EFFECTS OF FORMATION BRINE COMPOSITIONS ON EFFECTIVENESS OF LOWSALINITY BRINE INJECTION IN CARBONATE RESERVOIR

Low Salinity WaterFlooding (LSWF) has been intensively studied in worldwide. The method is simply performed by injecting water with much lower salinity compared to formation brine in the reservoir to induce wettability alteration through multi-component ion exchange mechanism. In this study, ionic ratios of formation water in carbonate reservoir is investigated as most study only emphasize on effects of ion composition in injected water and most explanations are already available for sandstone reservoir. Experiments are performed with dolomite rock as it contains both calcium and magnesium ions and ratio of both ions in formation water is one of study parameters. Spontaneous imbibition test, coreflood test and complexometric titration are performed to investigate effects of ion composition in formation brine on effectiveness of LSWF. The results show that LSWF is effective when formation brine contains calcium ion to magnesium ion ratio of 1:1 which is the least portion of calcium ion in this study. This ratio allows calcium ion to diffuse to injected low salinity water and adequate amount of calcium ion can still form calcium carboxylate complex and at same time, dissolution of calcium from rock surface is facilitated. Hence, oil is easily liberated from both mechanisms. The ratio of divalent ion to monovalent ion of 1:3, which is the smallest amount of monovalent ion is the most effective formation brine for LSWF. As hydrated size of monovalent, which is much smaller than that of divalent ions, surface is easily adhered by oil, leading to a more oil-wet condition and hence oil can be difficultly liberated. Small injection rate is an appropriate condition for LSWF. In this study, injection rate of 0.2 cm3/min, which is the smallest value, yields the best benefit of LSWF as potential determining ions have more time to complete multi-component ions exchange mechanism onto rock surface. In addition, the highest oil recovery is obtained at high temperature. A confirmation of complexometric titration showing disappearance of magnesium ion in effluent explains the oil recovery mechanism by means of forming magnesium carboxylate complex at high temperature.