THE RELATION OF LABORATORY PHYSICAL-PROPERTY MEASUREMENTS TO THE PRESENCE AND BEHAVIOR OF NATURAL GAS HYDRATES

Although gas hydrate occurs in a wide variety of sediment types, properties of the host materials influence the type and quantity of hydrate formed and whether a particular deposit may be an economic resource or a geohazard. The U.S. Geological Survey's (USGS) gas-hydrate program is, in part, studying this interaction between natural geologic conditions and the presence, properties, and location of natural gas hydrate at the intergranular scale. In addition to involvement in terrestrial drilling operations in the Mackenzie Delta, NWT, Canada, we have worked offshore to recover and study sediment samples that contain natural gas hydrate. We transported some Canadian samples back to a shore-based laboratory for determination of acoustic, strength, and other physical properties. Subsequent modeling indicates that sediment containing natural-gas hydrate behaves acoustically as if the hydrate does not contain cemented grains. In contrast, gas hydrate formed (using different techniques) in the laboratory at simulated in-situ conditions within reconstituted porous media does tend to cement sieved Ottawa-sand grains. Therefore, caution must be exercised when extrapolating laboratory-derived data to field conditions.

Additionally, giant piston cores, up to 38-m long, were recovered in the northern Gulf of Mexico to determine the lateral and subbottom extent of gas hydrate in different geologic settings. The sediments immediately adjacent to the recovered massive gas-hydrate layers are visually similar to surrounding sediments and, therefore, are unlike cores recovered from the Mackenzie Delta, where the pore-scale gas hydrate distribution is significantly influenced by lithologic differences.