Direct injection of carbon dioxide (CO2) into deep geological reservoirs creates acidic brines near well bores that can promote dissolution of carbonate minerals, altering pore space geometry and geomechanical integrity. This is considered a major obstacle for successful geological carbon sequestration (GCS). In this study, the effects of an anionic surfactant (i.e., internal olefin sulfonate (IOS C15-18)) on calcite dissolution in simulated reservoir brine was investigated in a high pressure and temperature batch reactor, and complemented with density functional theory (DFT) calculations. The reservoir brine was either saturated with carbonic acid (H2CO3*), adjusted to the same pH with HCl, or set at circumneutral pH. High-pressure high-temperature batch dissolution experiments show that H2CO3* or H2O driven dissolution is strongly inhibited by the adsorption of IOS, while H+ driven dissolution is not. Etch pit morphology obtained by laser profilometry shows IOS preferentially inhibits dissolution of obtuse edges in etch pits for H+ and H2O driven dissolution, compared to acute edges. They also show that IOS more uniformly inhibits etch pit growth at all edge and kink sites for H2CO3* driven dissolution. DFT results indicate that the binding energies of IOS to calcite terrace, obtuse edge, and three calcium kink (i.e., acute-acute, obtuse-obtuse, acute-obtuse) sites are much more favorable than binding energies of H2CO3*, H+, or H2O at these same locations, but less so at acute edge sites. This explains, at least in part, why IOS preferentially inhibits the growth of calcite etch pits near obtuse edge sites for H+ or H2O driven dissolution. In addition, according to DFT studies, IOS and H2CO3* preferentially bind to surface Ca, but H2O binds to both surface Ca and CO3 and H+ preferentially binds to surface CO3. This can explain why IOS has little effect on H+ driven dissolution, and appears to completely inhibit H2CO3* driven dissolution. This study sheds light on calcite dissolution mechanisms, and suggests the role that additives can play in inhibiting carbonate mineral dissolution during GCS to mitigate pore space alteration and geomechanical integrity loss.

Anionic surfactant inhibition mechanism of calcite dissolution at reservoir conditions: A combined experimental and DFT study

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