Abstract

This case history develops through the drilling of a relatively underexplored and less promising limestone formation in the Gulf of Suez area of Egypt. The target formation, known as Thebes of Eocene Age, was "faulted" and the two separated pay sections presented different rock compositional scenarios. Even though the geological description broadly classified these zones as carbonates, the stimulation treatments designed for the two zones were totally different. The process that was undertaken in determining the best solution was based on the introduction of some novel analytical techniques and a com-prehensive approach to acidizing-treatment design.

This paper details some of the new approaches that were used, including a composite log developed for carbonate-stimulation design. The emphasis on reservoir characterization in terms of its petrophysical properties and production capability is paramount in carbonates because of the extreme reservoir heterogeneity and a general noncorrelation between rock porosity and permeability. Furthermore, a new matrix acidizing simulator was run with input data from the composite log for modeling fluid distribution and skin evolution from the treatment. On the basis of these simulation runs, a couple of preemptive measures were taken to improve the treatment-fluid distribution profile.

Bottomhole pressure gauges in conjunction with real-time monitoring of skin evolution during the treatment were analyzed for determining the effectiveness and applicability of foamed treatments, in-situ crosslinked acid systems, and maximum pressure differential and injection rate (MAPDIR) techniques.

The post-stimulation production results and pressure-buildup surveys confirmed the success and effectiveness of the stimulation treatments.

Background

The Thebes formation is of early Eocene Age in the Zeit Bay field (Fig. 1) of the Gulf of Suez, Egypt. The field is mainly a horst block, bounded to the east and west by two normal faults, further intersected by strike-slip faults. Depending on the location of the well, the Thebes formation can occur at varying depths between 3,500 and 4,400 ft true vertical depth (TVD), or between 3,800 ft and 6,000 ft (MD KB). The formation thickness also varies widely, reaching more than 600 ft in some areas of the field.

Geisum Oil Company first evaluated 1 the Thebes formation in 1998 as a secondary target while it was exploring for oil in the deeper Upper Cretaceous sandstones and fractured granitic Basement. On this exploration well (Well A-8), about 527 ft TVD of pay was encountered that failed to produce in its original completion. subsequently, a small acid treatment was performed, and a short flow test was conducted on the well. The production estimates were 600 to 700 BOPD of 20° API oil. On the basis of volumetric calculations, the reservoir was estimated to have 5.7 million stock tank barrels (MMSTB) of oil-in-place with possible recoverable reserves of 1.1 MMSTB of oil. A new well (Well B-1) was drilled the following year for evaluating and developing the Thebes reservoir. In drilling and completing this new well, the operator encountered the Thebes formation 450 ft shallower than expected, while the intermediate-hole section of 12¼ in. was being drilled. The drilling prognosis at that stage was that the formation had "faulted," and a repetition of the Thebes existed at a deeper depth ( Fig. 2). The operator confirmed this theory when he encountered the Thebes formation again 440 ft deeper. The two sections were designated as Thebes I (the upper zone) and Thebes II (lower zone) with net pays of 195 ft and 163 ft, respectively.

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