Experimental study of combining low salinity water flooding and preformed particle gel to enhance oil recovery for fractured carbonate reservoirs

Oil recovery from carbonate reservoirs is usually low due to their extreme heterogeneity caused by natural fractures and the nature of the oil-wet matrix. Low salinity water flooding (LSWF) and preformed particle gels (PPG) control conformance are two novel technologies that have recently drawn great interest from the oil industry. Theoretically, LSWF can only increase displacement efficiency, and it has little or no effect on sweep efficiency; PPG can plug fractures, they can improve sweep efficiency, but they have little effect on displacement efficiency. We developed a cost-effective, novel, enhanced oil recovery (EOR) technology for carbonate reservoirs by coupling the two technologies into one process. The objective of this paper is to provide a comprehensive understanding of the combined technology and to demonstrate how the coupling method can improve oil recovery. The oil-wet carbonate cores provided a higher improved oil recovery than water-wet carbonate cores during LSWF. The decrease in fracture width resulted in a higher oil recovery factor. Compared to traditional bulk gel treatments, PPG forms stronger plugging but will not form an impermeable cake in the fracture surface; therefore, PPG allows low salinity water to penetrate into the matrix to modify its wettability, thereby producing more oil from the matrix. Results also show that oil recovery increased by 10% during LSWF after the second water flooding. Additionally, when PPGs were injected, another 4% of oil recovery was gained. As a result, the combined LSWF and PPG increased oil recovery by 18%. A full-factorial experimental design was performed to investigate the influence of the PPG-placed injection pressure (which refers to the maximum pressure used to inject PPG for each experiment), water salinity, and fracture width. Experimental results tell that PPG-placed injection pressure is the factor that strongly influences both oil recovery factor and residual resistance factor; fracture width is the least influential factor among the three. Experimental results prove that the coupled method bypasses the limitations of each method when used individually and improves both displacement and sweep efficiency.