Abstract

Carbonate reservoirs are generally highly heterogeneous and complex because many phenomena, such as diagenesis and tectonism have modified primary depositional structure and texture. Many carbonate reservoirs in the Shuaiba Formation of north Oman are characteristically heterogeneous with complex reservoir architecture. It is common to observe dual-media and natural fracture system behaviors with large variations in reservoir properties. Many of these reservoirs have been developed with horizontal wells, so they require application of advanced reservoir evaluation and management practices. Many of the horizontal wells produce at high water cuts, despite apparent initial high oil saturations. The need for data is paramount for effective management of these reservoirs, and to understand factors affecting the productivity of horizontal wells - both at the wellbore and within the reservoir itself. The first stage of this process is to define an accurate structural model incorporating all available reservoir, wellbore (2D modeling) and seismic data. The second stage, involves integrating this model with the inflow profile derived from advanced production logging measurements. We describe several field examples from a complex and heterogeneous carbonate reservoir in Oman, and discuss how they may be used to identify a range of important needs relating to horizontal well placement, improving completion strategy, and optimizing water shut-off activities.

Introduction

Short to long radius horizontal wells have been drilled in many types of oil and gas reservoirs, in both carbonate and siliclastic formations. A variety of technical advances in drilling, including logging while drilling and formation evaluation have meant that the application of horizontal wells for reservoir development in both primary recovery and enhanced oil recovery (EOR) applications is now a common practice. There are many options available for completion of horizontal wells, such as open hole (barefoot), cased hole (e.g. cemented, expandable and perforated liner), slotted liners and gravel-pack completions. In the Middle East and North America a large proportion of horizontal wells have openhole completions. Since horizontal wells have a much greater reservoir contact area than vertical wells, for reservoirs with lateral heterogeneity, fault and fracture networks or structural compartmentalisation, this can result in well performances that range from the spectacular to the very mediocre.

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