Residual crude oil suppresses silicate dissolution: Implications for CO2 storage in depleted oil reservoirs

Depleted hydrocarbon reservoirs are increasingly being considered for geological CO 2 storage because of their well-characterized geology, favorable flow properties, and existing infrastructure. However, residual hydrocarbons may alter mineral–fluid reactions and thereby influence long-term storage performance. Here, we quantify the effect of prior hydrocarbon exposure on silicate-rock reactivity using a series of batch-reactor experiments conducted in acidic aqueous solutions at 50 °C. Powdered anorthosite and basalt were exposed for 7 days to decane, light crude oil, or medium crude oil and then drained before reaction; untreated powders served as controls. The release of Si, Ca, Al, and Mg from decane-exposed powders was indistinguishable, within analytical uncertainty, from that of untreated powders, consistent with electron microscopy observations indicating little persistent decane coatings. In contrast, element release from powders exposed to light and medium crude oils was approximately one order of magnitude slower. This pronounced rate reduction is attributed to residual crude oil adhering to mineral surfaces and limiting fluid–mineral contact, consistent with a surface-passivation effect. These results provide direct experimental evidence that residual crude oil can substantially suppress silicate dissolution without completely arresting fluid–rock reactions. The findings indicate that conventional reaction-rate models based on clean mineral surfaces may overestimate mineral reactivity and the rate of mineral trapping in depleted oil reservoirs considered for CO 2 storage.