Effect of Methyl Orange on the Wettability of Organic-Acid-Aged Sandstone Formations: Implications for CO₂ Geo-storage

Carbon capture and sequestration are feasible techniques for achieving net-zero carbon emissions. In this respect, the storage capacity and fluid flow dynamics during carbon geo-storage are controlled by the wettability of the geo-storage rock. However, as a result of the presence of organic acid contamination in sandstone geo-storage formations, the rock may attain a CO₂-wet state, which would significantly reduce the CO₂ storage capacity. While nanofluids and surfactants have been recently utilized as wettability modifiers for enhanced CO₂ geo-storage capacity in sandstone formations, the impact of methyl orange (MO) on the wetting behavior of organic-acid-contaminated quartz has not been reported thus far. MO is a toxic dye that is normally discharged in considerable amounts into the environment. In the present study, MO is used for the wettability alteration of quartz substrates, which are representative of sandstone geo-storage formations. The quartz substrates are aged in n-decane/stearic acid solutions (10^-2 mol/L) for 1 week to mimic the real geo-storage conditions. Thereafter, quartz substrates are aged in various MO concentrations (10-100 mg/L) at 50 °C for 1 week. Subsequently, the advancing (θ_a) and receding (θ_r) contact angles of CO₂/brine on the stearic-acid-aged quartz substrates with and without MO are measured at various pressures (10, 15, and 20 MPa), brine (NaCl) salinities (0, 0.1, 0.2, and 0.3 M), and temperatures (25 and 50 °C), using the pendant drop tilted plate technique. The results show that the hydrophobic surface of stearic-acid-aged quartz becomes hydrophilic in the presence of MO. Moreover, θ_a decreases from 97° to 65°, while θ_r decreases from 90° to 56°, as the MO concentration is increased from 10 to 100 mg/L. Therefore, to enhance the CO₂ storage capacity and mitigate CO₂ emissions, it is recommended that MO be disposed of in deep underground sandstone geological formations instead of discharging it into the environment.