Abstract

The Intisar 'D' reservoir is a carbonate reef with a maximum thickness of approximately 1000 feet (305 meters). The reservoir was developed in late 1960's with bottom-water injection for pressure maintenance and crestal gas injection for EOR and gas conservation. The project is one of the largest EOR projects implemented in any reservoir to date. As of December 31, 1988, cumulative oil production was 1043 MMSTB (166 MM-m3) or 62% of the original oil-in-place.

This paper reviews the reservoir performance. Slim tube test results and the various gas compositions injected into the reservoir are presented. Gas-oil and oil-water contacts and their distributions are reported, Recovery efficiencies in the water and gas-swept zones are analysed and compared. The best production and injection strategy to optimize the oil recovery at the final stage of depletion is outlined.

Better performance was observed in the gasswept zone of the reservoir, therefore the feasibility of extending the current high pressure gas injection scheme to the waterswept zone is discussed.

Introduction

The Intisar 'D' field is located in Concession 103 of the Socialist People's Libyan Arab Jamahiriya (Fig.l). The reservoir was discovered in October 1967 and placed on production in October 1968. The reservoir is highly under-saturated with an initial pressure of 4257 psia (29351 kPa) and a bubble point pressure of 2244 psia (15472 kPal).

The bottom water injection, started almost simultaneously with the first production, was accomplished by using lower dump flood wells, and later supplemented with surface injectors. In December 1969, a high pressure crestal gas injection program was implemented to improve the oil recovery. Better performances were observed in the gas-swept Zone (GSZ)., therefore water injection was completely terminated after September 1981.

As of December 31, 1988, cumulative oil, water and gas production were 1043 MMSTB (166 MMm3), 89 MMBBL (14 MMm3[) and 1567 BCF (44391 MMm3), respectively. Water and gas injections totalled 677 MMBBL (108 MMm3) and 2152 BCF (60963 MMm3).

In this paper, major results from recent studies on performance interpretation, production optimization and water-swept zone (WSZ) EOR feasibility evaluation are presented.

Field Development

Planning and development of the Intisar ‘D’ reservoir were discussed extensively in early papers (Refs. 1–5). During initial development 36 wells were drilled, of which 13 were original producers, 16 were water injectors and 7 were gas injectors. Development drilling was completed by May 1970.

The majority of the early producers were cornpleted openhole with 9 5/8" casing set at or near the top of the reef. Since early 19705, 7"liners were landed in most of the producers to control the producing GOR. Between 1970 and 1984, eight wells were converted from water injectors(or shut-in water injectors) to producers. At the end of July 1980, control of well D9 was lost during a workover. At that time the field was temporarily shut-in for about a month for safety reasons, and efforts to kil1 well D9 were not completed successfully until January 1980. Currently, there are twenty producers and six gas injectors active in the field.

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