Physics
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufm.h11b0481d&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #H11B-0481
Physics
3389 Tides And Planetary Waves, 3653 Fluid Flow, 5114 Permeability And Porosity, 7212 Earthquake Ground Motions And Engineering Seismology, 8010 Fractures And Faults
Scientific paper
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 sigmaxz 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 qth, 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 Piñon 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.
Agnew Duncan C.
Brodsky Emily E.
Doan M.
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