HR: 10:50h
AN: T42C-03    [Abstracts]
TI: The Effects of Carbon Films Deposited on New Fracture Surfaces on Rock Strength and Electrical Conductivity
AU: * Roberts, J
EM: roberts17@llnl.gov
AF: Lawrence Livermore National Laboratory, POB 808, L-201, Livermore, CA 94550,
AU: Duba, A
EM: duba@amnh.org
AF: Dept. of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024,
AU: Karner, S
EM: stephen.l.karner@exxonmobil.com
AF: ExxonMobil Upstream Research Co., POB 2189, Houston, TX 77252,
AU: Kronenberg, A
EM: kronenberg@geo.tamu.edu
AF: Center for Tectonophysics, Dept. of Geology and Geophysics, Texas A&M University, College Station, TX 77843,
AU: Mathez, E
EM: mathez@amnh.org
AF: Dept. of Earth and Planetary Sciences, American Museum of Natural History, New York, NY 10024,
AB: Hollow cylinders of Sioux quartzite, jacketed by silver, were hydrostatically loaded to failure at temperatures up to 400 °C by applying pressurized Ar gas at the outer diameter (reaching ~290 MPa at a rate of 0.1 MPa/s) while maintaining a constant pore pressure at the inner diameter. Pore fluids consisted of CO, CO₂, CH₄, a 1:1 mixture of CO₂ and CH₄ (each with pore pressures of 2.0 to 4.1 MPa), and air (at atmospheric pressure). Biaxial-stress states were calculated using elastic-stress solutions that account for the applied pressures and hollow-cylinder dimensions. For the inner wall of the cylinders, effective radial stress was zero and calculated effective differential stress reached 1225 MPa. Failure occurred by the formation of mode II shear fractures that transected the hollow cylinder walls. The distribution of carbon in the run products was mapped by scanning electron microscopy and electron probe. Samples deformed in CO₂ and air contained little or no carbon above the small amount that exists in the undeformed rock. Samples deformed in CO contain ubiquitous carbon films on the fracture surfaces that formed during deformation. Because carbon is absent on other free quartz surfaces present in the experiments, we conclude that the carbon films formed preferentially on the fractures as they formed. The radial resistivity of dry, undeformed Sioux quartzite cylinders is extremely large in the ambient laboratory atmosphere (>23 MØmega- m). The radial resistivity of Sioux quartzite cylinders that failed in pore fluids that promote carbon deposition is relatively low (2.9 to 3.1 MØmega-m for CO tests; 15.2 to 16.5 MØmega-m for CO₂:CH₄ tests). The results of this study help to isolate the role of carbon deposition on fresh fracture surfaces in altering the electrical properties of rocks with little initial porosity from that of carbon deposition on fractures and preexisting equant voids of porous rocks. Taken together, our results and those of Roberts et al. (1999) indicate that electrical conductivity in rocks may be enhanced due to carbon deposition on grain and/or fracture surfaces. Our results are important for studies linking variations of crustal electrical properties to seismogenesis, as well as to illuminate physico-chemical mechanisms that may be exploited to monitor injection sites for carbon sequestration. No decrease in rock strength was observed owing to weakening caused by the presence of these carbonaceous pore fluids. This work was supported by the National Science Foundation and the Department of Energy. Work performed under the auspices of the U.S. Department of Energy by the University of California Lawrence Livermore National Laboratory was carried out under contract W-7405-ENG-48 and supported specifically by funding from the Office of Basic Energy Sciences.
DE: 8004 Dynamics and mechanics of faulting (8118)
DE: 8010 Fractures and faults
DE: 8020 Mechanics, theory, and modeling
DE: 8030 Microstructures
DE: 8045 Role of fluids
SC: Tectonophysics [T]
MN: 2007 Fall Meeting