Enhanced Oil Recovery by Combined Low Salinity Water and Polymer Flooding
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World energy demand is continuously increasing as the developed and developing countries consume more energy to keep their growing industries running. Significant portion of the energy demand is supplied by fossil energies, especially crude oil. Thus, to meet the world energy demand, it is necessary to increase the oil reserves and oil production capacities. This goal is achievable by either exploration/development of new oil reservoirs, or by improving the recovery efficiency of the current producing or mature reservoirs by applying enhanced oil recovery (EOR) processes.
Enhanced oil recovery processes are applied to recover oil not produced by natural energy drives of the reservoir. The natural reservoir drives could normally produce one third of the oil, usually two third of oil in the reservoir is left behind and trapped in pore structures of the reservoir due to rock/fluid interactions. The trapped oil could be mobilized and recovered if the rock/fluid interactions altered to favourable state. The favourable state may be achieved by implementing EOR processes and through, among others, wettability alteration, weakening the capillary forces, strengthening the viscous forces.
During the last 10-15 years low salinity waterflooding (LSW) as an emerging EOR process, has been extensively investigated both in experimental and field pilot scales. Although the EOR potential for LSW has been proven, however the mechanism or mechanisms behind low salinity effect (LSE) is still open for further discussions. So far, several mechanisms have been proposed as the reasons for LSE, but no one conclusively explains all observations reported for low salinity injection.
This research study has mainly concentrated on the improving the understandings about mechanisms believed to be the reason for low salinity effect with stress on wettability alteration mechanism. Also the aim in this work was to investigate if combination of low salinity water and polymer results in synergistic effect on residual oil mobilization and final oil recovery.
It is generally reported in the literature that low salinity shifts the wettability towards more water-wet state, and therefore oil polar components are released, mobilized and produced by low salinity water. The results from this study shows that the initial wettability state of the porous media is a key parameter for low salinity effect as less water-wet cores gave better oil recovery by LSE. Moreover and contrary to the general belief in the literature, wettability indicators in this study showed shift to less waterwet state by low salinity injection. The wettability indicators were used in this study include end point relative permeabilities, frontal advancement behaviour obtained by in-situ saturation monitoring, and relative permeability and capillary pressure curves obtained from simulation of experimental results. This study concludes that depending on the initial wettability of the porous media the wettability shift by low salinity effect could be either towards more water-wet or less water-wet state. The optimum wettability state is where the minimum residual oil saturation and maximum oil recovery may be obtained. This study also showed that injection of low salinity water at initial water saturation (secondary mode LSW) is more efficient than low salinity injection at residual oil saturation (tertiary mode LSW).
Furthermore, combination of low salinity water and polymer injection or alternatively nano-sized polymer particles revealed considerable synergy between two EOR processes. The combined effect was even more significant (more than 50% reduction in residual oil saturation after waterflood) when the low salinity environment was established at initial water saturation (Swi) rather than at waterflood trapped oil saturation (Sor). This synergistic effect can be attributed to improved banking of low salinity mobilized oil that is a combined effect with low salinity oil mobilization. The results are encouraging as the concentration of the polymer used in this study was very low (300 ppm) and only gave slight change in mobility ratio.
Under the experimental conditions in this study, also nano-size polymer particles (LPS: Linked Polymer Solution) exhibit similar EOR potential. Low salinity combined with LPS gave similar additional oil recovery effect as low salinity polymer, even though the mechanisms for LSW and LPS are different than for LSW and polymer.