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Rock physics of a gas hydrate reservoir

Stanford University, California, U.S.

Richard Uden and Turhan Taner

Rock Solid Images, Houston, Texas, U.S.

Corresponding author: jack@pangea.stanford.edu

The first 20% of the full text of this article appears below.

Gas hydrates are solids composed of a hydrogen-bonded water lattice with entrapped guest molecules of gas. There are convincing arguments that vast amounts of methane gas hydrate are present in sediments under the world's oceans as well as in onshore sediments in the Arctic. This hydrate is possibly the largest carbon and methane pool on earth. As such, methane hydrate may be the principal factor in global climate balancing. One may also treat this methane pool as a potential energy source. These considerations ignite the scientific and business community's interest in quantifying the amount of methane hydrate in the subsurface.

Gas hydrate reservoir characterization is, in principle, no different from the traditional hydrocarbon reservoir characterization. Similar and well-developed remote sensing techniques can be used, seismic reflection profiling being the dominant among them.

Seismic response of the subsurface is determined by the spatial distribution of the elastic properties. By mapping the elastic contrast, the geophysicist can illuminate tectonic features and geobodies, hydrocarbon reservoirs included. To accurately translate elastic-property images into images of lithology, porosity, and the pore-filling phase, quantitative knowledge is needed that relates the rock's elastic properties to its bulk properties and conditions. Specifically, to quantitatively characterize a natural gas hydrate reservoir, we must be able to relate the elastic properties of the sediment to the volume of gas hydrate present and, if at all possible, the permeability. One way of achieving this goal is through rock physics effective-medium modeling.

	Rock physics models in perspective

 
Several attempts to construct a relation between hydrate concentration and the compressional velocity in sediments have followed the path of modifying the popular Wyllie's time average equation which states that total traveltime through rock is the volume-weighted sum of traveltimes through the solid phase and the fluid phase considered independently of each other; i.e., V_P^–1 = (1-) V_PS^–1 + . . . [Full Text of this Article]