Abstract

In Yibal, a mature oil field in Oman, a number of optimisations have been implemented as a result of constantly challenging historically accepted procedures and by aligning the objectives of PDO, the Drilling Contractor and Service Contractors through the use of incentives. Although these may not include any step changes in technology, in combination they have realised a 25% reduction in drilling costs over a 2 year period (see fig. 1).

Introduction

The Yibal field, situated in the desert of North Oman, was discovered in 1962. The main oil reservoir is located within the Upper Shuaiba formation at 1370m TVD. The oil column attains a maximum thickness of 100 m with no primary gascap present and hydrostatic pressures. The STOIIP is estimated at 550 million m3 with an expected ultimate recovery factor of 55 %.

Since coming on production in the early 1970's the field has produced approximately half of its ultimate recoverable reserves but it is still responsible for almost one third of the current production target of 135,000 m3/day.

Since the middle of 1994 additional horizontal drainage points in the field have been provided by drilling side-tracks instead of new wells (see figure 1), and with 438 wells present in Yibal and the rapid production rate decline there are many side-track candidates available. Any new Yibal well requires 5 or 6 casing strings (due to the presence of shallow gas, artesian flow, requirement for sweet aquifer isolation, and isolation of a gas reservoir) and costs in the order of US$ 1.6 million with an average duration of 30 days. In comparison, a side-track takes on average 18 days and costs just under US$ 1.0 million. In the first half of 1996, 15 horizontal sidetracks were drilled, each producing initially 500-1000 m3/day.

Although the wells are shallow their complexity is increased by the horizontal design and the frequent presence of total losses and unstable shales. In this environment of repetitive but complex and very intensive work, it is a continuous challenge to obtain increased optimisation of the drilling operation. However this challenge must be undertaken in order to reduce or even maintain the UTC per barrel of oil for PDO.

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