The importance of Enhanced Oil Recovery (EOR) technologies cannot be overemphasized, especially in the context of the surge in energy demand driven by rapid economic growth in developing countries as people strive to improve their living standards. Getting higher oil recovery from existing fields will be a key part of meeting the world's growing demand for energy. In the past decade, injection of brines of low salinity content and selected ionic composition in sandstone reservoirs has been developed into an emerging EOR technology. The advantage of low salinity waterflooding is that it is operationally comparable to conventional waterflooding and does not require expensive chemicals or carbon dioxide and nitrogen. The complexity of the crude oil/brine/rock interactions is well recognized and the mechanisms behind the low salinity EOR process have been debated in the literature for the last decade.;The objective of this work is to explore by experimental study the effect of low salinity waterflooding on different outcrop and reservoir cores. Investigation of increased oil recovery by injection of low salinity water such as coalbed methane production water has been extended to reservoir cores from the Tensleep, Minnelusa and Phosphoria formations in Wyoming and outcrop cores (Berea and Bentheim). The Tensleep and Minnelusa formations are eolian sandstones of comparable depositional environment that contain interstitial anhydrite, dolomite and occasional calcite cements. The Phosphoria dolomite has pin-point to coarse vuggy pores lined by sparry dolomite crystals and also features patches of anhydrite. All the cores taken from pay zones showed increased oil recovery ranging from 5 to 8% original oil in place through injection of low salinity water. Increase in sulfate ion content of the effluent brine confirmed the dissolution of anhydrite, for all three reservoir rock types. Proposed mechanisms of recovery by low salinity flooding of sandstones which are tied to the presence of clay cannot apply because none of these rocks have significant clay content. Further evidence of the role of anhydrite dissolution was provided by the recovery behavior of Tensleep cores taken from the water-saturated aquifer zone of an oil reservoir. Anhydrite cement was sparse and only visible in occluded regions of pore space but not in regions that were clearly permeable. For these cores, there was no additional oil recovery when the injected brine was switched to low salinity water. The release of dolomite crystals and other fine embedded minerals which is likely associated with dissolution of anhydrite, may be a factor in the observed response to low salinity waterflooding. The movement of cement components is a possible contributing factor in the wide variety of observed relationships between pressure drop and oil recovery. For example, significant variation of relative permeability to brine at constant saturation is often observed.
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