A common approach to identifying hydrate in situ is to directly measure the elastic wave properties of the formation using wireline logs. Distinct changes in the elastic properties are a critical indicator of the presence of hydrate and free gas. Sonic velocity decreases sharply in the presence of free gas, but increases in the presence of hydrate. The shear wave velocity, in particular, allows for the estimation of the shear strength of the formation that increases in the presence of hydrate [Guerin, et al., 1999]. Such observations from sonic logs aid in determining the nature of the BSR, interpreting VSP and seismic data, as well as directly estimating the concentration of methane hydrate and free gas.
Empirical relationships to estimate gas concentrations from Vp and Vs in a variety of hydrocarbon-bearing formations have been widely used in the oil and gas industry [e.g. Kuster and Toksoz, 1974], If the presence of hydrate and free gas can be accurately identified from VSP and velocity logs, then similar formulations could be used to estimate the volume of methane hydrate from surface seismic data alone. However, such relationships are empirical and a major limitation in using them is our poor understanding of the velocity in massive hydrate and different hydrate-bearing sediments. Zimmerman and King , Lee, et al. , Dvorkin and Nur , and Guerin, et al. , among others, all consider theoretical treatments of elastic wave propagation to predict the velocity and pore structure of hydrate-bearing sediments. Most of these models suggest that the microscopic distribution of hydrate infills sediment pores with high-velocity material or acts as an intergranular cement or both, increasing the compressibility and rigidity of the host sediment. Although these models all yield reasonable estimates of the methane saturation and satisfy field observations, they not agree quantitatively. A robust physical model for the velocity of hydrate and hydrate-bearing sediment is therefore still needed.
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