Improved oil recovery from tensleep sandstone - Studies of brine-rock interactions by micro-CT and AFM

Numerous laboratory studies show that oil recovery by waterflooding can depend on the composition of the injected water. In particular, low salinity water injection has been shown to improve oil recovery in both laboratory core floods and field pilot tests. While the presence of clay, crude oil, and an initial water saturation have all been identified as minimal and necessary conditions for increased recovery from clastic rocks, their relationship as well as other relevant conditions remain uncertain. Several recovery mechanisms have been proposed. In recent work improved recovery of Minnelusa crude oil was observed during injection of low salinity coal-bed methane water into Tensleep sandstone. This eolian sandstone is primarily composed of quartz and is pervasively cemented with dolomite and anhydrite. The sandstone has low clay content and is a potential exception to the presence of clays as a necessary condition. The presence of dolomite crystals was previously shown to be associated with the improved oil recovery mechanism. In this paper we make a range of measurements at the pore and molecular scale on Tensleep sandstone to pin down more precisely the possible improved recovery mechanism. Tensleep sandstone was imaged via micro-CT before and after low salinity brine flooding. Local mobilization of the dolomite crystals and anhydrite within the pore space was observed after changing from high to low salinity injection. The effluent water was analysed for cations and anions during high and low salinity injection; results indicate the dissolution of dolomite occurs during low salinity flooding. These results indicate dolomite may adhere to quartz surfaces at high salinity conditions but releases at lower salinity conditions. We have systematically explored the role of surface forces in the adhesion of silica to carbonate surfaces with the Atomic Force Microscope (AFM). AFM studies of the force interaction between quartz and calcite surfaces are reported for a large number of salt pairs at high and low salinity conditions. Little change in the adhesive forces is measured with salinity and no divalent cation effect was observed; increased adhesion was observed only in the presence of sulfates of monovalent cations. DLVO theory and hydration forces do not explain the force measurements. From these results we postulate that the dolomite surface becomes covered in a molecular layer of sulfate. This layer improves adhesion at high salinity, however under low salinity conditions dissolves and increases the opportunity for the dolomite to detach from the quartz surface.