Reevaluating Wettability in Depleted Sandstone Reservoirs: Implications for Subsurface Gas Storage

Depleted hydrocarbon reservoirs are well known for their impermeable seal integrity and secure trapping through structural and residual mechanisms. In such systems, wettability plays a critical role in governing fluid distribution, capillary pressure efficiency, and overall gas storage capacity in underground gas storage (UGS). Therefore, it is crucial to study wettability characteristics under realistic reservoir conditions. While mimicking reservoir conditions in the laboratory is a common practice, researchers have traditionally relied heavily on contact angle measurements despite their well‐documented limitations, including sensitivity to surface roughness, contamination, and neglect of pore‐scale dynamics. In contrast, capillary pressure curves provide more representative and realistic insights for reservoir simulation modeling. In this study, the porous plate method was employed to measure Pc–Sw relationships to assess wettability alteration in Berea Brown sandstone under depleted reservoir conditions, complemented by surface‐level adsorption analyses. Quartz substrates and powdered quartz were treated with low concentrations (0.028 M) of hexanoic, stearic, and lignoceric acids dissolved in toluene, with adsorption characterized using time‐of‐flight secondary ion mass spectrometry (TOF–SIMS) and Fourier transform infrared (FTIR) spectroscopy. Both techniques confirmed the adsorption of organic acids, with adsorption intensity increasing with alkyl chain length. However, the Pc–Sw curves of six sandstone core samples (BB1–BB6) revealed no significant shifts after treatment. In addition, residual water saturations remained within experimental uncertainty (49%–62%), indicating that the cores retained their initial water‐wet state. These results highlight a critical discrepancy: while contact angle measurements reported in the literature suggest wettability alteration even at minimal organic acid concentrations, capillary pressure analyses demonstrate that such concentrations are insufficient to induce measurable pore‐scale wettability changes in low‐permeability sandstone. This study emphasizes the importance of capillary pressure curve measurements over contact angle or surface adsorption analyses for realistic evaluation of wettability alteration, providing more reliable insights for assessing CO2 and H2 storage performance in depleted sandstone reservoirs.