The issue of reliability of residual water saturation estimation by logging methods in hydrophobic reservoirs is often an intricate one due to high values of producing formation resistivity.The authors carried out an experimental study of residual water saturation formation in such reservoirs by NMR method.Samples of reservoir rocks with different wettability were selected.The authors varied water content in rock by capillary displacement via a semipermeable membrane,changing displacement pressure.NMR spectra of transverse relaxation time Т2 were obtained at every displacement stage.Analysis of the obtained results reveals significant differences of drainage process in hydrophilic and hydrophobic rocks.Disappearance of the longest components of relaxation time as the water content decreases is typical of hydrophilic rocks.The above suggests that water is always displaced out of the largest pores.In contrary in hydrophobic rocks water displacement out of large pores is accompanied by its transfer from the smallest pores to relatively bigger ones.It is demonstrated by the fact that with the decrease of water content in rock in the spectrum of relaxation time there is observed simultaneous gradual disappearance of the longest components as well as the shortest ones.In the result with residual water saturation the character of water distribution in the pore space differs considerably in hydrophilic and hydrophobic rocks.The conclusion is: an ordinary method of cutoff in the spectrum of relaxation time cannot be used for determination of residual water saturation in hydrophobic reservoirs in contrast to hydrophilic ones.Additionally,electric measurements showed that these effects can lead to the understatement of Archie’s parameter n value in resistivity index versus water saturation relation.In laboratory conditions true resistivity values of actual deposits might not be obtained due to inadequacy of processes of residual water saturation formation.
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