Rock-physics Models for Gas-hydrate Systems Associated with Unconsolidated Marine Sediments
Diana Sava, Bob Hardage, 2009. "Rock-physics Models for Gas-hydrate Systems Associated with Unconsolidated Marine Sediments", Natural Gas Hydrates—Energy Resource Potential and Associated Geologic Hazards, T. Collett, A. Johnson, C. Knapp, R. Boswell
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Rock-physics models are presented describing gas-hydrate systems associated with unconsolidated marine sediments. The goals are to predict gas-hydrate concentration from seismic attributes, such as primary (P)- and secondary (S)-wave velocities, and to analyze compressional-wave (PP) and converted-shear-wave (PS) reflectivity at the base of hydrate stability zones. Elastic properties of gas-hydrate systems depend on elastic properties of the host sediments, elastic properties of gas hydrates, concentration of hydrates in the sediments, and geometrical details of hydrate morphology within the host sediments. We consider various scenarios for hydrate occurrence, including load-bearing gas hydrate, pore-filling gas hydrate, and two different thin-layered modelsofgas hydrate intercalated with unconsolidated sediments. We show that the geometrical details of how gas hydrates are distributed within sediments have a significant impact on relationships between gas-hydrate concentrations and seismic attributes. Therefore, to accurately estimate gas-hydrate concentrations from seismic data, we need to understand how hydrates are formed and distributed within marine sediments. The modeling results for thin-layered hydrated morphologies show significant S-wave anisotropy, which may be used to infer gas-hydrate distributions and concentrations in alternating thin layers of hydrate-bearing sediments if multicomponent seismic data are available.
We compare the theoretical predictions of the isotropic rock-physics models with published laboratory measurements on synthetic gas-hydrate formed in unconsolidated sands. We find good agreement between the rock-physics model of disseminated, load-bearing gas hydrate and laboratory measurements, which suggests that, in this case, gas hydrates may act as part of the mineral frame of the unconsolidated sediments.