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AAPG Bulletin, V.
Natural fractures in the Spraberry Formation,
Midland basin, Texas: The effects of mechanical stratigraphy on
fracture variability and reservoir behavior
1Sandia National Laboratories, Department
6116, Mail Stop 0750, Albuquerque, New Mexico, 87185; email: [email protected]
2Department of Hydrology, New Mexico Institute of Mining and Technology, Socorro, New Mexico, 87801; current address: Irell & Manella, LLP, Newport Beach, California, 92660; email: [email protected]
3Petroleum Recovery Research Center, Socorro, New Mexico, 87801; current address: Harold Vance Department of Petroleum Engineering, Texas A&M University, College Station, Texas, 77843; email: [email protected]
4Pioneer Natural Resources, Midland, Texas, 79701; current address: Concho Resources, 110 West Louisiana, Suite 410, Midland, Texas, 79701; email: [email protected]
5Harold Vance Department of Petroleum Engineering and Geology and Geophysics, Texas A&M University, College Station, Texas, 77843; email: [email protected]
John C. Lorenz is a Distinguished Member of Technical Staff at Sandia National Laboratories, where he has worked on sedimentary and natural-fracture reservoir characterization issues. His work has focused on the Rocky Mountain region but has extended to reservoirs in other parts of the world. Lorenz earned a Ph.D. from Princeton University and joined Sandia in 1981. Previously he was with the Peace Corps, Morocco, and worked for the U.S. Geological Survey.
Jenny L. Sterling is a hydrogeologist who is working as a consultant and assistant for a leading environmental mediator at Irell & Manella, LLP. Jenny received her master's degree in hydrogeology from the New Mexico Institute of Mining and Technology in 2000, where her thesis work included study of the Spraberry Formation and fracture generation. Prior to her employment in the legal field, she worked as an environmental consultant at Daniel B. Stephens and Associates, in Albuquerque, New Mexico. Experience there included technical assistance to the mediator in a multiparty environmental mediation at a Superfund site, as well providing support as an advocate for several companies involved in environmental disputes.
David S. Schechter received his B.S. degree in chemical engineering from the University of Texas (1984) and his Ph.D. in physical chemistry from Bristol University, England (1988). He was a postdoctoral research associate and acting assistant professor at Stanford University from 1989 to 1993. Schechter was a senior scientist at the Petroleum Recovery Research Center at New Mexico Institute of Mining and Technology from 1993 to 2000. He is currently an associate professor of petroleum engineering at Texas A&M University. Schechter is author and principal investigator for a Department of Energy/National Petroleum Technology Office Class III Field Demonstration project in the naturally fractured Spraberry trend area, although his interests extend to all facets of characterization and reservoir engineering in naturally fractured reservoirs.
Chris L. Whigham is a reservoir engineer for Concho Resources, Inc., in Midland, Texas. He previously was a reservoir engineer for Pioneer Natural Resources and Parker and Parsley Petroleum Company and a vice president of First City National Bank of Midland. While at Pioneer, Whigham was the project manager of Pioneer's Department of Energy project assessing the economic feasibility of CO2 flooding Spraberry reservoirs in the Midland basin. Whigham holds a B.S. degree in petroleum engineering from Texas A&M University.
Jerry L. Jensen is an associate professor at Texas A&M University. His interests include integrating petrophysical data and geological information for reservoir characterization. He works with probe permeameter data, evaluating the amount and types of data that are necessary and reliable, and the diagnostics of data to visualize geological structure and organization. Jensen has 10 years' industry experience as a field engineer for Services Techniques Schlumberger in Paris and as a research engineer for Gearhart Industries in Texas. During 13 years at Heriot-Watt University in Scotland, he continued his industry involvement by developing and teaching commercial courses on the integration of petrophysics and geology and open-hole well-log interpretation. After leaving Heriot-Watt, he served as an associate professor at the University of Alaska, Fairbanks before coming to Texas A&M.
We would like to thank Martha Cather for insights into the petrology of Spraberry fractures; David Holcomb for assessment of the stresses of the formation; Paul McDonald, the original project manager; Brian McPherson for guidance on J. Sterling's thesis; Carl Rounding, lead geologist for the project at the time of writing; Tom Sheffield, original lead geologist; and Charlie Sizemore, the operations engineer for the project. J. Sterling also thanks the staff at the Geomechanics Laboratory at Sandia National Laboratories for their help with the mechanical measurements. The manuscript has had the benefit of a review by Chris Zahm and fire-breathing reviews by Al Lacazette and Steve Laubach. Acceptance and the final decision to publish this article was made by Neil Hurley, AAPG editor during the time of manuscript submittal and review. This project was supported by the U.S. Department of Energy at several levels, including support of Lorenz's work by the National Petroleum Technology Office of the National Energy Technology Laboratory in Tulsa (Bob Lemmon, contract manager), and contract DE-FC22-95BC14942 to the Petroleum Recovery Research Center/Pioneer Natural Resources (Dan Ferguson, contract manager).
Horizontal cores from sandstone-siltstone reservoirs in the Spraberry Formation (Midland basin, west Texas) have documented two systems of dramatically different yet dynamically compatible natural fractures, in reservoirs separated vertically by only 145 ft (44 m). Each system is capable of producing a different degree of the northeast-trending permeability anisotropy recognized in Spraberry reservoirs. One fracture system consists of two vertical fracture sets with an apparent conjugate geometry (striking north-northeast and east-northeast). The other system consists of evenly spaced, northeast-striking vertical fractures, nearly bisecting the acute angle of the first system. Although lithologically similar, differences in quartz-overgrowth and clay content in the layers resulted in a yield strength of the lower bed that is only half of that of the upper layer, producing different fracture systems in the two reservoirs despite their proximity. Such differences in the mechanical properties, due to variations in diagenetic and depositional histories of the strata, are probably widespread within the formation. They have the potential to cause significant vertical and lateral variation in the Spraberry fracture system across the basin. Low present-day in-situ stresses in the reservoirs allow the fractures to open, to become more conductive, and even to propagate, under very low injection pressures.
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