Methane Hydrate in Marine Sands: Its Reservoir Properties, Gas Production Behaviors, and Enhanced Recovery Methods

Natural gas hydrates are crystalline solids composed of water and gas1). Gas molecules, such as methane, are trapped in cavities composed of hydrogen-bonded water molecules. Natural gas hydrates have been a subject of concern for the natural gas industry from 1930s due to their role as a cause of flow assurance failure. Conversely, from 1960s onward, methane hydrate discovered in the Arctic, together with deep water environments, has offered both a means of determining past and future climate change and a potential energy resource, since a large fraction of the Earth’s fossil fuels is considered to be stored in hydrates2). Present estimates of global hydrate-bound methane in nature are on the scale of at least 3000 trillion m3 (1.5×103 Gt of carbon)3). Large volumes of methane hydrate exist in oceanic environment. Until the late 1990s, oceanic gas hydrates were thought to exist primarily in low permeability, unconsolidated muds; however, extensive methane hydrate deposits were discovered in sand reservoirs at the Nankai Trough off the coast of Japan in 19994). Methane hydrate accumulating in sand reservoirs at high saturation appears to be a promising energy resource because its greater permeability enables hydrate dissociation and gas production by using systems of the oil and gas industry3). Following this discovery, methane hydrates in marine sands have received widespread attention as an alternative natural gas resource. Recent studies have indicated that the amount of gas-in-place in global gas hydrates in sand reservoirs is in the order of 300 trillion m3 (1.5×102 Gt of carbon)3). To date, Japan and China have performed offshore methane hydrate production tests in the eastern Nankai Trough and South China Sea, respectively. The world’s first offshore production test in 2013 and the second production test in 2017, both at the eastern Nankai Trough, confirmed continuous gas production from oceanic methane hydrate accumulated in a sand (fine [Review Paper]