Planning of an Appraisal/Development Program for the Complex Khuff Carbonate Reservoir in the Yibal Field, North Oman

Abstract

In 1986, oil was discovered in the Permo-Triassic Khuff carbonates of the Yibal Permo-Triassic Khuff carbonates of the Yibal Field in North Oman. This formation which covers extensive areas of the Arabian Peninsula and Arabian Gulf, had hitherto never Peninsula and Arabian Gulf, had hitherto never tested oil and the Yibal Khuff accumulation is therefore unique in the region. Dry but sour oil rates of up to 1 200 m/d [7 550 bbl/d] were tested at a depth of 3 036 m [9 960 ft]. Subsequent appraisal wells, however, revealed the reservoir to be highly complex and dramatically downgraded initial expectations of an early, large scale development. This paper describes the stepwise approach adopted to meet the dual objective of better defining the reservoir whilst at the same time developing the reserves in a cost effective way. To this end, an appraisal/development plan has been designed consisting of the installation of limited liquid and gas handling facilities, further appraisal drilling, coring and extensive production testing. The synthesis of production test, reservoir geological and petrophysical interpretations has allowed the construction of a conceptual reservoir model consisting of a prolific, fractured crest where early gas breakthrough hampers the oil productivity and marginal, low permeable flanks. Areas of non-producible oil permeable flanks. Areas of non-producible oil have also been identified. Introduction The Permo-Triassic Khuff Formation forms part of a ramp type carbonate wedge thickening from less than 300 m [1 000 ft) on the Arabian Shield towards the NE into the present Arabian Gulf where more than 1 000 m [3 300 ft) have been recorded and major gas-bearing reservoirs have been discovered. The sequence, (total average thickness of 865 m [2 840 ft) at Yibal), may be subdivided into five zones, K1-K5, of which the top three (K1-K3) form the subject of this paper. The base of each of the K1-K3 zones consists of a tight, non-reservoir, lime-mudstone deposit of shallow marine/tidal flat origin. The overlying sequence is dominated by oolitic grainstones deposited as oolitic bars in a high energy, shallow subtidal environment. The Khuff reservoir of the Yibal field (Figs. 1 and 2) was discovered in 1977 by well Y-85 which tested sour gas from the K2 unit of the Khuff formation at a depth of ca. 2 910 m [9 550 ft)- The presence of oil was not identified and, hence the gas was classified as non-associated. Y-192, drilled end 1985 as a gas exploration well, tested oil from the K2, K3 and K4 Khuff units, gas/condensate from the K1 and K5, and volatile oil from the deep Gharif sands at ca. 3 940 m [12 900 ft]. A dry oil rate of ca. 1 200 m/d [7 550 bbl/d] was obtained while testing the K2. The crude is sour with H2S and CO2 contents of ca. 4 and 6 mole% in the gas phase respectively. As a result, the K1, K2, K3 reservoir was re-classified as a saturated oil reservoir with a gas cap. P. 631

Development of mature oil fields — A review

“…In 1989, an appraisal strategy consisting of the evaluation of the liquid-gas handling facilities, appraisal drilling, and western area development was developed. It was concluded that the initial conceptual reservoir model might not be representative for the total structure (Bos, 1989). Eckford (1999) proposed an optimal plan for the surface facilities to prepare the field for the next 30 years.…”

A laboratory feasibility study of dilute surfactant injection for the Yibal field, Oman

“…After an extensive field scale reservoir simulation study, Blaskovich et al (1985) suggested a pattern type waterflooding scheme for the field. Later, Bos (1989) and Eckford (1999) evaluated the existing field performance and recommended long term development scenarios. Waterflooding performance during different periods of the production was also evaluated and the results were published (Grant, 1981;Mugheiry et al, 2001).…”

The late Permian carbonates (Khuff Formation) in the western Arabian Gulf: Its hydrocarbon parameters and paleogeographical aspects

“…An alternativesourcemaybe the thermalconversionof oil to gas as proposedby Gronau (1977).However, if Grunau's (1977) mechanism is correct, the composition of the gas might be expected to change due to an increase in temperature if thermal cracking occurred much below 5300 m with hydrocarbons beginning to be replaced by~S, CO 2 , and N 2 • This does not appear to be the casefor the Khuff gas in the Arabian Gulf region, whose analytical results were publishedby Janahi and Mirza (1991), Bos (1989), Kasnick and Engen (1989) and El Bishlawy (1985). They show in general that methane, the principalgas, amounts to as much as 89% of the total gas content,with smallamountsof carbon dioxide, nitrogen,and heavier hydrocarbon compounds.…”

“…The exploratory appraisal well Y-230, drilled in 1987, established the fluid contacts in the eastern part of the field and tested oil at the rate of 1,250-1,890 barrels per dayin the lowpermeability K2 bed.WellY-236,drilled near the end of 1987, foundgas in Kl and the upper part of K2. The lowerpart of the K2 bedand K3 werefound to bein the oil column (Bos, 1989).…”

“…Porosityisprimarily Bos(1989)reported that gas was first discovered in oomoldic, partiallyenhancedbyintercrystalline porosity, and 1977 in well Y-85 from the K2 bed. Gas was next found in permeability ranges from less than 1.0 to more than 100 md 136 1(XD Figure 8.…”