Introduction

The oil bearing reservoirs of the Al Shaheen field, Block 5 offshore Qatar were discovered in the mid-seventies in connection with appraisal drilling on the underlying North Field Khuff gas accumulation. However, at the time, development of the Lower Cretaceous Al Shaheen reservoirs was deemed unattractive due to their limited thickness and tight nature.

Development of the reservoirs commenced in 1992, when Qatar Petroleum entered into an Exploration and Production Sharing Agreement with Maersk Oil Qatar AS.

This extended abstract presents highlights of the development and describes some of the technological advances, which have been applied to attain a commercially viable development.

Challenges

The giant Al Shaheen field comprises a series of thin, stacked reservoirs, dominated by tight carbonates with more permeable carbonate facies occurring over a limited part of the field. The reservoir sequence further comprises a very thin sandstone unit of variable quality.

Following the discovery of the Cretaceous oil accumulation in 1974, a number of appraisal wells were drilled by various operators and comprehensive studies were conducted at the time in order to establish the feasibility of development. However, well tests in the vertical appraisal wells yielded discouraging results and it was concluded that development was not economically feasible.

The main problem was that the majority of the oil in-place existed in thin carbonates with low permeability. Combined with a relatively high oil viscosity, the vertical wells were unable to sustain natural flow and gave very low rates with artificial lift. Further, the hydrocarbon distribution was identified to be governed by non-horizontal fluid contacts due to considerable lateral pressure gradients in the liquid phases. This made prediction of the extent of the accumulation challenging.

Another difficulty was that the reservoirs covered substantial areas, and development would require a very large number of platforms, each connecting to a limited oil in-place volume. This was further aggravated by the fact that the individual reservoirs did not overlay completely but rather spread over different areas.

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