Abstract

Petroleum Development Oman (PDO) issued a challenge to the service industry to place a well near the roof of a reservoir without touching the unstable cap shale. The well had to stay in the prolific Zone 1 of the carbonate Shuaiba reservoir to optimize well productivity and minimize the oil left behind after production in the "attic" between the wellbore and the overlying shale. Previous attempts to achieve these objectives had penetrated the mechanically unstable Nahr Umr cap shale. Traditional logging-while-drilling (LWD) propagation resistivity measurements had proven insufficient to steer the wells owing to their lack of directionality and limited depth of investigation. One contractor promised a technological breakthrough-a directional deep LWD propagation measurement that would determine, in real time, the distance between the wellbore and the conductive shale above the well and the distance between the wellbore and the less-permeable Shuaiba Zone 2 below the well. This permitted real-time placement of the well at an optimal distance from the shale.

To maximize the value of this approach, commercial innovation was required to leverage the expertise and capabilities of the solution provider. Three major key performance indicators (KPIs) were identified:

  • Rate of penetration.

  • Exit avoidance.

  • Minimized ceiling-to-wellbore separation.

Based on a risk/reward structure using these KPIs, the provider accelerated technology development and drilled the first well 6 months after the initial challenge.

The results have been very encouraging. Seven wells have been drilled for PDO using this technology, all demonstrating a step change in reservoir navigation and structural delineation capabilities in a wide range of resistivity contrasts.

Introduction

PDO, the primary oil production company in Oman, comprises the Omani government (60%) and a number of private shareholders (Shell, 34%; Total, 4%; and Partex, 2%). PDO currently produces approximately 700,000 bpd from 123 fields. The low-relief carbonate fields of central Oman, termed "pancake fields" (Saih Rawl, Musallim, and Burhaan,), have been developed by drilling producer/injector pairs. Despite the presence of waterdrive, electrical submersible pumps (ESPs) are used to improve oil evacuation.

The Shuaiba carbonate reservoir is composed of a relatively low-porosity transition zone (Zone 2) and a weathered upper interval (Zone 1), which lies unconformably below the mechanically unstable Nahr Umr cap shale. This sequence is shown schematically in Fig. 1. Because the upper interval has been exposed to weathering for an extended period prior to the deposition of the Nahr Umr shale, the surface between them has been karstified and modified to create features that are unrelated to the internal structure of the Shuaiba formation. (A photograph of a contemporary karst surface is shown in Fig. 2.) Consequently, attempts to maintain a distance to the Nahr Umr interface by correlation with internal features in the Shuaiba formation have been unsuccessful. A number of techniques have been developed to drill these pancake wells.

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