Potential impact of gas hydrate and its dissociation on the strength of host sediment in the Krishna–Godavari Basin

Abstract

During the Indian National Gas Hydrate Program (NGHP) Expedition 01, conducted between April–August 2006, a number of pressure cores containing gas hydrate-bearing sediments were recovered under in-situ pressures. Two of these cores (NGHP-01-10B-08Y and NGHP-01-21C-02E), after initial characterization, were sub-sectioned. They were then rapidly depressurized and frozen in liquid nitrogen. A number of sub-sections were delivered to the University of Southampton for further analysis. High-resolution 3-D X-ray computer tomography carried out in the first instance revealed that the structure and form of the hydrate within the host sediment had been preserved. However, the exsolution of free gas from the pore water during depressurization, and before the subsequent freezing, had caused the formation of significant voids within each sub-sections, precluding detailed testing of intact cores.Following this initial appraisal detailed soil characterization, such as particle size analysis, water content and salinity, were carried out on sub-samples from the core sections following thawing and dissociation of the hydrate. In addition, specific gravity and strength tests, including liquid and plastic limit and undrained triaxial compression tests, were carried out on the combined sediment from all the core sections. The results of these tests showed the sediment to be a high plasticity clay. The remoulded combined sediment had a relatively high water content, and low shear strength, considering the vertical effective stress to which the in-situ sediment had been subjected, therefore creating significant under-consolidation of the in-situ sediment.In this paper a number of hypotheses are presented in an attempt to explain this under-consolidation, including factors, such as the change in moisture content due to hydrate dissociation, over-pressure, the influence of salinity on sediment strength, as well as the effect of hydrate veins. It is considered likely that hydrate veins, enhancing sediment strength, were the main contributing factor to the apparent under-consolidation. Hydrate dissociation induced by production of gas hydrates within this under-consolidated sediment may potentially lead to seafloor instability and further research is required to assess this likelihood.