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Rock Physics Characterization for Gas Hydrate Reservoirs: Elastic Properties

Abstract: We offer a first‐principle‐based, effective medium model for elastic‐wave velocity in unconsolidated, high porosity, ocean bottom sediments containing gas hydrate or free gas. The dry sediment frame elastic constants depend on porosity, elastic moduli of the solid phase, and effective pressure. Elastic moduli of saturated sediment are calculated from those of the dry frame using Gassmann's equation. To model the effect of gas hydrate on sediment elastic moduli we use two separate assumptions: (a) hydrate modifies the pore fluid elastic properties without affecting the frame and (b) hydrate becomes a component of the solid phase, reducing porosity and modifying the elasticity of the frame. The goal of the model is to predict the amount of hydrate in sediments from sonic or seismic velocity data. We apply the model to sonic and VSP data from ODP Hole 995 and obtain hydrate concentration estimates from assumption (b) that are consistent with estimates obtained from resistivity, chlorinity, and evolved gas data.