Dolomitisation favoured by Lewis acidic background compounds in saline fluids

Abstract

Predicting the type and rate of reactions between minerals and fluids is of utmost importance in many applications. Due to the presence of background ions, natural environments are often much more complex than laboratory experimental conditions that are used to derive mineral dissolution or precipitation rates. Dolomitisation is one of the most important diagenetic processes affecting carbonate rocks. Still, its underlying mechanisms are not yet completely unraveled. Here, we test the impact of background ions in saline solutions on the dolomitisation rate. Using batch reactor experiments at 200 °C and mineralogical characterisation, we demonstrate that the presence of background ions influences the fluid starting pH and specific ion effect, both impacting the dolomitisation rate. The results indicate that ions with a stronger hydration enthalpy correlate with a shorter dolomitisation induction time, and that Lewis acid AlCl3 is more effective than Brønsted acid HCl. Importantly, dolomitisation occurred at a slightly acidic pH, and carbon speciation modelling showed that carbonate ions did not dominate in any of the experiments. Hence, dolomitisation in our experiments is faster in saline, slightly acidic rather than alkaline solutions and the rate is influenced by the solution composition, with specific ion effects influencing dolomite surface charge, interfacial tension and the structure of water. These new insights have implications for interpretations on natural environments, such as deep reservoirs with saline, slightly acidic formation water, and predictions related to geological carbon dioxide storage.