This document is an expanded abstract.

Summary

The capillary transition zone (TZ) in carbonate reservoirs can contain very large amounts of oil and it is essential to predict saturation profile versus depth to estimate the oil in place volumes in order to design oil recovery schemes. However, the characterization of TZ is extremely difficult, because it may vary in thickness from a few feet in high permeability reservoirs to over hundreds of feet in some low permeability, thick and tight reservoirs. Previous works mainly focused on lab measurements which can provide, relatively, accurate rock properties for reservoir study, but from the lab measurements, three-dimensional (3D) information on rock microstructures hardly can be known, which is very important for a better understanding of physical phenomena and fluid flow behaviour. Digital rock analysis (DRA) can directly image rock microstructures from nano scale to millimeter scale and accurately predict rock properties such as porosity and permeability. Therefore, it can be used to better characterize the heterogeneity and evaluate the properties of capillary TZ rock samples. In this paper, we use lab measurements and rock images, including thin section (TS) images and scanning electron microscope (SEM) images, Micro computed tomography (MCT) scans, to characterize the heterogeneity and estimate TZ rock properties. By analyzing these rock images, lab measurements and simulations, the heterogeneity and rock properties of TZ are well characterized.

Introduction

The transition zone (TZ) of a hydrocarbon reservoir is a mixing zone between immiscible oil and water. It usually produces both oil and water in different proportions. Specifically, transition zone of a carbonate reservoir is the depth interval from where oil and water mixture is produced at the zero time intervals after starting of the production. Newly reported incidents of dry oil production from TZ, augmented by the fact that oil is thought to be more mobile in these zones, makes it very interesting to overturn the conventional wisdom and start thinking seriously of finding a way to optimize oil recovery from TZs which contain very large amount of oil (Bera and Belhaj, 2016).

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