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AAPG Bulletin

Abstract

AAPG Bulletin, V. 98, No. 10 (October 2014), P. 20572079.

Copyright copy2014. The American Association of Petroleum Geologists. All rights reserved.

DOI: 10.1306/05061413173

A scaling law to characterize fault-damage zones at reservoir depths

Madhur Johri,1 Mark D. Zoback,2 and Peter Hennings3

1Department of Geophysics, 397 Panama Mall, Stanford University, California 94305; present address: BP America, 501 Westlake Park Blvd, Houston, Texas 77079; [email protected]
2Department of Geophysics, 397 Panama Mall, Stanford University, California 94305; [email protected]
3ConocoPhillips Technology and Projects, 600 N. Dairy Ashford, Houston, Texas 77079; [email protected]

ABSTRACT

We analyze fracture-density variations in subsurface fault-damage zones in two distinct geologic environments, adjacent to faults in the granitic SSC reservoir and adjacent to faults in arkosic sandstones near the San Andreas fault in central California. These damage zones are similar in terms of width, peak fracture or fault (FF) density, and the rate of FF density decay with distance from the main fault. Seismic images from the SSC reservoir exhibit a large basement master fault associated with 27 seismically resolvable second-order faults. A maximum of 5 to 6 FF/m (1.5 to 1.8 FF/ft) are observed in the 50 to 80 m (164 to 262 ft) wide damage zones associated with second-order faults that are identified in image logs from four wells. Damage zones associated with second-order faults immediately southwest of the San Andreas Fault are also interpreted using image logs from the San Andreas Fault Observatory at Depth (SAFOD) borehole. These damage zones are also 50–80 m wide (164 to 262 ft) with peak FF density of 2.5 to 6 FF/m (0.8 to 1.8 FF/ft). The FF density in damage zones observed in both the study areas is found to decay with distance according to a power law BLTN13173eq1. The fault constant BLTN13173eq2 is the FF density at unit distance from the fault, which is about 10–30 FF/m (3.1–9.1 FF/ft) in the SSC reservoir and 6–17 FF/m (1.8–5.2 FF/ft) in the arkose. The decay rate BLTN13173eq3 ranges from 0.68 to 1.06 in the SSC reservoir, and from 0.4 to 0.75 in the arkosic section. This quantification of damage-zone attributes can facilitate the incorporation of the geometry and properties of damage zones in reservoir flow simulation models.

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