Seismic Property changes in Methane Gas Hydrate Bearing Sediments During Geomechanical Testing

Geomechanical properties of methane hydrate bearing sediments can impact oil and gas well stability and production performance of hydrate-bearing reservoirs in permafrost regions. Methane hydrate stability in permafrost environments occurs at lower pressures than those of deep sea deposits. Consequently, changes in temperature conditions that occur during hydrocarbon exploration can have a larger impact on methane hydrate stability than in deep sea fields. In recent years, the understanding of the geomechanical properties of hydrate bearing sediment has advanced significantly. However, geophysical signatures of the changes occurring within the sediment undergoing mechanical changes and failure (which are necessary for remote detection and monitoring of the well and reservoir) are not well understood, primarily because of limited available experimental and field data. We conducted a series of triaxial compression tests on methane-hydrate bearing sediment samples in an X-ray transparent triaxial test cell. The cell can apply high pressures under controlled temperatures (5MPa and 2°C were our typical test parameters). This capability allows us to conduct experiments under the realistic temperature, pressure, and chemical conditions of a hydrate reservoir, which is important for studying the behavior of hydrate bearing sediments subjected to changes in the reservoir environment. The cell is also capable of propagating 6-to-12 kHz compression and torsion waves along the sample axis during the test, due to a piezoelectric source and receiver embedded in the top and bottom loading pedestals. Methane hydrate was first synthesized inside a sediment pack under isotropic stress, after which additional stress was applied to bring the sample to failure. Concurrently, seismic velocities of the sample were monitored throughout the process. Before and after the experiment, X-ray CT images were taken to examine the heterogeneity of hydrate distribution within an intact sample and the distribution of fractures and shear/localization bands in a failed sample. In this presentation, we will discuss the results of these experiments including the changes in seismic signatures during sediment failure and the recovery of seismic signatures within hydrate bearing sediments over 24 hours after failure. In addition, we will also show the effect of dissociation rate on the strength of sediments when the sediment is brought to failure by hydrate dissociation rather than by mechanical loading.