Miscible Flood Performance of the Intisar "D" Field, Libyan Arab Republic
Authors: DesBrisay C.L. , Gray J.W. , Spivak A. 
Published: December 2007

Introduction
Pressure maintenance by both crestal-gas and bottom-water injection was Pressure maintenance by both crestal-gas and bottom-water injection was initiated early in the life of the Intisar "D" reservoir to maximize oil recovery and to conserve produced gas. As of June 1974, cumulative oil recovery amounted to 496 million bbl and no significant breakthrough of injected gas was observed. A reservoir simulator confirms uniform downward miscible displacement of oil and predicts that continued high-pressure gas injection, combined with recompletion of wells, will yield a recovery efficiency of 80 percent.

Introduction
This paper reviews the performance of a combination miscible gas and bottom-water displacement program begun almost coincidentally with first production from the Intisar "D" field. This project is one of the largest miscible gas displacement projects implemented in any reservoir to date, with 300 to 400 MMcf of gas being reinjected daily. Water injection operations were begun in Nov. 1968, 1 month after production started, and crestal-gas injection was initiated in Dec. 1969, when the first of three 40,000-hp gas compressors was commissioned. This early supplemental recovery effort was necessary because recovery of the highly undersaturated oil in this reservoir by primary means alone would have been low.
A compositional reservoir simulator was used to match the 5 years of available performance history. This match was accomplished easily and indicates that a high-pressure, vaporizing gas-drive process, which generates miscibility through interphase mass transfer, is performing very efficiently. This is the result of effective gravity drainage in the permeable and vertically homogeneous carbonate reef and of attainment of a pressure level high enough to assure miscibility. The history match provides the basis for model prediction runs that indicate that continued injection of gas at miscible conditions will result in an ultimate oil-recovery efficiency of about 80 percent of the stock tank oil originally in place in the percent of the stock tank oil originally in place in the reservoir. This is comparable with the high recovery efficiencies being achieved in some Canadian reef reservoirs where crestal miscible gas displacements have been conducted previously.

Discovery and Development
The Intisar "D" field was discovered in Oct. 1967 by the fourth well drilled on Occidental of Libya's 465,000-acre Concession 103. It is located in the Libyan Arab Republic's prolific Sirte Basin, 220 miles south of Benghazi and 525 miles east-southeast of Tripoli (see fig. 1). The field discovery well, Well D1-103, penetrated Paleocene carbonate reservoir rock at a depth of 8,946 ft Paleocene carbonate reservoir rock at a depth of 8,946 ft and found the oil-water contact 888 ft lower, at 9,834 ft. Production tests of this well yielded oil rates of up to Production tests of this well yielded oil rates of up to 75,000 B/D, and its productivity index was calculated to be 423 B/D/psi.
Development of the field was begun in June 1968 and completed in May 1970 with the drilling of 13 producing wells, 16 water injection wells, and seven gas injection wells (see Fig. 2). In addition, 24 shallow-water source wells also were completed. The field went on stream in Oct. 1968, when production-handling facilities were completed.

Geology
The reservoir is almost circular in shape, with a diameter of about 3 miles, as shown in Fig. 2.
JPT
P. 935