Calcite Dissolution Behaviour During Low Salinity Water Flooding in Carbonate Rock

Low salinity water flooding (LSF) is a promising technology for improving oil recovery. However, the efficiency of LSF in carbonate reservoirs has not become well established because of the uncertainties about the LSF recovery mechanism. Some studies attributed the recovery improvement with LSF to calcite dissolution in coreflood experiments. Furthermore, calcite dissolution can influence the salinity and pH of injected brines at the reservoir scale, and hence impact the oil recovery process. To better understand calcite dissolution during LSF and the possible effects on oil recovery, the interaction between low salinity brines (LS) and carbonate rock was studied. Bulk tests and one-phase and two-phase coreflood experiments were performed. Bulk experiments were executed by adding crushed calcite material to a jar of brine, and were kept until equilibrium between rock and brine was reached. Coreflood experiments were performed on chalk and limestone core materials. Both types of experiment were conducted at different conditions to test the impact of different parameters such as temperature, pH, CO2 partial pressure and brine composition. Furthermore, the experimental data was history matched using PHREEQC software. The experiments showed that LS can dissolve calcite and equilibrium can be achieved in a relatively short time for the bulk experiments. In addition, equilibrium can be reached during coreflood as well at low rate injection. Increasing the injection rate reduced the interaction time, and therefore calcite dissolution did not reach equilibrium. The experimental results showed that the amount of calcite dissolved by LS increased with increasing CO2 partial pressure and decreasing the pH. The two-phase coreflood experiment confirmed that calcite dissolution also occurs if oil is present in the porous media, but the dissolved amount was lower than in single-phase coreflood experiments. The increase in salinity of injected brines, due to calcite dissolution, was not significant. However, calcite dissolution resulted in a major increase in the brines’ pH, which can affect the rock and oil surface charges, and hence oil recovery. The bulk experimental results were in a good match with simulation data from PHREEQC at equilibrium state.