The water composition can affect the wettability of oil reservoir sandstone rocks. If several injection water compositions are available, the composition with highest oil recovery potential should be selected. Alteration of wettability to more water-wet conditions has the potential to accelerate the oil production and thereby reduce the water production. The main objective for the presented work was to develop a method for fast screening of the potential for available injection water compositions to alter wettability to more water-wet.
The wettability of reservoir rock samples was characterized by using a flotation method which relies on the affinity of minerals to either the brine or crude oil phase. Crushed reservoir rock samples were aged at reservoir temperature using brine and crude oil of different compositions, and the amounts of oil-wet particles in the oil-phase were determined. The experimental results were compared with geochemical simulation of rock-brine interactions.
In the flotation study, the potential for the injection water compositions to alter the wettability to more water-wet was shown to depend on the crude oil compositions. By geochemical simulations including both ion-exchange and solubility of carbonate minerals, the amounts of divalent cations onto clay surfaces (M2+) were found to vary with the brine composition. Some of the possible injection water compositions were found to reduce M2+ and thereby reduce the potential for cation-bridging. The main trend was decreasing concentration of oil-wet particles (more water-wet conditions) in flotation experiments with decreasing M2+ estimated by simulations. The dominating wetting mechanism for the studied rock-brine-oil systems appeared to be cation-bridging.
The study has shown that screening of the potential for available injection water compositions to alter the wettability to more water-wet can be carried out by combining flotation experiments and geochemical simulations. This screening is much less time consuming than standard experiments, and only small amounts of rock samples are required. The potential for the most promising injection water compositions can then be further evaluated by core flooding experiments.