Mechanical Strength and Seismic Property Measurements of Hydrate-Bearing Sediments (HBS) During Hydrate Formation and Loading Tests
Authors: Nakagawa Seiji , Kneafsey Timothy J. , Moridis George J. 
Published: October 2008

Abstract
An on-going effort of conducting laboratory triaxial compression tests on synthetic methane hydrate-bearing sediment cores is presented. Methane hydrate is formed within a sand pack inside a test cell under a controlled temperature and confining stress, and triaxial compression tests are performed while monitoring seismic properties. A unique aspect of our experiment is that the formation and dissociation of hydrate in a sediment core, and the failure of the sample during loading tests, can be monitored in real time using both seismic waves and x-ray CT imaging. For this purpose, we built a specially designed triaxial (geomechanical) test cell. This cell allows us to conduct seismic wave measurements on a sediment core using compressional and shear (torsion) waves. Concurrently, CT images can be obtained through an x-ray-transparent cell wall, which are used to determine the porosity distribution within a sample owing to both original sand packing and formation of hydrate in the pore space. For interpreting the results from both seismic measurements and geomechanical tests, characterization of sample heterogeneity can be critically important. In this paper, we present the basic functions of our test cell, and the results of preliminary experiments using a sandpack without hydrate and a sandstone core. These measurements confirmed that (1) clear x-ray images of gas-fluid boundaries within a sediment/rock core can be obtained through a thick aluminum test cell wall, (2) the test cell funcions correctly during loading tests, and (3) both compressional and shear waves can be measured during a loading test. Further experiments using methane hydrate-bearing samples will be presented at the conference.

Introduction
Because of geomechanical stability concerns, the placement of wells and seafloor platforms associated with oil production is strongly influenced by the presence of gas hydrates on the sea floor or within the sediment lithology. These concerns will be far more pronounced if gas production from oceanic gas hydrate accumulation is to become an economically viable option in the future. Thermal loading and dissociation caused by warm reservoir fluids (originating from a deeper conventional reservoir under production) ascending through wellbores that intersect hydrate-bearing sediments (HBS) can also have adverse consequences for the HBS stability. The current state of knowledge on the properties of HBS that have a direct impact on the seafloor stability is still in its infancy. Additionally, for remote detection, resource evaluation and monitoring of in-situ HBS during exploration and production of oil and gas (including gas production from the hydrates), it is essential to establish concurrently the quantitative relationships among index properties (sediment porosity, hydrate saturation, gas saturation, etc.), geophysical properties (seismic velocities and attenuation, in particular), and geomechanical properties (mechanical stiffness, strength, time-dependent behavior) of HBS.
Detailed strength measurements that were conducted on laboratory-made, pure methane (CH4)-hydrate samples (Durham et al., 2003; Stern et al., 1996). Stern et al (1996) indicated that the mechanical, plastic flow properties of CH4 hydrates are very different from those of water ice. Thus, unlike ice (which exhibits yielding and softening), CH4-hydrates exhibit continuous hardening, and solid-state disproportionation and exsolution. These results suggest that geomechanical tests using water ice as an analogue may result (e.g., Nagaeki et al., 2004) in erroneous conclusions for the mechanical properties of oceanic HBR.