OS2B-3:SEISMIC AMPLITUDES AND ATTENUATIONS OF GAS HYDRATE-BEARING SEDIMENTS IN FRACTURED AND SAND RESERVOIRS

Zijian ZHANG1, Dan MCCONNELL1, Qiuliang YAO2
1 Fugro GeoConsulting, Houston, U.S.A.; 2 University of Houston,  U.S.A.

    An interpretation method is being developed to predict the gas hydrate occurrence in fractured clay reservoirs and sand reservoirs based upon an acoustic model for seismic amplitude versus hydrate content and seismic attenuation. During the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II expedition, fractured gas hydrate in clay-rich sediments and pore space gas hydrate in sands were identified from logging-while-drilling data in GC955 area.

    Seismic amplitude is principally affected by velocities above and below an interface. However, the amplitude presents attenuation on the hydrate bearing sediments. Thus, seismic interpretation relied on amplitude in elastic wave behavior can not completely reveal seismic characteristics of the hydrate bearing sediments, especially if multiple hydrate layers are present. In the paper, we discuss the physical mechanisms of intrinsic and scattering attenuations. Some previous lab measurements and viscoelastic modeling studies show attenuation behaviors on low frequency and synthetic seismic. Two offset wells provide log data of resistivity, porosity, density, and sonic velocities for identifying hydrate layers and, therefore, allow us to investigate the behaviors of attenuations. The intrinsic and scattering attenuations are significantly in strong dissolved or free gas filling fractures/faults. Hydrate filling fractures/faults are expected to have the effects of intrinsic and scattering attenuations, however it is difficult to distinguish them from no-hydrate filling fractures/faults. In no-fracture/fault zone above the BSR, significant amplitude reduction comparing to background indicates possible fracture-filling hydrate or pore-filling hydrate.

    This approach, considering intrinsic and scattering attenuations, is an improvement over existing seismic interpretation, assuming that gas hydrate-bearing sediments are isotropic and elastic. Our study shows high potential to account for the attenuation effect on prediction of hydrate saturation from seismic inversion.