The underground storage of CO₂ in a depleted carbonate formation is a suitable method for limiting its anthropogenic release and minimize global warming. The rock wettability is an essential factor controlling the mechanisms of CO₂ trapping and its containment safety in the geo-storage formation. The geo-storage rock contains innate organic acids which alters the wettability of the rock surface from the hydrophilic condition to the hydrophobic state, thus reduce the CO₂ storage capacity. In this study, methyl orange which is a toxic dye that is generally released into environment was used as wettability modifier to change the wettability of stearic acid aged calcite (oil wet) to water wet. This study uses the contact angle technique (sessile drop method) to examine the effects of various concentration of methyl orange (10-100 mg/L) on the wettability of the CO₂/brine/stearic-acid aged calcite system under geo-storage conditions (i.e., temperatures of 25 and 50 ℃ and pressures of 5-20 MPa). The results indicate that the advancing and receding contact angles $({\theta }_a$ and ${\theta }_r)$ of the organic-acid contaminated rock surface were drastically reduced upon exposure to methyl orange, attaining the minimum values of 62° and 58° respectively, in the presence of 100 mg/L methyl orange at 20 MPa and 50 ℃. Thus, the present results suggest that rather than discharging methyl orange into the environment, it could be injected into underground reservoirs in order to reduce the level of environmental pollution and at the same time increase the CO₂ storage capacity of carbonate formations.

Fossil fuels are the primary global energy source, and their improved production will ensure a balance between the increasing energy demand and supply. Chemical-enhanced oil recovery has been well thought of as a promising method for increasing hydrocarbon production. However, the effectiveness of this method depends on wettability of rock-oil-brine systems’ Previous studies have shown that oil-wet rock demonstrated a water-wet state when treated with surface active chemicals like surfactants, nanofluids. Moreover, increasing attention has become focused on the application of hazardous pollutants such as methyl orange and methylene blue to enhance the CO₂/H₂ containment security of the host rock by altering its wettability. Nevertheless, the capacity of methylene blue to modify the rock wettability for the production of trapped hydrocarbons in sandstone reservoirs is yet to be explored. Thus, in the present study, methylene blue is used as a wettability modifier to enhance the oil production from quartz rocks that have been aged with stearic acid solution (10-2 mol/L). First, the organic-aged quartz is treated with various concentrations of methylene blue (10-100 mg/L) for one week at 60 ℃. Then, contact angle measurements are performed at different temperatures (25 and 50 ℃) under various pressures (10-20 MPa) and brine salinities (0-0.3 M). Thus, the quartz is found to turn hydrophobic when aged in organic acid/n-decane solution at 20 MPa and 50 ℃. However, when the rock is treated with various concentrations of methylene blue, the hydrophobicity is found to decrease, thus suggesting that oil recovery will be promoted by methylene blue treatment. Overall, the results demonstrate that the most favourable condition for reducing the hydrophobicity of the sandstone rock is via treatment with 100 mg/L methylene blue. Hence, the injection of methylene blue into deep underground sandstone reservoirs has the potential to produce more residual hydrocarbons.