Numerous laboratory studies have shown that water composition can substantially affect oil recovery by waterflooding. 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. The Tensleep and Minnelusa formations are eolean 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 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. In addition to effluent composition, dissolution of anhydrite cement within Tensleep sandstone was previously demonstrated through pore level micro X-ray CT imaging. Further evidence of the role of anhydrite dissolution was provided by the recovery behavior of Tensleep cores taken from the water-saturated zones of an oil reservoir. Anhydrite cement was sparse and 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.

You can access this article if you purchase or spend a download.