Prediction of Wettability Variation Within an Oil/Water Transition Zone and Its Impact on Production

Abstract

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.

The Impact of Tides on the Capillary Transition Zone

“…Jackson et al 2005)-higher up in the transition zone the rock becomes more mixed wet whereas our results were obtained for a strongly water-wet sand. This may reduce the influence of the tidal changes on the transition zone by reducing the imbibition of water into the upper parts of the transition zone or conversely, because the tides will have influenced the fluid distribution since the reservoir filled, the tidal influence may affect the change in wettability higher up in the transition zone.…”

“…For this investigation, we decided to approximate the key features of the reservoir rather than exactly reproduce reservoir properties and conditions. For example, real transition zones in oil-water systems may have a vertical thickness of >10 m (Fanchi et al 2002;Jackson et al 2005) whereas in the laboratory, it is impractical to pack a column of this height. In the reservoir, oil-water gravity drainage will occur over millions of years after reservoir filling whereas the objective of this investigation was to observe the interaction of gravity drainage and tidal pressure variations over a period of several months.…”

“…In practice, however, the saturation versus depth inferred from log data is often very different from that based on capillary pressure curves measured in the laboratory (Fanchi et al 2002;Masmaleh et al 2007). Often this difference is ascribed to formation heterogeneity and changes in wettability with depth and saturation (Fanchi et al 2002;Jackson et al 2005;Masmaleh et al 2007;Bunn et al 2010). …”

“…Parker and Rudd 2000;Fanchi et al 2002;Jackson et al 2005) as a result of capillary gravity equilibrium (Dake 1983). This zone can contain significant volumes of oil, particularly in lower permeability formations.…”

Two-Phase Flow in Porous Media with Slip Boundary Condition

“…17) both have the structure of pressure-driven flow. Single-and two-phase flow in porous media are often described in pore-network models (Hui and Blunt 2000;Blunt 2001;Jackson et al 2003;Nguyen et al 2006) where pore bodies are connected via tubes in which the fluids flow. Therefore we draw an analogy between the flux increase in a capillary tube model with a slip boundary condition and k r > 1 in a porous medium.…”