Geological characterization of natural gas hydrate bearing sediments and their influence on hydrate formation and dissociation

Lithology of host sediments associated with gas hydrate reservoirs have a significant effect on the hydrate formation. The Krishna-Godavari (K-G), Mahanadi and Andaman basin in India have been proven to be rich in natural gas hydrate. In this study, a detailed investigation was carried out for the mineralogical analysis of the sediments collected from K-G, Mahanadi and Andaman basin along with hydrate formation in sediments. The study reveals that, the majority of the minerals in K-G basin samples are clay minerals and quartz. The presence of pyrite crystal (FeS2) in the K-G basin sediment is the indication of anoxic and sulfide environment. Spares fragments of microfossil like diatoms and spicules of fresh-water sponges have been identified from K-G basin sediments. Mahanadi basin is also consisting of clay minerals & quartz, and proportion of clay minerals are higher than K-G basin sediment. Andaman basin is mainly composed of clay and calcite minerals with little amount of quartz and feldspar. The rheological behavior of suspensions of sediments in water was investigated as it has significant effect on the drilling operation through the formation. Considering the similar mineralogical composition, gas hydrate formation and dissociation behavior were investigated in colloidal suspension containing sediments collected from K-G basin. Hydrate formation studies indicated that the presence of sediment solution significantly decreased pressure needed for hydrate formation compared to formation in pure water alone. It was observed that the salinity has delayed the hydrate formation as salt is considered as hydrate inhibitor. The rate of hydrate formation was observed to be high in case of sediment aqueous solution than the pure water. Whereas, in case of sediment solution with salinity shows lower formation rate than both pure water and sediment aqueous solution. The studies are relevant to the origin and stability of natural gas hydrate deposits known to exist in deep permafrost and marine sediments, where the effect of sediment surfaces is largely unknown.