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

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. Mohammed S. Ameen received his Ph.D. and Diploma of Imperial College in structural geology and geomechanics from Imperial College, London, 1988. He has more than 20 years of academic and industrial experience and has patented a new method for the characterization of microfractured reservoirs. He has published 25 articles on fractures and folds, and has edited three special publications for the Geological Society (London). He conducted the first classic work on the fractures and folds across the Taurus-Zagros Range, Iraq, covering 30 major fold traps. The work is published in the AAPG Bulletin, the Geological Magazine, and the Journal of Petroleum Geology. He joined the Reservoir Characterization Department at Saudi Aramco in 1998. Since 2004, he has been leading the Structural Geology and Rock Mechanics Group in the Geological Technical Services Division, Saudi Aramco. He is an active member of the AAPG, Society of Petroleum Engineers, European Association of Geoscientists and Engineers, and the Geological Society (London). Ismail M. Buhidma is a petroleum engineering consultant in the Gas Reservoir Management Division of Saudi Aramco with 32 years of diverse industrial experience. For the last 12 years, he has been actively involved in Aramco's nonassociated gas development program. His areas of interest include reservoir management, well performance analysis, well test analysis, phase behavior, geomechanics, reservoir simulation, reservoir characterization, and well stimulation. Prior to joining Aramco, Ismail worked for Exxon in Libya, Schlumberger and Atlantic Richfield in the USA, and Qatar Petroleum in Qatar. Ismail holds B.S. and M.S. degrees in petroleum engineering from the University of Tulsa. He is a member of Society of Petroleum Engineers (SPE) and has published numerous SPE appears. Zillur Rahim is a petroleum engineering consultant with Saudi Aramco working in gas field development. He received his B.S. degree from Algerian Petroleum Institute and M.S. degree and Ph.D. from Texas A&M University, College Station, all in petroleum engineering. He has 25 years of industry experience, has published more than 50 technical papers, and has taught numerous industry courses. Previously, he worked with Holditch and Associates Petroleum Consultants and with Schlumberger Reservoir Technology group. His area of expertise includes reservoir engineering, hydraulic and acid fracturing, and reservoir management.