M. FURUYA and S. TAKAHASHI, Zakum Development Company (ZADCO)

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Abstract

This paper presents the results and the use of the interpretation of pressure transient data of a horizontal well in a multilayered heterogeneous reservoir. The well was drilled as a pilot horizontal oil producer with objectives to understand both the reservoir characteristics and the mechanism of water breakthrough in nearby producers due to pattern water injection.

Interpretation was first conducted using well test analysis packages, then the analytical results were verified by numerical simulation models.

This paper addresses the test interpretation findings and sets planning guidelines for pressure transient tests of horizontal wells.

Introduction

Upper Zakum is an offshore oil field located about 80 km north-west of Abu Dhabi. The concerned reservoir in this study is a highly heterogeneous limestone reservoir having some interbeds of stylolite streaks. The reservoir is divided into seven geological layers. The reservoir oil is highly undersaturated with 320 API gravity.

ZADCO started oil production from this reservoir late 1982 and water injection late 1983 for the pressure maintenance. The reservoir has been developed mainly by five spot pattern water injection scheme combined with a ring of peripheral water injectors. Early water breakthrough was firstly detected in April 1991 in one of the pattern producers located in the vicinity of a high capacity water injector. Several intensive data gathering plans were implemented to identify the layer(s) responsible for the water breakthrough such as thermal decay logs, multiple formation pressure testers, selective completion, pulse testing, coring and so on. Integration of these data suggested the existence of a high permeable streak in the bottom layer of the reservoir possibly acting as a thief layer (L7). This layer is segregated from the upper layers (layers Li L5) by a tight stylolite in layer 6 (L6). Figure-1 presents typical open hole log results.

In late 1992, a pilot horizontal oil producer was drilled 270 feet apart from the water breakthrough well in order to obtain direct information of water front advancement in the reservoir. Figure-2 shows the pattern where the pilot well was drilled.

P. 637

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