Osmosis as Mechanism for Low-Salinity Enhanced Oil Recovery

Abstract

We believe that osmosis has been overlooked as a possible mechanism for observed low-salinity enhanced-oil-recovery (EOR) effects. Osmosis can occur in an oil/water/rock system when injecting low-salinity water, because the system is full of an excellent semipermeable membrane-the oil itself.In the present work, water transport through oil films was visualized both in 2D micromodels and in sandstone cores imaged in a microcomputed tomography (CT). After treating these model systems with hexamethyldisilazane (HMDS) to render them more oil-wet, water became discontinuous, and it was possible to establish osmotic gradients. Either expansion or contraction of the connate water was observed, depending on the direction of the imposed salinity gradient.Because osmosis could be the underlying mechanism for lowsalinity EOR, two changes in research strategy are proposed: Most importantly, the use of spontaneous-imbibition tests as evidence for wettability alteration in low-salinity water should be critically reinvestigated. This is because observed production could have stemmed from "osmotic expansion" of the connate water rather than wettability change. Second, much research focus should be shifted from sandstone reservoirs to fractured oil-wet carbonates. Osmosis potentially yields larger responses for the latter reservoir type, whereas from a mechanistic perspective the reason behind low-salinity EOR functioning in both sandstones and carbonates deserves further attention.

Comprehensive investigation of low-salinity waterflooding in sandstone reservoirs

“…Sandengen and Arntzen (2013) suggested that the increase in the osmotic pressure causes brine expansion and oil displacement. This mechanism was agreed by Sandengen et al (2016), Fredriksen et al (2016), and Fredriksen et al (2017). However, Bartels et al (2017) showed that the osmosis pressure can contribute in the incremental oil recovery, but it is not the primary mechanism of LSWF.…”

Experimental Investigation of Osmosis as a Mechanism for Low-Salinity EOR

“…It was observed that brine was effectively trapped by the oil phase in the oil-wet micromodel (see Figure 2.14a) while the water-wet micromodel was unsuccessful in establishing stable pore-sized encapsulations of connate water by oil due to continuous water films on the grain surfaces (see Figure 2.14b). [34] After stable encapsulations of connate water had been verified, low-salinity waterflooding was initiated in the oil-wet micromodel. Deionized water was used as the low-salinity water.…”

“…Stable encapsulations of connate water were observed in the oil-wet micromodel, but not the water-wet micromodel. [34] shows the complete waterflooding sequence initiated after the visual verification of the stable connate water encapsulations. Connate water contraction was observed in a similar experiment performed when using deionized water as connate water and high-salinity brine as injection fluid.…”

“…Sandengen et al (2016) also extended the visualization experiment to 3D using 5 mm diameter sandstone cores [34]. Bentheimer sandstone core plugs were treated to obtain plugs with three different wetting regimes (water-wet, oil/mixed-wet, and oil-wet), two 18: The time sequence shows connate water expansion during low-salinity flooding in an oil-wet silicon-wafer micromodel.…”

“…N aI solution, which was used as connate water, has high X-ray attenuation meaning that preserved pockets of N aI will be visible as bright areas on the micro-CT images. [34]…”