Dissolution and Restructuring of Calcite Surfaces in Contact with Electrolyte Solutions - Implications for Enhanced Oil Recovery

Injecting diluted brine into carbonate (e.g., calcite) oil reservoirs has been shown to enhance oil recovery; yet, the mechanism(/s) behind this ‘dilution effect’ remains elusive. Using a Surface Forces Apparatus (SFA), we studied the dissolution and restructuring of calcite surfaces at 75°C that are sequentially exposed to electrolyte solutions of decreasing concentration, starting from highly concentrated brine (e.g., formation water with composition similar to that is found in oil reservoir) via seawater and different dilutions of seawater, to diluted monovalent solutions. Calcite surfaces are observed to restructure and roughen (100s of nanometers) within seconds, and persisted over several hours, after contact with electrolyte solutions and after refluxing with diluted brine solutions. This restructuring of the calcite surfaces due to dilution of the injection brine can have equally large – or even larger – effect as the changes in the physical/colloidal (electric double layer, van der Waals, and hydration) forces on the water contact angle, wettability, and – by implication – the oil recovery.