Influence of salinity on gas-water capillary pressure behavior in carbonate reservoirs: Implications of underground gas storage

Wettability is a key factor influencing fluid distribution and storage efficiency in subsurface reservoirs. Understanding how water salinity affects the wettability of carbonate rocks is critical for predicting multiphase flow and optimizing gas storage capacity. The contact angle technique is a standard approach for assessing wettability, but it suffers from significant limitations. Its sensitivity to surface imperfections and contamination, along with its localized nature, often leads to results that do not accurately represent reservoir-scale behavior. Additionally, it overlooks the bulk characteristics of the porous medium. This study instead recommends capillary pressure curve analysis as a more robust and representative method for evaluating wettability under realistic reservoir conditions. This work investigates the impact of salinity—specifically deionized water (DI) and seawater (SW)—on the capillary pressure behavior of pure calcite Indiana limestone samples, assessed through capillary pressure curve measurements to evaluate wettability alteration. According to the literature, Zeta potential of DI with calcite is exhibit a highly negative charge (–22 mV), while Zeta is significantly reduced (–3.25 mV) in SW due to the suppression of surface deprotonation and competitive adsorption of multivalent cations. In addition, capillary pressure curve experiments using nitrogen gas as the non-wetting phase were conducted in three stages: four rock core samples were sequentially saturated with DI (1DI), dried and desaturated using seawater (2SW), and then dried again before being re-saturated with DI (3DI). Results indicate that the low-salinity aquifer (1DI) exhibits the highest initial gas storage capacity (∼90 %), which drops significantly to ∼62 % with increased salinity (2SW). Furthermore, reintroducing DI water after seawater exposure improves capacity by only 10 %, implying that seawater irreversibly alters the wettability of calcite surfaces in a way that reduces gas storage efficiency. These findings underscore the importance of salinity in governing calcite surface charge and wettability, with direct implications for multiphase flow behavior in carbonate reservoirs.