Abstract

The Yibal carbonate reservoir in North Oman is developed using horizontal wells. The completion technique has evolved from open hole to cemented and perforated liner. In parallel, the stimulation technique developed from spotting small volumes of acid across barefoots to selective acid treatment of perforated liner sections.

This paper describes the completion and stimulation techniques and compares their production performance based on a 55 well data set.

Introduction

The Yibal field, operated by Petroleum Development of Oman (PDO), is situated in the North of Oman, 300 km southwest of Muscat. The field came on stream in 1969 producing from the Shuaiba Carbonate of the Lower Cretaceous. Yibal Shuaiba is the largest producing reservoir in Oman: to date 175 Million m3 have been produced (30% of STOIIP).

Yibal produces more than 32,000 m3/d of light, low viscosity crude (40 deg API, 0.6 cp) with an average water cut of 56% from 260 vertical and 60 horizontal gas lifted wells. The horizontal wells contribute more than 60% to the current production. Compared to the vertical wells, the horizontals provide better reserves recovery and higher production at substantially lower cost.

The domal structure chalky limestone Shuaiba reservoir is characterized by complex faulting systems and limited lateral fluid movement. The permeability ranges from 1 to 100 mD. The field was initially developed by depletion soon followed by lateral water flooding, subsequently converted to pattern bottom water flooding after ten years. Conductive features, mainly faults, charge the overlying reservoir with water creating so-called water fingers. Horizontal producer completions aim to avoid and isolate such conductive features and water fingers to minimize water production and maximize oil production. Note that low water cut production has the added benefit of reducing lift gas requirements and increasing effective facilities ullage.

This paper describes the development of the Yibal horizontal well completion and stimulation techniques. The production performance of the various completion and stimulation types is presented and analysed.

Horizontal Well Completion

Yibal horizontal well completions have evolved from barefoot to cemented and selectively perforated liners as described below and summarized in Table 1. Hassan and Abu-Sharkh describe a similar development in Abu Dhabi [1].

Barefoot.

Early horizontal wells were completed barefoot for low initial well costs and simplicity as well as to gain experience with their production behaviour. The horizontal section length was on average around 780 m; a tapered 4 1/2" - 3 1/2" gaslift tubing string with a completion packer was run. Once sufficient experience had been gained with drilling 8 1/2" horizontal sections, the design was slimmed down to a 6 1/8" reservoir section (Figure 1).

Barefoot.

Early horizontal wells were completed barefoot for low initial well costs and simplicity as well as to gain experience with their production behaviour. The horizontal section length was on average around 780 m; a tapered 4 1/2" - 3 1/2" gaslift tubing string with a completion packer was run. Once sufficient experience had been gained with drilling 8 1/2" horizontal sections, the design was slimmed down to a 6 1/8" reservoir section (Figure 1).

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