Investigating the Effect of Potential Determining Ions on the Improved Oil Recovery by Spontaneous Imbibition

Recovery of oil from fractured reservoirs is dependent on the water spontaneous imbibition in the matrix and depletion of oil contents into the fractures. However, this process is feasible only where the matrix blocks would be water-wet. The carbonate reservoirs are mostly oil-wet and thus applying spontaneous imbibition in the fractured carbonate reservoirs is dependent on the wettability alteration during the water flooding. In this work, imbibition experiments were performed using solutions incorporating various concentrations of potential determining ions present in the sea water (SO42-, Ca2+, and Mg2+) at four temperature levels (40, 60, 80, and 90 °C). The effects of ions, temperature, and salt concentration on the wettability alteration of carbonate reservoirs during spontaneous imbibition process were investigated. Calcite outcrops from a limestone mine in Limburg, the Netherland, and crude oil of Bangestan formation, Iran, with an API of 22.3, were applied. It is shown that the presence of SO42- ions The strongly affects the spontaneous imbibition to a maximum extent, in other words, the 75 percent removal of SO42- from imbibition solutions leads to 2% reduction in the oil recovery. Moreover, by increasing NaCl concentration the oil recovery decreases (2% at 90 °C), while by removal of NaCl from the solution the oil recovery increases (30% at 90 °C). No variation in the oil recovery is observed where variations in the concentration of Ca2+ and Mg2+ ions at 40, 60, and 80 °C are applied. However, at 90 °C, the recovery of oil is increased with increasing the concentration of potential determining ions.