HR: 16:35h
AN: S34A-05    [Abstracts]
TI: Case Study of Injection and Induced Seismicity at Paradox Valley, Colorado
AU: * Mahrer, K D
EM: kmahrer@do.usbr.gov
AF: US Bureau of Reclamation, USBR (D-8330) Bldg 67, Rm 1000, Denver Federal Center, Denver, CO 80225 United States
AB: The US Bureau of Reclamation's Paradox Valley Unit (PVU) extracts aquifer brine from 9 shallow wells along the Dolores River, Paradox Valley, southwestern Colorado and, after treating, high-pressure injects the brine 4.3 to 4.8 km below the surface. PVU injects at rates between ~800 and ~1,300 l/min. Since 1991, PVU has emplaced over 4.4x10⁶ m³ of fluid and induced more than 4,000 surface-recorded seismic events. The events are recorded on the local, 15-station, Paradox Valley Seismic Network. The induced seismicity at Paradox separates into two distinct, source zones: a principle zone (>95% of the events) asymmetrically surrounding the injection well to a maximum radial distance of ~3+ km and a secondary, ellipsoidal zone, ~2.5 km long and centered ~8 km northwest of the injection well. The expansion of these zones has stabilized since mid-1999, ~3 years after the onset of continuous injection. Within the principal zone, hypocenters align in distinct, linear patterns showing at-depth stratigraphy and the local Wray Mesa fracture and fault system. The primary faults of the Wray Mesa system are aseismic, striking sub-parallel to the inferred maximum principal stress direction, with one or more faults, probably, acting as fluid conduits to the secondary seismic zone. Individual seismic events, in both zones, do not discernibly correlate with short-term injection parameters; however, a 0.5 km2 region immediately north west of the injection well responds to long-term, large-scale changes in injection rate and the surpassing of a threshold injection pressure. Focal mechanisms of the induced events are consistent with simple double-couple, strike-slip moments and sub-horizontal extension to the northeast. In addition, the fault planes are consistent with principal stress directions determined from borehole breakouts. More than 99.9% of the PVU seismicity is below human detection (~M 2.5). However, approximately fifteen events have been felt locally with the largest being a magnitude M 4.3. Because of the M 4.3 and earlier, felt M 3.5 and M 3.6 events and injection economics, PVU changed injection strategies 3 times, since 1996. These changes reduced seismicity from ~1,100 events/year to as low as ~60 events/year. Finally, it is important to recognize that PVU is not a research operation, but an economically-governed project to remove and dispose brine. Hence, the project has two goals: (1) maximize brine disposal while minimizing objectionable (i.e., felt) seismicity and (2) characterize the reservoir (e.g., lifetime, injectability, etc.) within the parameters dictated by (1). Analyzing and interpreting the induced seismicity helps meet these goals.
DE: 7299 General or miscellaneous
SC: Seismology [S]
MN: 2005 Joint Assembly