiment investigation was conducted by carrying out core flooding experiments at supercritical conditions of CO2 (P=10 MPa and T= 323 K). Notably, it was observed that the sample saturated with NaCl showed a greater potential to store higher CO2 volumes, 38% of the core is saturated with CO2, rather than the sample saturated with formation brine where the CO2 saturation reaches only 22%. It can be concluded that conducting experiments using NaCl brine solution saturated cores leads to an underestimation of CO2/brine IFT, which results in an overestimation of the CO2 geo-storage capacity. The findings of this paper reveal new understandings about interfacial properties and how they can affect the storage capacity of CO2 in the underground saline aquifers.