Residual gas saturation is known to be a key factor in evaluating gas recovery from a lean gas reservoir invaded by water.The large scatter in maximum trapped gas saturation(SgrM),the existence of two opposite SgrM/porosity trends and the key controls of the variability of SgrM were illustrated by two recent studies(Suzanne et al.,2001;Hamon et al.,2001).This paper tackles the influence of irreducible water saturation on the variability of trapped gas saturation.Sgr measurements were performed by controlled evaporation and capillary imbibition or by capillary drainage/imbibition.Sixty samples were selected from sandstone formations to account for the largest scatter in SgrM observed in our previous study and cover a very large range in porosity and permeability.The main results of this study are:-The fluid distribution after controlled evaporation was checked with NMR and X-ray scanner measurements and was found homogeneous.-The Sgr values obtained by evaporation-imbibition were found in very close agreement with those achieved by capillary drainage-imbibition on eight reservoir samples-The presence of irreducible water prior to the imbibition does not change the existence of two opposite Sgr trends as a function of porosity(or permeability).-Sgr at irreducible water saturation,Sgr@Swir,may decrease as porosity decreases.This relationship is shown to be related to increasing clay content,decreasing pore size or increasing amount of microporosity as SgrM values.-Maximum trapped gas saturation and Sgr at irreducible water saturation were found equal.So,Sgr may either increase or decrease as a function of irreducible water saturation,showing that Sgr is not controlled only by initial water saturation.And,the frequent extrapolation of Land’s empirical relationship to the interval [SgrM,Sgr@Swir] is not correct.
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