Prediction of Wettability Variation Within an Oil/Water Transition Zone and Its Impact on Production
Authors: Jackson Matthew D. , Valvatne Per H. , Blunt Martin J. 
Published: April 2006

Summary
We use a pore-scale network model in conjunction with conventional reservoir-scale simulations to investigate wettability variation within an oil/water transition zone. If the initial water saturation within the transition zone is controlled by primary drainage, we predict that initial production behavior is the same regardless of wettability. However, if the initial water saturation has been modified by movement of the free water level(FWL) following reservoir filling, then both the initial water saturation and production behavior are different depending upon wettability. In this case, the wettability of the reservoir may be estimated using in-situ measurements. Moreover, wettability variation may yield anomalous dry oil production from the transition zone. Over longer production timescales, wettability variation can result in high displacement efficiency during waterflooding. Assuming that the reservoir is uniformly water-wet or oil-wet, or using empirical hysteresis models, leads to a significant underestimate of recovery.

Introduction
Many oil reservoirs contain a significant transition zone, in which fluid saturations and production characteristics vary with depth.1-5 Typically, the top of the transition zone contains oil in the presence of connate (immobile)water, while the base of the transition zone is fully water saturated. Within the transition zone, both oil and water are usually mobile. The transition zone may vary from a few meters to several hundred meters in thickness and contain a significant proportion of the oil in place. The transition zone often exhibits variable wettability, with the most water-wet conditions found at the base and the most oil-wet conditions at the top.4–9The wettability of a crude oil/water/rock system depends upon factors such as the mineralogy of the rock, the composition of the oil and water, the temperature, and the initial water saturation.9–15 Wettability variations within a transition zone are principally controlled by the increase in water saturation with depth.4-9 At the top of the transition zone the oil saturation is high, so more pores and throats are contacted by oil. These pores and throats can become oil-wet if surface-active components such as asphaltenes within the oil are adsorbed onto the mineral surfaces.9–15At the base of the transition zone the pores and throats are not contacted by oil, so they remain water-wet. The reservoir is least water-wet (most oil- or mixed-wet) at the top of the transition zone, becoming progressively more water-wet with depth as the water saturation increases.4-9
Wettability can have a significant impact on flow during oil recovery, and upon the volume and distribution of the residual oil.16-19 However, the impact of wettability variation associated with an oil/water transition zone is poorly understood. Masalmeh4 measured oil relative permeability in cores that had been aged at different initial water saturations. He found that the oil relative permeability at a given water saturation increased with initial watersaturation (increasingly water-wet conditions).Based on these findings, he suggested that oil may be more mobile toward the base of a transition zone, yielding higher oil recoveries than conventionally predicted. However, he did not measure the variation in water relative permeability with initial watersaturation (and therefore wettability), which is much more significant and can have a profound influence on waterflood efficiency.20Parker and Rudd5suggested that wettability alteration may yield anomalous dry oil production from a transition zone, but provided only a qualitative explanation of the pore-scale mechanisms responsible.
The aim of this study is to investigate and predict the effect on production of wettability variation associated with an oil/water transition zone, using a pore-scale network model in conjunction with conventional reservoir-scale simulations. We use a 3D network model, which combines a physically based pore-scale model of wettability alteration21 with a network representation of a Berea sandstone.22,23 The network is reconstructed directly from a sample of the sandstone, so the pore-size distribution and coordination number are fixed and are not "tuned" to match experimental data. We demonstrate that this network model can successfully predict experimental relative permeability data for water-wet Berea sandstone24 and waterflood recoveries for mixed-wetBerea.19We therefore have confidence in its ability to capture and predict the effect of wettability alteration on relative permeability and capillary pressure. The network model is a tool that allows us to investigate wettability variations much more quickly and efficiently than laboratory experiments.
We begin with a detailed analysis of initial production behavior from a transition zone. We find that, if the initial water saturation is governed by the balance of gravity and drainage capillary forces, wettability variation makes no difference to production behavior, because this is dictated by the drainage relative permeability curves. These are the same irrespective of wettability alteration following oil migration into the reservoir. However, if the FWL migrates upward following wettability alteration (for example, because of leakage of hydrocarbons from the reservoir), then the initial water saturation is governed by the balance of gravity and waterflood capillary forces, and production behavior is dictated by the waterflood relative permeability curves. These depend upon the wettability of the reservoir. Consequently, the initial water saturation and production behavior is different depending on whether wettability alteration has occurred. In this case, the wettability of the reservoir may be determined from in-situ measurements. Moreover, wettability alteration may yield anomalous dry oil production from the transition zone.
We then perform a simulation study to investigate longer production timescales and find that wettability variation can result in high displacement efficiency during waterflooding. Assuming that the reservoir is uniformly water-wet or oil-wet, or using empirical hysteresis models, leads to asignificant underestimate of recovery.