Abstract

Even though the proposed screening criteria for low salinity water flooding (LSWF) are fulfilled, improve recovery is not always obtained. The LSWF mechanisms are therefore still discussed. The objective for the study was to describe the brine-rock interactions at high and low salinity. Reservoir core plugs were flooded either by formation water, sea water and low salinity waters in succession or by low salinity water directly from initial water saturations. Effluent samples were analyzed for ionic concentrations and pH. Relative permeability (kr) and capillary pressure (Pc) curves were obtained at the core scale by history matching the experimental production and differential pressure across cores using a simulation tool. A developed two-phase model was used to predict the release of divalent cations from the rock during LSWF, and to relate this to the oil recovery.

The high salinity water flood with formation water was found to give close to piston-like displacement, while the oil was produced over much longer periods in LSWF. The estimated kr- and Pc-curves indicated that the rock was water-wet in the high salinity floods and mixed-wet in the low salinity floods. The experimental results were in accordance with the modeling of the brine-rock interactions. When the formation water was replaced with the low salinity water, increase in the concentrations of divalent cations onto clay surfaces was predicted for the selected brine compositions. Higher concentrations of polar oil components can then be bonded to the clay surfaces by the divalent cations and make them less water-wet. It is concluded that the low salinity water altered the wetting state of the rock. The direction of alteration can be explained by ion exchange taking place on the clay surface. The low salinity water potential for improving recovery should be considered on a case by case basis based on the interactions between the formation brine, injected brines, oil components, and rock type.

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