Role of Organic Acids and Organosilanes on Wettability and Implications for Underground Gas Storage

Depleted carbonate reservoirs are increasingly utilized for underground gas storage due to their proven ability to retain hydrocarbons over geological time scales. In this context, understanding the wettability characteristics of these reservoirs is crucial, as wettability influences fluid distribution, capillary forces, and gas trapping efficiency. Researchers have attempted to replicate oil-wet rocks in the lab using various organic materials and concluded that even traces of these organics can alter the rock’s wettability to oil-wet. However, they relied on contact angle measurements to confirm wettability and adsorption, a method that carries significant uncertainties. Furthermore, the capillary pressure curve, rather than contact angle, provides essential data for reservoir simulation modeling in the oil and gas industry. Therefore, this study investigates the effects of wettability alteration in carbonate rocks treated with various organic acids and organosilanes by employing capillary pressure curves to assess their impact. Nine chemicals─three organic acids (hexanoic acid, stearic acid, lignoceric acid) and six organosilanes (dichlorodimethylsilane, chloro(dimethyl)thexylsilane, trichlorododecylsilane, diethoxydimethylsilane, triethoxy(octyl)silane, and dodecyltriethoxysilane)─dissolved in toluene were applied in this investigation. The results showed that capillary pressure curves revealed changes ranging from significant to negligible in wettability using organic acids and organosilanes regardless of chain length. An exception was observed with lignoceric acid, which, contrary to expectations, exhibited a shift due to residual water. The adsorption of carbon compounds on calcite surfaces was corroborated by ATR-FTIR and TOF-SIMS analyses, confirming the presence of organic films. Nevertheless, the extent of this adsorption was insufficient to induce significant wettability alterations as inferred from capillary pressure measurements. This study emphasizes the need for complementary methods, such as capillary pressure analysis, to assess wettability changes more comprehensively in carbonate reservoirs, especially for underground gas storage applications. These insights aid in optimizing gas storage strategies by identifying treatments that alter wettability more effectively in carbonate reservoirs.