Experimental and numerical modeling of low-salinity waterflood in a low permeability carbonate reservoir

Low-salinity waterflooding in carbonate reservoirs is of great interest because of the potential for increased recovery of oil. However, the exact mechanism of low-salinity oil recovery is not well understood. It has been suggested that the main mechanism is wettability alteration resulting from the interaction of Na⁺, Cl^-, Ca²⁺, Mg²⁺, SO4 ^2-, and RCOO^- in the electrical double layer (EDL) near the surface of carbonate pores. In this paper, we present laboratory experiments and numerical modeling in low-permeability carbonate cores to assess this assertion. Seawater followed by series of low-salinity waterflood experiments were performed on several facies of carbonate reservoir. The permeability of the cores is in the range of 0.5 to 1.5 mD and porosity is from 0.18 to 0.25. Cores were aged for eight weeks at reservoir pressure and temperature. The API gravity of the oil is 32°. The IFT and contact angle measurements were performed at several brine salinities. A two phase numerical model was used to simulate oil recovery and molecular transport of salt components to investigate the wettability alteration effect. The low-salinity waterflood experiments yielded incremental oil recovery of up to nine percent. Low-salinity waterflood was observed to be more significant in less-heterogeneous and less-vuggy cores. Moreover, low-salinity waterflood altered the wettability of several one-inch diameter, crude-aged, carbonate core discs from oil-wet to intermediate-wet. Hence, the main mechanism of incremental oil recovery during low-salinity waterflood in this carbonate reservoir is wettability alteration. The mathematical model matched the oil recovery when the relative permeability functions were adjusted to account for wettability alteration effects.