Review of rock physics theories for quantifying gas hydrate and associated uncertainties

The importance of gas hydrate as an alternative energy resource is ascending because of its worldwide distri bution along the continental margina, in the permafrost regions and deep-water lakes, its richness, and high fuel productivity. Seismic survey is used predominantly to identify gas hydrate deposits at the regional scale, whereas drilling and coring are used for ground-truthing at the local scale. However, reliable quantification is the utmost priority to evaluate its resource potential commercially and understand its effect on the environment. Pressure core analysis is a direct way to know the morphology and amount of gas hydrate in a reservoir, while rock physics modelling is used widely to estimate gas hydrate saturation from remotely sensed geophysical data. Elevated velocity and resistivity are two parameters mostly used in rock physics theory to estimate the gaz hydrate concentration in sediments, whereas, pore-water salinity is one of the important proxy methods used to estimate gas hydrate saturation. Several rock physics models based on the empirical, semi-empirical, and late of physics exist in the literature that relate the observed anomalous physical properties like velocity and resistivity to reservoir properties like porosity, permeability, saturation etc. However, both direct and indirect methods have certain limitations, which are important for accurately quantifying gas hydrate. We review in detail the existing rock physics theories with possible uncertainties to estimate gas hydrate saturation using velocity, which provides reliable estimates with less uncertainty than that from resistivity.