Winters, W. J.¹, W. F. Waite¹, B. Dugan¹, D. H. Mason¹, I. A. Pecher² 
(1) U.S. Geological Survey, Woods Hole, MA 
(2) Institute of Geological and Nuclear Science, Lower Hutt, New Zealand

ABSTRACT: Strength and Physical Properties of Sediment Containing Laboratory-Formed and Natural Gas Hydrate

The U.S. Geological Survey is studying the formation, behavior, and properties of gas hydrate and sediment mixtures. In addition to performing triaxial shear strength and physical property tests on laboratory-formed gas hydrate and on natural hydrate samples recovered from drilled wells, we have also performed tests on many other samples with pores (a) completely water filled, (b) completely dry, (c) filled with ice and, (d) pure ice specimens. These allow us to assess the importance of gas hydrate compared to other natural pore fillings. 
Models attempting to predict behavior of gas hydrate in the natural environment need to account for differences between natural and laboratory-formed hydrate because hydrate formed in the laboratory tends to cement sediment grains, whereas natural hydrate may not. We have used various methods to form hydrate within sediment pores in the laboratory to study formation effects on acoustic and shear strength behavior. 
During mechanical shear tests, the presence of hydrate significantly affects the measured sediment pore pressure and hence the stress-strain and strength properties. Sediment shear strength may be increased more than 500% by intact hydrate, but greatly weakened if the hydrate dissociates. Gas hydrate may also significantly influence acoustic velocity, permeability, and electrical resistivity. P-wave velocity may increase more than 130% during formation, but complete signal attenuation may occur during dissociation. These observations are important for models that (a) predict the location and quantity of natural gas hydrate using remote sensing methods and (b) predict pore pressure generation and resultant strength loss from dissociation-inducing events.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.