Experimental investigation of polymer flooding with low-salinity preconditioning of high temperature–high-salinity carbonate reservoir

Application of polymer flooding in high temperature–high salinity (HTHS) carbonate reservoirs is challenging due to lack of polymers that can withstand such harsh reservoir conditions. The traditional polymers are usually sensitive to high salinity, especially at high temperature. However, injection of low-salinity make-up brines may precondition high-salinity reservoirs before initiating polymer flooding which may reduce chemical degradation of polymer. This study aims to evaluate the effectiveness of a partially hydrolyzed polyacrylamide base polymer for mobility control application in a low-salinity preconditioned carbonate reservoir and hence on the improvement of oil recovery at HTHS carbonate reservoir. Core flooding experiments using unsteady-state technique were conducted on reservoir cores with permeability range of 10–100 mD. During each experiments, salinity of the make-up brines were changed to study the effect of preflush salinity and polymer flooding in HTHS reservoir. Oil production from water flooding for all the cases was found to be between 49 and 65%. Polymer helped to reduce the mobility ratio from 4.1 to less than 1 and additional 7–11% of oil was recovered from the remaining oil saturation after water flooding. Comparisons were also made between oil recovery results based on volumetric production and in situ saturation monitoring (ISSM) data, which were found to be matching. Additionally, the ISSM helped to understand the performances of fluids injected during oil recovery stages and captured front movement of the fluids at all time. Also, high capillary end effect was confirmed from the ISSM which may lead to underestimation of the oil recovery from water flooding in the absence of ISSM. Resistance factor and residual resistance factor were also calculated during all core flooding experiments and were found to be 7.0, 2.4, 36 and 3.7, 1.4, 8.9, respectively.