Practical Methods to Optimizing Production in a Heavy-Oil Carbonate Reservoir: Case Study From Issaran Field, Eastern Desert, Egypt

Abstract

The Issaran field, a heavy-oil reservoir with reserves of approximately 500 MM bbl of oil, was discovered in 1981. The producing horizons were carbonate formations of Miocene age occurring at an average depth of 2,200 ft. The oil being produced on artificial lift was of 9° to 12° API in a viscosity range of 3,000 to 5,000 cp. Before mid-1999, nine wells had been drilled in the field with a cumulative production of about 450 BOPD. This field did not command high priority because of its low productivity and the low prices that heavy crude attracts. However, with the oil price increases in 1999 and 2000, the Egyptian government made a concerted effort toward developing this field. New and applied technology provided considerable returns within 1 year of new investments. The field has been producing as much as 1,800 BOPD (a four-fold increase), from five new wells drilled in the area. Plans were made for thermal recovery in the form of steam stimulation for this heavy-oil reservoir, but first, it was necessary to produce under "cold production." Several industry publications1–3 have addressed cold production, predominantly pertaining to sandstone reservoirs. This paper highlights the experiences, challenges, and practical solutions in optimizing production from a fractured, carbonate reservoir. Several leading-edge technologies and optimization techniques that are effective in less-challenging reservoirs have contributed tremendously to enhancing production from this heavy-oil reservoir. The path to success began with the definition of the reservoir-flow mechanism, optimized perforating schemes, ehnanced carbonate stimulation, and improved completion designs. Introduction Field research1 reveals that most heavy-oil production comes from unconsolidated sands originating from fluvial-deltaic deposition. The successful mode of cold production in such wells has been to produce formation sand along with the oil. The use of well-completion designs that allow such recovery is known as sand management. Rate increases of up to 20 times the sand-free oil-production rate (with no sand control) have been reported.4 Several other nonthermal recovery mechanisms for increasing productivity have been identified, such as foamy oil and horizontal well completions (possible variations into multilateral completions) and hydraulic fracturing. In contrast to fluvial-deltaic sandstones, carbonate reservoirs are generally products of a much more serene depositional environment where processes such as cementation, compaction, and recrystallization of the rock resultin a tight and more rigid matrix. In such reservoirs, sand management techniques are not applicable. However, diagenesis in carbonates can sometimes open conduits to flow in the form of fractures or vugs. The task then becomes to identify such fractures and capitalize on their possible contribution to flow. Background The Issaran field was developed initially with the drilling of eight wells between 1981 and 1987. The Issaran oilfield is a major accumulation in terms of oil in place. It is estimated to have approximately 500 MM STB. The field is located 180 miles southeast of Cairo, and 2 miles inland of the western shore of the Gulf of Suez (Fig. 1, Page 8). The Issaran field is of the Miocene age. The field primarily consists of three oil-bearing reservoirs ranging in depth from 1,000 to 2,000 ft. These are the Upper and Lower Dolomite and the Nukhul formations (Fig. 2, Page 9). More recently, the Nukhul formation was further subdivided into the Gharandal formation. The references made to Nukhul in this paper also include the Gharandal formation, unless otherwise specified.

Enhanced Oil Recovery: An Update Review

“…Oman also announced steam injection project in limestone Fahud Field among other steam pilots [18].  Cyclic steam injection pilot in Issaran heavy oil field, Egypt [183].  Steamflood pilot at the giant Wafra Field, Kuwait-Saudi Arabia [184,185].…”

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“…The reservoir characteristics of the Nukhul formation, which lies deeper than the Upper and Lower Dolomite formations, is quite different from the more shallow ones. It consists of a tight limestone matrix [28]. The oil consists of 10% H 2 S, 10% CO 2 and asphalting content around 15%.…”

“…These large pores, or vugs, have extremely high permeabilities. The Nukhul reservoir accounts for 15% of the reserves for the Issaran field [28].…”

“…Porosity and permeability measurements made on several porosities ranged from 23% to 33%, and permeability ranged from 1.3 to 104 mD. The matrix permeability for the Nukhul formation is estimated to be very low, but the fracture permeabilities exceed several darcies [28]. One of the biggest challenges with this field is that it consists of several zones and each zone has its own unique properties and characteristics.…”

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“…Cold production could be achieved from these reservoirs, but this depended on identifying natural fractures in the field. The primary production occurred from natural fractures within these formations, but a determined effort was made at finding these fractures and applying stimulation techniques (Waheed et al 2001). After properly applied, the production was enhanced to more than 2,300 BOPD in 2002.…”