It is generally accepted that the volume of natural gas contained in the world's gas hydrate accumulations greatly exceeds that of known gas reserves. There is also growing evidence that natural gas can be produced from gas hydrates with existing conventional production technology. Advancements in nuclear-magnetic-resonance (NMR) logging and wireline formation testing have allowed for the characterization of gas hydrate reservoirs at the pore scale. Integrated NMR and formation testing studies from northern Canada and Alaska have yielded valuable insight into how gas hydrates are physically distributed in sediments and the occurrence and nature of pore fluids (i.e., free-water along with clay- and capillary-bound water) in gas hydrate-bearing reservoirs. Information on the distribution of gas hydrate at the pore scale has provided invaluable insight on the mechanisms controlling the formation and occurrence of gas hydrate in nature along with new data on gas hydrate reservoir properties (i.e., porosities and permeabilities) needed to accurately predict gas production characteristics for various gas hydrate production methods.
In recent years significant progress has been made in addressing key issues controlling the formation and occurrence of gas hydrate in nature. One of the most important factors that contribute to the formation of gas hydrate is the physical properties of the host reservoir. The study of gas-hydrate samples indicates that the physical nature of in-situ gas hydrates is highly variable (reviewed by Sloan and Koh, 2008). Gas hydrates are observed
occupying pores of coarse-grained rocks;
nodules disseminated within fine-grained rocks;
a solid substance, filling fractures; or
a massive unit composed mainly of solid gas hydrate with minor amounts of sediment. Most gas hydrate field expeditions, however, have shown that the occurrence of concentrated gas hydrate is mostly controlled by the presence of fractures and/or coarser grained sediments in which gas hydrate fills fractures or is disseminated in the pores of sand-rich reservoirs (Collett, 1993; Dallimore and Collett, 2005; Riedel et al., 2006; Collett et al., 2008a, 2008b; Hutchinson et al., 2008; Park et al., 2008; Yang et al., 2008; Fujii et al., 2008). Torres et al., (2008) concluded that hydrate grows preferentially in coarse-grained sediments because lower capillary pressures in these sediments permit the migration of gas and nucleation of hydrate. The growth of gas hydrate in clay-rich sediments, however, is more poorly understood and appears to be limited to mostly massive occurrences.