Single-String Multi-Zone Completions In The Bahrain Field
Authors: Ameer S.A. , Almoayyed A.K. 
Published: December 2007

Oil in the Bahrain field is produced from 12 zones and natural gas is produced from 6 zones. Gas has been injected into the principal oil producing zones for pressure maintenance since 1938.

Oil well completions between 1935 and 1967 were of two types: singles and duals, in which the latter completion used the tubing-casing annulus to produce the upper zones. The former completion had the problem of a weaker zone's inability to produce fully problem of a weaker zone's inability to produce fully against a stronger zone commingled downhole, while the latter completion had the same problem at surface. In addition, a major disadvantage of the latter completion was the potential of losing upper zones production due to casing failure by external corrosion production due to casing failure by external corrosion in shallow water zones.

Commencing in 1967, a single-string multi-zone technique has been employed in completing new wells and converting the majority of existing wells to multi-zone producers. With this technique, production from each zone is controlled downhole with a production from each zone is controlled downhole with a choke set in a sliding side-door sleeve (SSDS) or a landing nipple. From two to four zones per well have been completed in this manner.

The major advantage of this completion technique is its lower cost compared to that of drilling additional wells or larger-hole wells to accommodate multi-string completions. Another benefit is that weak zones in some wells can be gas lifted with gas from a high gas-oil ratio (GOR) zone without additional investment for surface gas-lift facilities.

Operationally, this completion technique has been successful in the Bahrain Field because of low corrosion from produced fluids and a minimum of downhole sand and scale problems.

Introduction

The total number of wells drilled in the Bahrain Field to the end of 1978 was 313. Of the operating wells there were 230 oil producers, 10 gas producers and 7 gas injectors. One fourth of the oil wells were single zone producers, while the remaining oil wells were single-string multi-zone completions.

A major workover program was initiated in 1967 and continued into 1970 to convert all tubing-casing annulus completions and a number of single zone wells to single-string multi-zone completions. As a result of this program, a significant gain in oil production was realized. production was realized. This paper describes the history of oil well completion practices in the Bahrain Field with emphasis on the single-string multi-zone completion.

BAHRAIN FIELD

The Bahrain Field is located on the principal island of the Bahrain archipelago and covers an area of approximately 18 miles by 9 miles. The structure which forms the field is a highly faulted, slightly asymmetrical anticline on a north-south trend. The general topology of the island conforms to the anticlinal trend.

The main hydrocarbon zones in this field are the Bahrain, Arab and Khuff zones (Figure 1). Natural gas is produced from three Khuff zones (I, II and Ill) and three Arab zones (A, B and C). The Bahrain zones contain the most significant oil reserves and are subdivided into a First Pay (3 zones) and Second Pay (5 zones). As of the end of 1978, they accounted for 96 percent of the cumulative oil produced and 87 percent of the daily oil production produced and 87 percent of the daily oil production during 1978. The Arab D zone contributed essentially the balance of 1978 production. Three other zones, the Magwa, Kharaib and Arab B made a minor contribution to production.