EOR Efficiency of Low-Salinity Polymerflooding in Acidic Carbonate Reservoirs

Abstract

Low-salinity polymerflooding (LSPF) is a hybrid EOR method which has synergy effect of both polymer injection method and low-salinity water injection method. It not only provides a favorable mobility ratio between oil and water by adding polymer into water, but also alters the wettability of carbonate rock surface with ion exchange and mineral dissolution by injecting low-salinity water. However, LSPF method has practical limitations for particular environments; some carbonate reservoirs contain acidic formation water due to the presence of hydrogen sulfide (H2S) and carbon dioxide (CO2). In this kind of acidic carbonate reservoirs when LSPF is adopted, LSPF efficiency can be severely deteriorated since the polymer is very sensitive to H+ ion concentration (pH). That is, when the reservoir condition is acid, polymer degradation is yielded, which causes the polymer adsorption. In these aspects, the main goal of this study was to investigate EOR efficiency by applying LSPF method to carbonate reservoirs containing acidic formation water. The coreflooding experiments were performed by injecting low-salinity polymer solution into carbonate cores for various levels of salinity and SO42- ion content. The experimental results were analyzed for carbonate oil reservoir containing acidic formation water during LSPF with three different types of water with respect to salinity level. As experimental results for the polymer adsorption during LSPF, the polymer retention phenomenon was identified by the measured pressure difference and effluent analysis. Although river water (RW) has the lowest salinity level as 120 ppm, the polymer adsorption with river water was large in comparison with sea water (SW) or diluted sea water (DSW) because the river water includes low concentration of SO42- ion. Here, the function of SO42- ion prevents the polymer degradation so that less polymer adsorption occurs. With regard to pH condition, it was found that the amount of polymer adsorption was larger in acidic reservoir condition than in neutral condition, indicating that the use of polymer in acidic reservoir was not efficient as much as in neutral reservoir. From the analysis of wettability alteration, the changes in residual oil saturation, relative permeability and contact angles measured before and after the LSPF were the largest for the diluted sea water case, while it was the smallest in river water case. This means that the wettability altered to more water-wet state in diluted sea water injecting case compared to river water case. In terms of pH condition, the wettability altered more greatly in acidic condition than in neutral condition. This study also investigated SO42- ion contents contained in injection water. From the results for the river water case, the residual oil remained in the core was larger and the wettability altered less than the other water types, in spite of its lowest salinity level. This is because SO42- ion concentration is the lowest as 10 ppm in river water. Since SO42- ion prevents the polymer adsorption and favorably facilitates the wettability alteration, it is essential to analyze about SO42- ion concentration. The effect of SO42- ion concentration of 500 to 3,000 ppm with the use of DSW was experimentally analyzed. The higher SO42- ion concentration induced the less polymer adsorption occurrence. This is because SO42- ion allows to maintain polymer chain structure by the repulsive interaction between SO42- ion and polymer molecule, which prevents the adsorption. In terms of the wettability alteration, when more SO42- ions were contained in the injection water, the greater alteration to water-wet state from oil-wet state was observed and thus less residual oil was remained in the core. In the analysis for pH condition of reservoir, when the reservoir is acidic condition that contains more H+ ion compared to neutral condition, the aforementioned effect of SO42- ion concentration on polymer adsorption was greater. On the other hand, the wettability altered more efficiently in acidic reservoir condition due to H+ ion activity. Therefore, in overall, the LSPF hybrid EOR method in a carbonate reservoir containing acidic formation water was found to be efficient as much as neutral reservoir when injecting fluid was the DSW, even though the reservoir was acid. Meanwhile, when the river water being available relatively easily in onshore fields is planning to be designed in acidic reservoir, the LSPF proposed in this study is a reliable method with the addition of optimum SO42- ion content into river water.