Effectiveness of Ammonium Ion in Low salinity Waterflooding in Shaly-sandstone Reservoir

Abstract

Low Salinity Waterflooding (LSWF) has been intensively studied worldwide as it is a low-cost technique. By injecting water with very low salinity compared to formation water in the reservoir, the method changes equilibrium between rock surface, oil and surrounding water, causing wettability alteration through Multi-component Ion Exchange (MIE) mechanism. In this study, ammonium ion is added into low salinity water to facilitate the MIE mechanism in shaly-sandstones reservoir. Experiments are performed with shaly-sandstone rock as it contains both calcium and magnesium ions as part of clays and ammonium ion can express its potential by triggering MIE mechanism. Stirring test, spontaneous imbibition test and coreflooding test are performed to research the effects of ammonium ion in low salinity water that could generate positive results in oil recovery mechanism.

From the experiments, it can be observed that ammonium ion is a foreign ion that can be easily adsorbed onto rock surface to displace divalent ions. Nevertheless, the ammonium ion itself tends to displace more calcium ion than magnesium ion on clay surface. Adding potassium ion into ammonium ion solution results in shifting from favorability of calcium ion dissolution toward magnesium ion dissolution. Since magnesium ion creates stronger bond compared to calcium ion, ammonium ion is therefore considered to be superior to potassium ion in oil recovery mechanism. The results from imbibition and coreflood test confirms that low salinity waterflooding in shaly-sandstone can be accomplished by the used on ammonium ion. However, ammonium ion should be prepared to favor the dissolution of calcium ion since oil that is attached through calcium ion is easier to be recovered compared to that of the dissolution of magnesium ion.

In this study, the solution of pure ammonium chloride and mixture of ammonium-calcium-sodium ion at the ratio of 40-20-40 show good potential in enhancing oil recovery. Nevertheless, the case of ammonium-calcium-sodium ion at the ratio of 40-20-40 might be suitable for the existing oilfield since sodium can be mixed with the water formulation at high portion and fresh water demand can be minimized. From the conditions tested in this experiment, oil recovery factor from the use of ammonium ion can be increased up to 26 percent.