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%.
Oil in transition zones has, in water-wet rocks, the tendency to fill the larger pores preferentially. As oil tries to enter a pore (originally water-wet) a certain threshold pressure has to be built up before the capillary pressure in the pore can be overcome and the oil can actually enter the pore. In the smaller pores the capillary effect is stronger and therefore higher pressure differentials are needed for oil to enter these pores, as a result larger pores are filled most easily. The largest pore throat will determine the minimum capillary rise above the free water level. The transition zone is the part of the reservoir where the saturations grade from 100% water in the water zone to an irreducible water saturation some vertical distance in the reservoir above the free water level, (Figure 1). In this interval both oil and water are mobile. Transition zone may vary in thickness from a few meters to over hundred meters and therefore it may contain a sizable part of the reservoir STOIIP. Usually the transition zone is not perforated for oil production because it is considered not economic. However, in the transition zone where only the largest pores are oil filled the relative oil mobility at a specific saturation is larger than at a similar saturation higher up in the reservoir where also small and poor conducting pores are oil filled. It has indeed been shown in this study that large volumes of oil can be produced from transition zones.
Measurements of residual oil saturations and associated water-oil relative permeabilities as a function of initial oil saturation have rarely been performed in the past because they are difficult and time consuming. Therefore, in reservoir simulations only estimates of the oil mobility as a function of depth have been used. In fact generally two approaches are used: The first and most common one, uses residual oil saturations and imbibition relative permeability curves that are measured by starting at the maximum initial oil saturation (Soi). In this case a possible dependency of residual oil or relative permeability on Soi is completely ignored and the recovery factor is highly under estimated. The second one is more sophisticated and does use a dependency of residual oil (i.e. Land model ) and relative permeability (i.e. Killough hysteresis model ) on initial oil saturation. However, these models have serious shortcomings:
initial/residual saturation correlation lack proper experimental data as only gas/water or gas/oil data are available,
all models for relative permeability dependency on initial oil saturations are either conceptual or based on very little data,
the models do not distinguish between rock types and
wettability characteristics of the rock are ignored as they assume water wetting .