Institute of Petroleum Engineering

Centre for Gas Hydrate Research Publications



Abstract 097
Sediment Geomechanical Response to Hydrate Dissociation by Depressurisation: An Experimental Study
Yang, J., Marinakis, D., and Tohidi, B.
European Geosciences Union 1st General Assembly, Nice, France, 25 - 30 April (2004).
Naturally-occurring gas hydrates present a potentially huge energy resource, and much scientific effort has been devoted to improving methods for identify/quantifying resources and designing mechanisms for gas production. However, there is still a poor understanding of how gas hydrates alter sediment physical properties and what role they may play in seafloor stability and methane release to the ocean/atmosphere. Accurate knowledge of the geophysical properties of marine sediments hosting gas hydrates is essential for both the reliable seismic identification and quantification of seafloor hydrate occurrences, and for predicting potential geohazards resulting from hydrate destabilisation. It is believed that gas hydrates may significantly increase the strength of sediments, either by acting as a pseudo-cement, or intra-granular pore-fill, restricting the normal processes of compaction and cementation. If this is the case, then the dissociation of solid hydrate to liquid and gas may significantly reduce the shear strength of sediments, increasing the potential for seafloor instability, so presenting a major geohazard. To predict potential seafloor instability resulting from hydrate destabilisation due to either long-term natural (e.g. changes in sea level/temperature) or anthropogenic (e.g. gas production schemes) causes, it is necessary that we improve our knowledge of how hydrates alter sediment physical properties. In this work, we present the results of an experimental investigation into the geomechanical response of hydrate-bearing sediments to hydrate dissociation by depressurisation. Hydrates of a multi-component natural gas were formed in a synthetic glass bead (0.1 mm diameter) cylindrical core, simulating coarse-grained, unconsolidated sediments. Dissociation was induced by controlled pore fluid release, reducing pore pressure. The relationship between stress and strain was determined during depressurisation by measuring sediment compaction rates at constant overburden pressure. Results show that gas hydrates significantly alter the elastic properties of sediments, and play an important role in stress-strain relations. Deformation of sediments during depressurisation was most pronounced when hydrate dissociation was occurring, and generally took the form of significant strain build-up followed by sudden release and associated sediment compaction. In line with previous studies, results suggest that hydrate acts as a pore fill rather than as a grain-contact cement, and prevent sediment compaction by acting as an additional pseudo-grain phase.