OIL RECOVERY IN THE TRANSITION ZONE OF CARBONATE RESERVOIRS WITH WETTABILITY CHANGE: HYSTERESIS MODELS OF RELATIVE PERMEABILITY VERSUS EXPERIMENTAL DATA

摘要

Due to its moderate permeability and/or very similar oil–water densities,the oil-water transition zone can extend over a large height and therefore contain a sizable amount of STOIIP. In the literature there is very scarce experimental data available for the oil-water system describing a drainage-imbibition process in the transition zone and practically none of them takes into account the variation of wettability on relative permeabilities. Most of the hysteresis models are based on simple extrapolations and do not incorporate wettability changes along the transition zone which is a key point especially in carbonate reservoirs. In a previous study [1] we performed steady-state core floods experiments with crude oil/brine on limestone cores over a large range of initial oil saturations and observed that wettability varies with height above the oil-water contact and has a strong impact on both oil and water imbibition relative permeabilities. Moreover we showed that there is no unique relationship between initial and residual oil saturations while the most used hysteresis models([2],[3],[4])are based on the same Land’s residual versus initial oil saturation relationship. The most sophisticated models incorporating wettability,such as Skjaeveland’s relative permeability hysteresis model [4] show better predictions but still need a lot of inputs that are not always available at laboratory scale. In this study,we compare our experimental data with the most used hysteresis models of relative permeability and we estimate the uncertainties on predicting oil recovery. We also present a new Kr hysteresis model,using the bounding Kr(relative permeability)curves and incorporating wettability change,which best fits our experimental data.