Abstract

A pattern waterflood based on dual lateral horizontal injectors is planned for the Saih Rawl Shuaiba reservoir in order to restore its pressure and increase the oil recovery. The dual lateral design is utilized to reduce field development cost and accelerate water injection.

This paper discusses the innovative drilling of five of such wells, a first in PDO, as part of the waterflood pilot project. Each lateral is drilled 'S' shaped in order to achieve a tuning fork profile. The paper covers well design, actual experience including directional drilling challenges and solutions, and the novel process of selective re-entry into the legs that does not require a permanent downhole mechanical depth reference.

The five wells have been completed successfully and at some 30% cost savings per dual well relative to two single laterals. Based on the length of exposed reservoir, the dual lateral cost per metre was 75% cheaper than a single lateral design. On the basis of the excellent overall project economics using this dual lateral technology and water injection results, a full field development comprising more than 200 km of horizontal well section is planned.

Introduction.

Horizontal well technology is being applied to several of PDO's fields such that more than 90% of development wells are drilled as single lateral horizontal wells annually. One of the fields is Saih Rawl in its Shuaiba reservoir. Resulting from the more than ten fold increase in the production rates per well using horizontal well technology in this reservoir, horizontal single lateral producers are drilled on a continuous basis for the last 3 years to develop the Shuaiba reservoir. At present, about 27 horizontal producers have been completed in the Shuaiba.

Previous in-house study on well cost reduction have identified the cost saving benefit of multi-drainhole as the next step in horizontal well application in the Saih Rawl development in 1993. In order to achieve this, a multidisciplinary team was set up to investigate the technology requirement for the Saih Rawl development and broader application in PDO. Due to the rather rapid decline in Shuaiba reservoir pressure with the many horizontal producers, multi-drainhole technology was introduced as a strategy to accelerate water injection while reaping the expected cost savings.

Therefore a programme of five horizontal dual lateral injectors was established as a waterflood pilot project to test the concept of pressure support. This drilling campaign was part of the introduction of multi-drainhole technology in PDO.

This paper describes the innovative drilling of these wells which is a first in PDO. It covers well planning, implementation, innovations and the challenges of being a pioneer. The results of the campaign are presented. The expected cost saving over single lateral was achieved and the five wells completed on time and within budget. An overview of a novel selective re-entry technique that was made in course of this project is also presented in the paper.

This paper concludes with recommendations on the design of a new whipstock system. Based on the good drilling performance in conjunction with actual effective pressure support, the field development plan has adopted the line drive water injection scheme using multi-drainhole technology.

Geological Overview

The Saih Rawl field in North Oman was discovered in 1971. The development of one of the productive zones, the Shuaiba limestone, is the focus of this paper. The Shuaiba reservoir is some 60m thick with maximum oil column of 32m

The overburden of Aptian (Lower Cretaceous) Shuaiba consists of tertiary and cretaceous deposits. The lithological sequence consists of 1350m carbonates (UeR, Natih) and shaly strata (Shammar, Shargi). The Albian Nahr Umr shale is located directly above the Shuaiba and seals the reservoir. Shuaiba itself is mainly a porous wackestone with an average porosity of about 25% and permeabilities ranging from 2 to 10 mD.

Field Development Plan

Up to the early 90's, the Shuaiba was developed with few vertical wells that suffered from early water coning due to the rather thin oil column. This situation made the field an attractive target for horizontal drilling with wells located at the very top of the reservoir. Developing the field by horizontal wells commenced in 1993 and was an immediate success. Well bore (horizontal barefoot) lengths up to 1871m have resulted in a productivity improvement from 40 for vertical producers to horizontal well initials above 700 m3/d.

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