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AAPG Bulletin

Abstract

AAPG Bulletin, V. 88, No. 2 (February 2004), P. 133-151.

Copyright copy2004. The American Association of Petroleum Geologists. All rights reserved.

Incremental oil recovery using a horizontal drainhole in the San Andres Formation, Olson field, west Texas

Neil F. Hurley,1 Sara Ustabas2

1Colorado School of Mines, 1516 Illinois Street, Golden, Colorado; email: [email protected]
2Turkiye Petrolleri Anonim Ortakligi (TPAO), Ankara, Turkey; email: [email protected]

AUTHORS

Neil F. Hurley is a professor of geology at the Colorado School of Mines. He received B.S. degrees from the University of Southern California (1976), an M.S. degree from the University of Wisconsin, Madison (1978), and a Ph.D. from the University of Michigan (1986). He is a past editor of AAPG and has been an AAPG and Society of Petroleum Engineers distinguished lecturer. Specialties include carbonate geology and reservoir characterization.

Sara Ustabas has worked for Turkiye Petrolleri A. O. as an exploration geologist since 1999. She received her B.S. degree from Karadeniz Technical University (1996), and she completed her M.S. degree at the Colorado School of Mines (1999).

ACKNOWLEDGMENTS

The authors acknowledge Marathon Oil Company for permission to publish this report. Kurt Miller, Will Tank, and V. G. Odorisio have done significant work on Olson field, and some of their results have been incorporated in this study. We gratefully acknowledge Turkiye Petrolleri A. O. for financial support for Ustabas' master's thesis at the Colorado School of Mines. Reviewers Rick Major, Bill Morgan, Jeff Dravis, and Lee Billingsley provided helpful comments.

ABSTRACT

Oolitic carbonates and associated rocks in the San Andres Formation produce hydrocarbons at Olson field, west Texas. Mudstones, dolomitized fusulinid-peloid packstones and wackestones, ooid-peloid grainstones and packstones, and minor siliciclastic sediments occur in the field. The reservoir is stratigraphically compartmentalized by lenticular oolite deposits that are overlain by algal-laminated mudstones and siltstones. Pore-filling anhydrite cement, common in all lithologies, adds to porosity heterogeneity. Interwell communication is poor, as indicated by variable bottomhole pressures and erratic waterflood response.

A medium-radius lateral hole was drilled to a total depth approximately 50 ft (15 m) from an offset vertical well. At this depth, the horizontal well intersected a previously existing artificial fracture that occurred in the nearby vertical well. The intersection of this fracture by the lateral borehole had a significant economic impact. The production rate in the vertical well jumped from a few barrels per day to an average of 70 BOPD and less than 20 BWPD. The ensuing years maintained relatively high flow rates. Projected incremental oil recovery is 153,000 bbl, roughly equivalent to production from an average well drilled during the early life of the field.

Other unsuccessful horizontal wells have been drilled at Olson field, but they have not targeted preexisting hydrofractures in offset vertical wells. Therefore, the concepts presented in this paper have not been retested. This approach, intentionally drilling a drainhole to intersect a preexisting hydrofracture, could add new life to many older, compartmentalized reservoirs.

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