Formation & dissociation of methane gas hydrates in sediments: A critical review

Methane Gas Hydrates, MGHs, are perceived as a future source of energy for the sustenance of mankind owing to their abundance, usually found in continental margins and permafrost regions. However, identification of potential MGHs reserves, characterized by their high saturation in sediment pores, S-h, establishing their spatial distribution in hydrate bearing sediemnts, HBS, and their extraction, is extremely challenging. This calls for a proper understanding of the mechanisms and factors, associated with sediments and their matrix characteristics, viz., the pore-diameter, d(p), particle diameter, d, specific surface area, SSA, initial water saturation, S-w, salinity of the pore-water, S, mineralogy, porosity, eta, and density, rho and thermodynamic conditions, affecting the formation and dissociation of MGHs in sediments. In this context, several laboratory investigations have been conducted to mimic the natural process of MGHs formation, with an intention to gain an insight into (i) mechanisms involved in formations, (ii) response of HBS during dissociation, and (iii) CH4 gas recovery rate under prevailing thermodynamic conditions. However, such studies have, usually, focused on the simulation of 'specific reservoir conditions' (viz., P-T, S, d) and hence acquiring a comprehensive picture about the (i) MGHs formation and dissociation process, and (ii) influence of above-mentioned parameters on these processes, becomes necessary. With this in view, a critical review of the literature pertaining to the synthesis of MGHs has been conducted and it has been demonstrated that in addition to thermodynamic conditions, d, d(p), S-w, SSA and mineralogy of the sediments have a significant influence on the formation as well as dissociation process. In this context, the discussion, analysis and relationship proposed in this manuscript prima facie will aid in estimating S-h and understanding the mechanisms governing its dependency on sediments and their matrix characteristics.