High Oil Recoveries from Transition Zones

Abstract

Abstract Oil-water transition zones may contain a sizable part of a field's STOIIP, specifically in low permeable sandstone and carbonate reservoirs. The amount of recoverable oil in a transition zone depends–among other things- on the distribution of initial oil saturation (Soi) as a function of depth and the dependency of the oil's mobility, i.e., the residual oil saturation (Sor) and relative permeability, on initial oil saturation. In this paper we present laboratory measurements of residual oil saturation and oil relative permeability as a function of initial oil saturation to properly characterize oil mobility in transition zone. We found that the residual oil saturation after water flooding showed, for the example studied here, no dependence on initial oil saturation. On the other hand we found that there is a clear trend in the imbibition oil relative permeability for decreasing Soi, i.e., for a given oil saturation oil mobility increases as initial oil saturation decreases. In other words, laboratory measurements show that the mobility of oil in the transition zone is much higher than conventional analysis would suggest. Consequently, in a given time span more oil can be produced from the transition zone than generally assumed and potentially large volumes of reserves can be added to reservoirs with large transition zones. The impact of the measured relative permeabilities and residual oil saturations on oil recovery has been quantified for a generic field example by numerical modeling using MoReS, the Shell group reservoir simulator. The recovery factor was found to increase from 32% using a single set of relative permeability curves for the whole field independent of initial oil saturation to 56% using the measured Soi dependent relative permeability curves. The water cut at abandonment was for both cases taken at 95%.