The article is devoted to examining results of laboratory experiments concerned with re-saturation of washed porous media and second oil displacement. First experiments were carried out in the 1970s at UfNII – BashNIPIneft by V.M. Berezin and V.F. Usenko. The task was to investigate in the laboratory the mechanism of science-based picture of impact on pillars of residual oil in inhomogeneous porous and permeable medium produced in waterflooding. It was believed that produced oil accumulations might be efficiently involved in the process of displacement by water during cyclic stopping-restart of waterflooding combined with changing filtration flows. Core column having been composed of sandstone sample from an oilfield and having been flooded upon primary oil displacement was saturated with oil once more and subjected to re-displacement by water. Measured residual oil saturation turned out to be higher than that after primary displacement. The cycles were reproduced by other researchers (V.G. Panteleev, G.N. Pilyakov, E.V. Lozin, A.M. Kuznetsov) also performing the same result. The same one was demonstrated on carbonate medium. The process was further investigated by B.I. Levi by means of numerical modeling. To reduce the observed negative effect, adding reagents to injected water have been investigated, not yielding considerable reduction of residual oil saturation.
A possible explanation of the described phenomenon is being considered. If the same result will be obtained when repeating the experiments on hydrophobic porous medium, the answer will consist in the process of oil capillary entrapment during re-saturation. If the picture is vice versa, the answer will follow from wettability change.
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