Calcite Dissolution Behaviour During Low Salinity Water Flooding in Carbonate Rock

Abstract

Low salinity water flooding (LSF) has been proved to be a promising enhanced oil recovery method for sandstone reservoirs. The efficiency of LSF in carbonate reservoirs has not been proven completely yet, because there are still uncertainties about the LSF recovery mechanism in carbonate. From experimental studies, it was shown that wettability alteration, due to salinity reduction, is able to mobilize the remaining oil. Some studies attributed the wettability alteration by LSF to Calcite dissolution. Furthermore, Calcite dissolution can influence the salinity and pH of injected brines, and hence the oil recovery. However, there is a lack of experimental data on the Calcite dissolution effect of LSF in carbonate rock. The objective of this experimental study was to get a better understanding of the Calcite dissolution behaviour and how it might affect the oil recovery. Bulk and coreflood experiments have been performed; the experimental data was history matched with the use of PHREEQC. Calcite dissolution decreased with decreasing PCO2 and increased with decreasing pH and increasing NaCl salinity from 500-2000PPM. Increasing the injection rate reduced the interaction time and therefore Calcite dissolution did not reach equilibrium in coreflood at high rates. Calcite dissolution from chalk matched with the Calcite dissolution from pure Calcite, while Calcium concentration in effluent from limestone was higher. The two-phase coreflood experiment confirmed that Calcite dissolution occurs as well if oil is present in the porous media, but showed lower values than in single-phase coreflood experiment. pH increased from 5-6 to 9.5 due to Calcite dissolution, which might have an effect on the rock and oil surface charges, and hence oil recovery improvement. The effluent of the two-phase coreflood showed that an emulsion was formed, which suggested formation of in-situ surfactants. From CT scans, micro-CT scans and material mass balance calculations an increase in porosity around 1 % was observed. The increase in the total dissolved species due to Calcite dissolution was not significant, which will not harm LSF effect.