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.
In the presented work the wettability conditions of reservoir chalk plugs have been characterised after water flooding with formation water, after spontaneous imbibition and water flooding by sea water, and after the first cycle of water alternating carbon dioxide flooding (CO2-WAG). Core plugs from a fractured chalk reservoir in the North Sea were used in the experiments carried out at reservoir conditions. Easily accessible sulphate in the core plugs was removed before the core plugs were drained to initial water saturation by using the porous disc method. Wettability acquisition was then carried out using stock tank oil. The wettability conditions of the chalk plugs were characterised by the spontaneous imbibition of water and the water-wet area determined by the sulphate wettability test. The water-wet area of reservoir chalk plugs water flooded by formation water was found to be rather small. The spontaneous imbibition of formation water was also rather low for companion core plugs. The prepared reservoir chalk plugs therefore appeared to be close to mixed-wet or preferential oil-wet. Sea water has earlier been found to improve the spontaneous imbibition of water into reservoir chalk plugs from the same field. After spontaneous imbibition and viscous flooding with sea water, the water-wet area of the core plugs was found to be on the average slightly larger than for the core plugs water flooded by the formation water. When the same core plugs were flooded with CO2, most of the oil was produced. After the following sea water injection, the water-wet area was found to be larger than before the simulated first cycle of the CO2-WAG flooding. During long term spontaneous imbibition experiments followed by viscous water flooding by sea water, slightly alteration of the wettability conditions towards more water-wet was observed. In the first cycle of a CO2-WAG process, the reservoir chalk plugs were found to become even more water-wet. Alteration of the wettability conditions to more water-wet during CO2-WAG processes can be important for the oil recovery in fractured chalk reservoirs, and should therefore be studied further.
Change of injection brine composition from high to low salinity has been reported to improve the oil recovery in some field pilots and core flooding studies. In other studies low salinity water flooding (LSWF) has not been successful. It has also been reported that LSWF can be successful in secondary floods but not in tertiary floods. The main objective for the reported study has been to investigate the oil recovery and cation concentrations in effluent samples during secondary and tertiary LSWF experiments using brines of different compositions. These results have been combined with evaluation of cation exchange during these floods by simulations and history matching to estimate the saturation functions. When diluted formation water (LSW2) was injected in secondary floods, the oil production was only slightly higher than the total oil production during secondary flooding with formation water (FW) followed by tertiary flooding with LSW2. In the secondary flood with LSW2 the oil was produced during a long period, and wettability alteration appeared to occur during the flood. LSW2 was also found to give less water-wet conditions than FW. When LSW2 was injected to a core aged with LSW2, the oil production was slow and rather low remaining oil saturation was obtained. In a secondary flood with optimized low salinity water composition selected based on evaluation of cation exchange on clay surfaces, the oil recovery was found to be faster than in the secondary LSWF with LSW2 (diluted FW). By modeling of the cation exchange in the flooding experiments, it was found that the change in oil recovery profile can be explained with the change in the concentrations of divalent cations on the clay surfaces. It is concluded that the potential for secondary and tertiary LSWF, will strongly depend on the compositions of rock, FW and LSW. The potential for tertiary LSWF will also strongly depend on the composition of the high salinity brine injected in the secondary flood. Modeling of cation exchange during the reservoir history can be used to evaluate the potential for LSWF of the oil reservoirs and to optimize the LSW compositions.
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