Effects of Formation Brine Compositions on Low Salinity Waterflooding Using Seawater

Abstract

Water injection is a process to maintain reservoir pressure and at the same time to sweep remaining hydrocarbon in reservoir when forces provided by primary recovery is not adequate. Recent studies concluded that injecting Low Salinity Brine (LSB) could yield additional oil recovery. In this study, effects of ion compositions in formation brine on low salinity seawater injection are emphasized. The composition of formation brine with total salinity of 100,000 ppm is modified in various formulations. First, physical properties of sandstone core samples are measured prior to core saturation process with different formation brines. After that, core samples are altered to oil-wet condition by flushing with organic-acid oil. Sandstone core samples are then displaced by synthetic seawater. The results show that rate of oil recovery is improved when the ratio of Calcium ion to Magnesium ion is increased. Due to the larger ionic size of Calcium ion compared to Magnesium ion, the ionic strength between oil and Calcium ion is weaker than of with Magnesium ion. However, if Calcium ion is excessive in formation brine, the abundance of Calcium ion will result in high rate of mineral dissolution and this will obstruct the substitution of monovalent ions from injected seawater to divalent ions on sandstone surface, resulting in decreasing of rate of oil recovery. Low Potassium ion concentration in formation brine causes diffusion of Potassium ion from injected seawater back to formation brine, substituting divalent ions between oil and surface and this increases rate of oil recovery. In addition, formation bine concentration is diluted to 75,000 and 50,000 ppm to study effects of total salinity in formation brine. The highest salinity contrast between formation and injected brines favors dissolution of bridging divalent ion through MIE mechanism and this yields the highest oil recovery factor. Lastly, when comparing seawater and diluted formation brine, the result shows that seawater yields more benefits in terms of higher oil recovery factor and rate of oil recovery than using diluted formation brine. This can be concluded that impoverishment in Calcium ion and enrichment in Potassium ion in seawater is recommended condition for injected fluid. Consequently, high amount of Calcium ion and low amount of Potassium ion in formation brine are considered as favorable conditions for seawater injection.