INITIAL WATER SATURATION AND OIL RECOVERY FROM CHALK AND SANDSTONE BY SPONTANEOUS IMBIBITION

摘要

Spontaneous imbibition is of special importance to the efficiency of oil recovery from fractured reservoirs.Many factors affect the process of spontaneous imbibition.This paper presents an investigation of the effect of rock properties and initial water saturation,S_(wi).In all,59 imbibition curves are presented for various types of chalk and sandstone.Final recovery for Berea sandstone expressed as percent of original oil in place(OOIP)showed only a few percent variation with change in initial water saturation.Final recovery for chalk showed much greater variation;for two of the chalk sample sets,oil recovery decreased systematically with increase in S_(wi).The largest variation was for Rordal chalk;increase in S_(wi)from 7.5 to 51% resulted in decrease in recovery from 67 to 30% of original oil in place.Results for very strongly water wet chalk and zero S_(wi)were closely correlated with results for a wide range of other types of porous media.For the chalk cores,there was an overall tendency for imbibition rate to first increase with increase in initial water saturation and then decrease slightly with increase in S_(wi)above about 34%.For Berea sandstone the rate of imbibition decreased with increase in S_(wi)from 0 to 6%,was at a minimum for the range 6 to 15%,and then increased with increase in S_(wi)from 15 to 30%.These changes are ascribed to the net effect of initial water saturation and subsequent increase in water saturation on imbibition capillary pressure and resistance to flow of water and oil.