Fluid-rock interaction during low-salinity water flooding of North Sea chalks

The injection of low-salinity water into carbonate oil-reservoirs has been the subject of much interest as a potential method to enhance oil recovery (EOR). Low salinity water injection has been shown to alter the geochemical equilibrium in the reservoir, and consequentially potentially change the surface wettability of the reservoir rocks. Nevertheless, the geochemical reaction pathways during low salinity water flooding in carbonates remain poorly understood. In order to provide a more comprehensive insight into the geochemical reaction pathways, a series of batch and core-flooding experiments have been performed at 70 °C. The experiments made use of analogous and authentic reservoir material from the Danish North Sea oil reservoirs, and brines with chemical composition similar to the formation water and injection water available in offshore flooding operations. Results show that both mineral dissolution and the precipitation of new secondary phases can accurately describe the geochemical changes observed in the experimental effluents during the course of a low-salinity water core-flooding scenario. The dissolution of amorphous silica (SiO₂) present in the chalk, and the precipitation of a Mg–Si clay mineral, identified here as sepiolite, are key reactions that take place during low-salinity water core-flooding in chalks. The suggested geochemical pathway was successfully fitted by a simplified geochemical simulation which can accurately describe the experimental observations. Surface reactions which take place during modified-salinity water flooding were put into perspective by quantifying their contribution to the overall effluent composition.