HR: 0800h
AN: H11B-0481    [Abstracts]
TI: Mechanisms of Permeability Enhancement by Seismic Waves at the Pinon Flat Observatory
AU: * Doan, M
EM: mdoan@pmc.ucsc.edu
AF: Univerty Joseph Fourier - Grenoble, LGIT - BP 53, Grenoble, 38041, France
AU: Brodsky, E E
EM: ebrodsky@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth and Marine Sciences 1156, High Street, Santa Cruz, CA 95064, United States
AU: Agnew, D C
EM: dagnew@ucsd.edu
AF: University of California San Diego, IGPP 0225, La Jolla, CA 92093-0225, United States
AB: Seismic waves increase permeability. This was demonstrated by the change in tidal response of the water levels of several wells monitored for more than 20 years at the Piñon Flat Observatory in Southern California. But how is the permeability affected by seismic waves? Do the shear waves permanently mismatch the fractures extending below the observatory? The linear relationship between the amplitude of the shaking and the change in permeability favors this explanation. However, the major fractures of the hydraulic system are horizontal. As they are also only 100m deep, the sigma_xz stress is negligible. Moreover, a fracture mismatch does not explain the recovery observed within the 6 months following the disturbing earthquake. Does the fluid flow induced by the earthquake explain the change in permeability ? In poroelastic media, the seismic waves can induce pressure changes up to 10⁴Pa. With the presence of large heterogeneity, like an open well, large fluid flow may circulate within the fractures. This may induce phenomena like fracture unclogging that temporarily changes the apparent permeability of the fracture network. We model the radial flow within the fracture medium by a finite difference code. If the flow exceeds a threshold value q_th, the local permeability is increased by a constant k. We model the progressive unclogging of the medium using the seimic data recorded on the site. The final unclogging front then extends to several meters, which is enough to significantly alter the tidal response of the well. With a threshold of 10^-9 m/s and a local permeability enhancement by a factor of 5, we model the changes in permeability observed in Piņon Flat Observatory. The response of the fractured system is thus partially controlled by the properties of the well. This has implication the production enhancement by seismic waves attempted by the oil industry. The control by large heterogeneity may also be an efficient process for inducing fluid flow along active and heteregeneous faults.
DE: 3389 Tides and planetary waves
DE: 3653 Fluid flow
DE: 5114 Permeability and porosity
DE: 7212 Earthquake ground motions and engineering seismology
DE: 8010 Fractures and faults
SC: Hydrology [H]
MN: 2007 Fall Meeting