HR: 09:50h
AN: OS41C-07    [Abstracts]
TI: Mechanical, Thermal and Electromagnetic Properties of Hydrate-Bearing Clay, Silt, and Sand at Various Confining Pressures
AU: * Santamarina, J
EM: carlos@ce.gatech.edu
AF: Georgia Tech, Civil and Environ. Engineering, Atlanta, 30332
AU: Yun, T
OS41C-07 AF: Georgia Tech, Civil and Environ. Engineering, Atlanta, 30332
AU: Lee, J
OS41C-07 AF: Georgia Tech, Civil and Environ. Engineering, Atlanta, 30332
AU: Martin, A
OS41C-07 AF: Georgia Tech, Civil and Environ. Engineering, Atlanta, 30332
AU: Francisca, F
OS41C-07 AF: Now at: Universidad Nacional de Cordoba-CONICET, Facultad de Ingenieria, Cordoba, 5016 Argentina
AU: Ruppel, C
EM: cdr@eas.gatech.edu
AF: Georgia Tech, Earth and Atmos. Sciences, Atlanta, 30332
AB: With sponsorship from the ChevronTexaco JIP, we have since 2002 conducted exhaustive laboratory experiments that provide critical results to inform interpretation of field- and lab-based analyses of natural methane hydrates. Specifically, we have measured the mechanical, thermal and electromagnetic properties of sediments containing different amounts of tetrahydrofuran (THF) hydrate and subjected to different effective confining stress. The use of THF allows us to control the concentration of hydrate in pore space; this is an important advantage for evaluating the impact of hydrate concentrations on the properties of hydrate bearing sediments. For these experiments, we formed gas hydrate in kaolin clay (specific surface S_a=36 m² g^-1), precipitated and crushed silt (S_a=6 and 0.11 m² g^-1, respectively), and sand (S_a=0.019 m² g^-1) to test the impact of specific surface, porosity, and grain size on the measured physical properties. The mechanical measurements show that, while the small strain shear stiffness of uncemented sediments is controlled by the effective stress, the stiffness becomes strongly dependent on hydrate concentration once hydrate formation promotes increased interparticle coordination. At intermediate and large strain, the normally consolidated sediments display strain-hardening behavior in the absence of hydrate. When hydrate is present, the quasi-elastic behavior extends to higher axial strains, and the stress-strain response becomes less sensitive to confining pressure. The failure mode is also affected by the presence of hydrates: Specimens with 50% or 100% hydrate-filled porosity develop vertical fractures when tested at low confinement (0.03 MPa), and an approximate tenfold increase in confining pressure is required for the development of shear planes. Thermal conductivity results reflect a complex interplay of particle size, effective stress, hydrate concentration, and hydrate formation effects that have not have been fully recognized in previous studies of thermal conductivity in hydrate-bearing sediments. Our results clearly indicate that, with or without hydrate, the thermal conductivity of sediments is controlled by interparticle contact conduction so that thermal conductivity increases with increasing effective stress not only due to higher mineral volume fraction, but also due to increased interparticle coordination and contact force. The increase or even decrease in thermal conductivity with hydrate formation depends on sediment type, effective stress, and hydrate volume fraction. In particular, there may be a loss in conduction once hydrate has formed, and this phenomenon is more likely in coarse-grained sediments or sediments subjected to low effective stress due to mineral-mineral contact loss that results from volume expansion during hydrate formation. The real permittivity measured at high radio-frequencies reflects the orientational polarization of free water. On the other hand, the imaginary part of effective electrical conductivity is determined by the volume fraction of electrolyte and its electrical conductivity. Therefore, both measurements closely track the decrease in volumetric free water content during consolidation and subsequent hydrate formation.
DE: 3004 Gas and hydrate systems
DE: 3021 Marine hydrogeology
DE: 3036 Ocean drilling
SC: Ocean Sciences [OS]
MN: Fall Meeting 2005