An enormous amount of methane has accumulated in the shallow subsurface in many part of the world. However, most of this natural gas resource is not economically recoverable at the present time as the methane is adsorbed in coal seams, trapped in methane hydrate (clathrate), or dissolved in saline groundwater. Shallow accumulations of methane pose the threat of potentially increasing the rate of global warming. Enhanced gas recovery by subsurface CO2 injection (CO2-EGR) is potentially an option for greenhouse gas control, combined with use of potential energy resources. CO2 injection coupled with extraction of coal mine methane and coal-bed methane may make emission-free closed circuit power plants possible. Coal seams in Japan and under the seabed around Japan could potentially adsorb about 10 Gtonnes of CO2 and in so doing displace 2.5 trillion cubic meters of coal-bed methane. About 12 Gtonnes of CO2 could potentially be sequestered in hydrate layers, displacing 6 trillion cubic meters of methane hydrate under the deep seabed around Japan. Similarly, about 26 Gtonnes of CO2 could potentially be sequestered in saline groundwater, displacing 6 trillion cubic meters of methane in sedimentary basins in Japan. CO2-EGR could potentially sequester a total of 48 Gtonnes of CO2 in and around the Japanese Islands, with the prospect of also having enhanced production of nearly 10 trillion cubic meters of methane. The extremely light isotopic compositions of carbon in methane suggest that methanogens formed many of the subsurface accumulations of methane-rich natural gas in the world. Chemolithotrophic methanogens form methane from CO2 , thereby obtaining energy without sunlight under anoxic circumstances. Methanogens are often blamed for greenhouse gas emissions as they produce methane in organisms and in rice paddies, for example. However, application of CO2-EGR for production of subsurface biogenic methane displaced by sequestration of CO2 might result in enhanced carbon recycling. Present-day subsurface ecosystems are probably somewhat similar to ancient ecosystems adapted to exist in an anoxic CO2 -rich atmosphere under high pressure and temperature. Biogenic methanogenesis is thought to occur even in deep basaltic aquifers offering the opportunity for deep subsurface biogeochemical carbon recycling utilizing CO2-EGR to produce enhanced sequestration of greenhouse gases accompanied by the development of potential new energy resources.
Key Words: CO2 aquifer, methane, coal-bed methane, hydrate, methanogen, sealing.