It is estimated that 60% of the world's remaining oil is held in carbonate reservoirs. Due to its moderate permeability, the transition zone can extend over a hundred meters and therefore contain a significant amount of STOIIP. The water-oil displacements behavior is not always well understood, especially when it occurs in the transition zone where capillary effects are dominant and both phases are mobile. The oil trapping and the rock wettability in this zone appear to be two key features to deal with. They must be studied as a function of parameters such as initial oil saturation, oil characteristics, rock properties etc. There is very little experimental data available in the literature that describes these features.
This study focuses on relative permeability and residual oil saturations during drainage and imbibition in carbonate reservoirs.
Steady-state core floods were performed with crude reservoir oil on outcrop limestone cores, some with moldic porosity, over a very large range of initial oil saturations. Cores were aged with crude oil before the imbibition process to allow wettability change at the initial oil saturations. Two main types of limestone have been studied: with unimodal or bimodal pore size distributions.
The two types of limestone cores exhibit very different responses to wettability alteration for the same oil/brine system while almost identical mineralogy. We attributed these differences to the vuggy structure of the Estaillade limestones, which might promote oil-wettability.
-The inspection of the water relative permeability curves show that water wettability decreases as Soi increases, i.e. as the elevation above the contact increases, for the two types of limestones. Therefore it is not correct to derive imbibition scanning Kr curves from the bounding Kr curve at high Soi, while assuming that wettability is constant.
- Hysteresis is observed for both the oil and water relative permeability curves as a function of saturation
- Non monotonic evolution of Sorw as a function of Soi has been observed for the limestone with bimodal pore size distribution. This behavior is ascribed to the combined effect of increasing fraction of micro porosity being filled by oil initially as well as wettability variation as Soi increases. On the other hand, Sorw increases monotonically as Soi increases, for the limestone with unimodal pore size distribution.
- The comparison between the experimental relative permeability curves and the ones derived from simple hysteresis models show that neglecting the variation of wettability along the transition zone leads to erroneous values in oil saturations thus on oil recovery.
In order to reduce the uncertainties in predicting oil recovery of carbonates reservoirs, core laboratory flooding experiments are conducted using representative rock samples and reservoir fluids at the same conditions as in the reservoir (pressure, temperature, wettability).
Numerical models are then developed to best fit the experimental data and to mimic as closely as possible the different scenarios. This is especially important when investigating the transition zone where it has been shown by several authors [1-4] that assumptions made to quantify the recovery factor can lead to erroneous values.
The oil-water transition zone (see Figure 1) is defined as the part of the reservoir between the free-water level (FWL) and the dry-oil limit (DOL)  where the water saturation reaches a near-constant irreducible value. In this zone, capillary effects are dominant and both water and oil are considered to be mobile. Its thickness can be more important (over a hundred meters) for low permeability reservoirs or/and similar water-oil density values. It could then contain a considerable amount of STOIIP.