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Abstract

AAPG Bulletin, V. 94, No. 1 (January 2010), P. 27–60.

Copyright copy2010. The American Association of Petroleum Geologists. All rights reserved.

DOI:10.1306/06160909012

The function of fractures and in-situ stresses in the Khuff reservoir performance, onshore fields, Saudi Arabia

Mohammed S. Ameen,1 Ismail M. Buhidma,2 Zillur Rahim3

1Structural Geology and Rock Mechanics Group, Geological Technical Services Division, Exploration Technical Services Department, Saudi Aramco, P.O. Box 2817, Dhahran 31311, Saudi Arabia; [email protected]
2Hawiyah Unit, Reservoir Management Department, Saudi Aramco, Saudi Arabia
3Uthmaniyah, Shedgum and Special Studies Unit, Reservoir Management Department, Saudi Aramco, Saudi Arabia

ABSTRACT

The highly variable performance of the Permian–Triassic Khuff reservoir in onshore Saudi Arabia has been attributed to the presence of natural fractures. Similar preproduction pressure profile and hydrocarbons in the different reservoir units in some fields have been attributed to vertical communication through large faults. To validate these assumptions, we studied the static and dynamic data from the Khuff reservoir in 19 major structural traps. We identified two distinctive fracture domains based on fracture orientation and density. Fracture evolution is mainly controlled by the extensional and consequent compressional plate tectonics instead of local structures. In-situ stresses are dominated by the Zagros plate tectonics and affect fracture aperture differently in the two fracture domains. The fracture impact on the Khuff reservoir performance is mostly subtle because of the nature and distribution of the fractures. High fracture-enhanced productivity occurs locally in some of the producing wells, and it results from high-density fracture clusters (including mesoscopic faults) with channel-type apertures.

The following findings challenge the perceived major functions of fractures in the Khuff reservoir performance in onshore fields: (1) Individual fractures are dominantly tensile and small (mesoscopic and microscopic); (2) individual faults are small and not readily resolvable at seismic scale; (3) the depth and carbonate nature of the Khuff reservoir make the fractures highly susceptible to fast healing unless preserved within the hydrocarbon column; (4) initial vertical pressure gradient changes with production indicate a lack of present-day communication across the anhydrite sealing layers, between the different Khuff reservoir units; (5) horizontal well direction does not generally have an impact on productivity; and (6) sustained and heavy losses of circulation are rarely encountered in the Khuff reservoir wells.

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