Effect of Acidity of Formation Water for Low-Salinity Polymer Injection Method in Carbonate Reservoirs

Abstract

The low-salinity polymerflooding (LSPF) is a promising enhanced oil recovery (EOR) method with a synergetic effect that combines advantages of the polymerflooding and the low-salinity water flooding. Since polymers are highly sensitive to both pH and salinity of the formation water, the efficiency can be deteriorated when the LSPF process is adopted to carbonate reservoirs with highly acidic or high-salinity formation water. In order to maximize the LSPF efficiency, properties of the injection fluid need to be properly designed in terms of salinity, ion composition, and ion concentration, while taking the polymer properties into account. In order to investigate efficiency of the LSPF for a carbonate oil reservoir that contains "acid" formation water, a set of coreflooding experiments was performed with low-salinity polymer solution in carbonate rock specimens containing acidic or neutral formation water. Not only were the polymer retention and permeability reduction by the polymer adsorption phenomenon analyzed with the measured pressure difference and ultraviolet-visible spectroscopy, but the wettability alteration during the LSPF was also examined by the relative permeability and contact angle changes. In addition, the enhanced oil recovery by the LSPF process was measured to identify the efficiency. Based on the calculated permeability reduction, it is found that the reduced magnitude was smaller for the diluted seawater than the seawater and river water, although the river water has the lowest salinity. This is because its SO42- ion content, which interferes adsorption of the polymer molecules on the grain surface, was lower than that of the others. Meanwhile, amount of the permeability reduction was larger in the acidic formation water than in the neutral formation water, as the polymer adsorption is facilitated when the formation water has a low pH value. On the other hand, the wettability alteration to water-wet in the diluted seawater is greater than in the seawater due to its low salinity. In spite of the lowest salinity, however, the results indicated the alteration of the river water was the smallest due to the extremely low SO42- ion content. Although the same alteration behavior was observed in the neutral formation water, it appears that the magnitude of the wettability alteration by the SO42- is higher for the acidic formation water. Therefore, we concluded that the wettability alteration was mainly affected by the SO42- ion contents of the injection water. Consequently, the diluted seawater with the high SO42- content yielded the best oil recovery for the acidic carbonate, as it prevents the permeability reduction and facilitates the wettability alteration. For an acidic reservoir, it is found that low-salinity water such as river water is not always desirable as an injection water unless it contains enough SO42- ion. Thus, not only the low salinity, but the SO42- ion content is also significantly important for the EOR efficiency, particularly in "acidic" reservoir.