Accurate and precise prediction of the temperature and pressure (P-T) conditions at the boundary of the methane hydrate stability field is an essential component of a variety of endeavors in the field of geochemistry. Kvenvolden (1988), Gornitz and Fung (1994) and others have used knowledge of the P-T stability conditions to define the geophysical limits of gas hydrates and thereby estimate the size of the global reservoir. As the thermal signature of global warming penetrates into the ocean (Levitus et al., 2000), precise knowledge of the stability of gas hydrates will be required to assess the risks of decomposition in this reservoir. Recently, Ruppel (1997) has suggested that a discrepancy exists between in situ temperature measurements on the Blake Ridge and the predicted base of the hydrate stability zone. This claim is based in part upon P-T predictions of gas hydrate stability. In our own research, we have conducted a series of in situ deep-sea gas hydrate synthesis experiments (Brewer, et al., 1998) and have begun using an ROV to prospect for gas hydrate out-crops and undersea gas vents, which potentially result from decomposing gas hydrate deposits. One of the goals of this field work is to explore for gas hydrates close to the limit of the stability zone and this creates the need for accurate and precise predictions. Given the small temperature gradients with depth in the deep-sea, an error of 0.5°C, could mean a depth error of more than 100 meters. With a shallow sloping bottom (1% grade), one could easily be ten kilometers or more off target if the wrong temperature is used.
A variety of empirical P-T relationships for predicting the stability of methane hydrates can be found in the literature. The purpose of this chapter is not to provide an exhaustive list of these relationships nor to resolve all the discrepancies among them. This would be an injustice to the varied purposes of the studies and the authors involved. Nor do we pretend to present here a
complete description of the thermodynamics of gas hydrates. That has already been done in an excellent textbook by Sloan (1990, 1998), and the reader is referred there if a more complete understanding is desired. Rather, our goal is to select several representative approaches, to examine their differences, strengths and weaknesses, and to present some practical tools for applying these methods to the study of methane hydrates in the natural environment.
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