Physicochemical and Interfacial Interactions among Carbonate/CO2/Brine systems: Implications for CO2 geo-storage

Abstract

Carbon capture and storage (CCS) constitutes a promising potential to contribute to the global net-zero targets. However, the efficiency of geological CO2 storage and the associated trapping mechanisms are a complex function of several physicochemical and interfacial factors, including but not limited to rock-fluid and fluid-fluid interactions. In this context, two critical parameters are the wettability of rock/CO2/brine systems and the CO2/brine interfacial tension (IFT) that directly impact the most dominant trapping mechanisms of CO2 in the underground: residual and structural trapping. Specifically, the capillary sealing efficiency of the seal is highly affected by prevailing wetting behavior and the IFT while wettability of the storage rocks controls the immobile CO2 saturation. In addition, the geochemical interactions in the rock/CO2/brine geosystem govern the permanent immobilization of CO2 in the porous medium. The associated geochemical interactions in carbonate aquifers are of special interest given their wider occurrence and relatively less attention received so far.
Thus, a precise characterization of these physicochemical and interfacial properties is essential to safeguard the subsurface storage in carbonate rocks. While the wetting behavior of the carbonate/CO2/brine systems can be altered by a range of factors, one key element that remains ambiguous is the effect of wettability alteration on the residual trapping mechanism in carbonate formations. Moreover, and importantly, in terms of the fluid-fluid interactions, despite the CO2/brine IFT being broadly reported, a true representative formation brine composition has not been experimentally tested, and a robust model to predict CO2/brine IFT as a function of influencing factors is lacking. Finally, the key factors influencing mineral trapping potential in carbonate rocks (e.g. the pH conditions, host rock mineralogy, formation temperature, impurities in CO2 stream, and brine composition and ionic strength) has not received notable attention – which is, nevertheless, vital for the long-term CO2 sequestration in carbonates.
Accordingly, following the aforementioned motivation, this study aims to: 1) provide a better understanding of the wettability alteration processes in carbonate/CO2/brine geosystems, 2) investigate the effect of diverse carbonate wettability and permeability on residual trapping efficiency via direct measurement of residual CO2 saturation, 3) the impact of mixed brine concentration on CO2/brine IFT to be elucidated via experimental sessile drop measurements, 4) perform predictive modeling of IFT in terms of several influencing factors using machine learning approach, and 5) evaluate the factors affecting the geochemical reactions in carbonate rocks and the associated mineral trapping capacity. This study, thus, utilizes a coupled experimental, simulation, and machine learning approach to address the scientific research objectives.
Our core-scale observations reveal that the wettability transition from water-wet to CO2-wet highly affects CO2 residual trapping, and it was found that the trapping efficiency of CO2 decreases when the contact angle increases (a ~60° increase in contact angle results in a 36% reduction in residual CO2 saturation). Artificial tailoring using stearic acid was found to be an effective chemical treatment to shift the wettability of limestone samples from water-wet to CO2-wet. Furthermore, the residual trapping efficiency (R = Sgr/Sgi) of limestone samples decreased with increasing rock permeability (28% reduction in ‘R’ for gas permeability increase from 5.24 mD to 680 mD) despite the highest residual CO2 saturation in most permeable samples. Additionally, using mixed salt brine tends to increase CO2/brine IFT by ~8 mN/m when compared to NaCl brine, mainly because of the presence of bivalent salts. Moreover, the storage capacity is 85% overestimated when using synthetic mono-salt brine compared to a realistic mixed brine case. IFT modeling predictions suggest that the Gradient Boosting (GB) method greatly improved the accuracy of estimating CO2/brine IFT under subsurface conditions (R2 = 0.964; 4% greater than the next best method). A robust and reliable correlation was also generated in this study to estimate CO2/brine IFT accurately as a function of all possible influencing factors. Finally, CO2 mineralization in carbonate formations and its associated geochemical reactions are found to be more pronounced with temperature (almost a double increase in precipitation potential for a shift from 50 °C to 120 °C), pH (highest precipitation recorded for pH=9), presence of impurities (1% H2S in CO2 stream leads to 6% reduction in precipitation potential), and bivalent salts (~40% increase in precipitation potential when MgCl2 is used instead of NaCl).
In summary, this dissertation addresses the key interactions among the carbonate/CO2/brine systems and their impact of CO2 storage potential using sophisticated experimental and computational approaches and thus has direct implications for CO2 geo-storage. The outcomes of this work contribute to the assessment of the storage potential of carbonates/CO2/brine geosystem by reducing the risks encountered in the realistic subsurface conditions, to maximize the storage capacity and enhance the containment security.

Date of Award	2025
Original language	American English
Supervisor	Muhammad Arif (Supervisor)