Abstract

This paper describes a creative and pragmatic approach that improved the production from one of the most difficult reservoirs in the Bahrain field. The Middle Cretaceous Ab zone is a thin, tight, highly faulted, and irregularly-fractured limestone reservoir. The difficulty with this 15 ft, 1 md reservoir has prevented an efficient recovery. The average production of wells is 15 bopd. This has prompted a detailed integrated study plan to increase the wells' productivity. Welltesting was key to understand the reservoir dynamics. A comprehensive welltesting campaign revealed a flow mechanism controlled by a fracture network. Fracture modeling and simulation failed to give clues on how to improve the productivity. However, the new approach that links transient welltesting and production data with fracture network indications derived from seismic interpretations has resulted in improving the productivity considerably. This was accomplished through re-entering old wells and designing special trajectories to intersect productive open fractures. The productivity was significantly increased to 60 bopd with sustained performance.

The paper describes in detail our approach and methodology to understand the reservoir and its drive mechanism and to increase productivity and recovery starting from analyzing core data up to designing special configuration wells. The paper further highlights the pitfalls of the conventional workflow approach in modeling such difficult reservoirs.

Introduction

The Bahrain Field lies beneath the center of Bahrain Island, to the east of Saudi Arabia and to the west of the Qatar arch (Fig 1). The Bahrain field was discovered in 1932 and is the oldest field in the southern part of the Arabian Gulf. The broad geological structure of the Bahrain field reservoirs consists of an asymmetrical anticline that is elongated in the north-south (N-S) direction. The fault pattern is characterized by a N-S graben with extensional faults along the crest of the dome. Additional set of later-formed NW- trending faults was seen on the flanks of the reservoir. Geology of the field is complex with a large number of faults occurring in the Wasia group formations, which contain the major oil reservoir of Bahrain field, the Mauddud reservoir. Another important member in this group is the Ahmadi formation, consisting of 2 reservoirs, Aa and Ab. These reservoirs are considered to be some of the most difficult reservoirs in the Bahrain field in terms of productivity.

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