Institute for Geo-Resources and Environment

Recent systematic explorations of oil and natural gas by Japan National Oil Corporation and other
oil companies revealed that huge volumes of coal seams lie in the depths of Paleogene and
Cretaceous sedimentary basins in central Hokkaido, off the Pacific coast of northeastern Honshu
and northwest of Kyushu. Deep unmineable coal seams and coaly shale layers may provide
possible sink for CO2 sequestration and source for coalbed methane.
Unmineable coal seams deeper than -1,200m and shallower than -3,000m in Japan exceed 300
Gtons that may store 10 Gtons of CO2 and produce 3 Tm3 of coalbed methane. Very deep
unmineable coal seams deeper than -3,000m in Japan are estimated to exceed 3 Ttons, which may
contain 24 Tm3 of coalbed methane. CO2 sequestration potential of very deep unmineable coal
seams deeper than -3,000m are more than 100 Gtons in Japan.
Total volumes of deep unmineable coalbeds exceed tens to hundreds times of the shallow official
coal reserves that are mineable with conventional drift mining or open-cut mining practices. Huge
volumes of deep unmineable coal seams are potential sinks for CO2 and also untapped enormous
energy resources. Injected pure CO2 behaves readily as a supercritical fluid around injection wells
in deep coal seams. By the supercritical CO2-enhanced coal seam gas recovery (EGR) and in situ
fire-free microbial gasification of coal, deep unmineable coal seams may provide CO2–sink and
methane source for the CO2 emission-free closed-circuit power plant. New type of “coal mine” with
borehole mining method is proposed to develop “deep unmineable coalbeds”.

Coal is generally used as solid fuel on the surface after open cut mining or underground mining. However, the coal seam mined economically is limited by the geological and technological conditions. In situ method is a promising method, which enables the energy recovery from deep coal seam and undersea coal seam usually noneconomic by conventional mining methods. In “in situ” method, coal is usually converted to the gaseous form underground (UCG: Underground Coal Gasification). One of features of this method is the CO2 storage in coal seam in parallel to the gas production. There are two methods for UCG: combustion and non-combustion methods. The former is the conventional UCG realizing the coal gasification by supplying heat through the combustion of coal seam. The other one is non-combustion method using microbial decomposition and supercritical CO2 extraction for gas production at temperatures of some 10 or 100 degrees Celsius. The CO2-reducer methanogenic archaea are found in underground coal seams, in petroleum reservoirs snd in saline aquifers in Japan. The underground bioreactors for CO2-reduction and methanogenesis may be realized in deep coal measures or in underground hydrocarbon accumulations. The former UCG is now booming up for the technology development in the world now towards commercialization. The later UCG is still in the R&D stage. This report summarizes the present state of the arts of these two UCG methods pressing the gas production and CO2 storage in coal seam.

The world aquifers are estimated to have total storage capacity larger than three trillion tons of carbon dioxide. Systematic drilling survey for oil exploration suggests that the annual 70 million tons carbon dioxide injection is possible longer than 22 years in aquifers with anticlinal geological structures in and around Japan.  The annual 70 million tons carbon dioxide injection separated from flue gas of fossil-fueled power plants into aquifers may reduce Japan's artificial carbon dioxide emission as much as 6% with annual cost of 420 billion yens that match 1,200 yens/c-ton carbon tax.
Extraction of methane dissolved in fossil groundwater squeezed out by the carbon dioxide injection makes possible to utilize low grade of natural gas resources. Hydrate-filled layers are formed under deep sea floor due to high pressure and cool temperature. Free natural gas tends to accumulate under gas-hydrate-filled layers and under permafrost layers, because the layers are perfect insulators of gas. Carbon dioxide injection is expected to stabilize gas-hydrate-filled layers and permafrost caps, because the carbon dioxide hydrate is stable at wider pressure-temperature range than methane hydrate and ice.
Carbon dioxide injection under gas-hydrate-filled layers or under permafrost layers may realize the greenhouse gas mitigation and recovery of unused natural gas. Autogenous sealing of carbon dioxide in deep and cool aquifers assures virtually complete and practically unlimited subsurface containment of carbon dioxide.
Chemoautotrophs fix carbon dioxide in deep aquifers even in the absence of sunlight. Thermophilic methanogens can convert the carbon dioxide into methane in anoxic and hot aquifers. Biogenic restoration of subsurface hydrocarbon deposits may be possible in CO2-injected aquifers probably after tens or hundreds of years. Microbiological recycling of carbon dioxide in aquifers is an attractive possibility for energy-short countries such as Japan. Important key for underground CO2 recycling is hydrogen supply, that is, energy sources from microbial decomposition of organic matter, from geochemical water-rock interaction and from deep geothermal activities. Wide diversity of underground microorganisms needs extensive search for favorable species and ecosystem.

Deep and intermediate earthquakes indicate mechanical instability of the upper mantle rocks. Velocity anisotropy of elastic wave propagation also suggests that mantle rocks can sustain differential stress and stress gradients at least over a short time interval. Release of those stresses due to sudden reduction of rigidity, volume expansion due to local inhomogeneity, and lowering of rigidity due to dehydration of hydrous minerals or phase transformations can induce onset of partial melting of upper mantle materials. The estimation of stress around inhomogeneities or pockets of pressurized fluid of various aspect ratios indicates that a thin, lenticular or spheroidal inhomogeneity induces the greatest pressure reduction around its tips. The maximum thermo-mechanical stress due to phase transformation around such inhomogeneities or pockets of pressurized fluids is obtained theoretically by the three dimensional theory of elasticity. A model for development and propagation of physical inhomogeneities (fractures or flaws) in the upper mantle, and possible onset of partial melting and its transport by cyclical stress release is postulated. Theoretical equations and examples are presented to show that phase transformations of stable and metastable phases in areas of tectonic instability and upper mantle thermal anomalies can produce rapid volume change and reduction of rigidity in upper mantle rocks. Such changes can propagate further physical inhomogeneities, pockets of partial melt and volatiles, and create conditions for rapid or explosive emplacement of volatile-rich magmas and diatremes.
As the mantle rock is denser than the fluid (i.e., magma with volatiles) in the fracture system, fractures (with fluid) propagate upwards because of higher effective stress. The upward propagation of fluid-filled fractures is accelerated as the vertical distance of fractures becomes larger. Thus rapid concentration of thin films of pressurized melt or volatiles (such as CO2, H2O) in weak, shear or deep fault zones can lead to the shattering of more rigid, surroundirrg rocks with numerous fluid-filled fractures. If fluid pressure in fracture zones under shear is lowered due to dilatancy, more fluid flows into deep fault or shear zones. Thus the accelerated propagation of pressurized fractures with increasing length and higher effective stress can lead to rapid or explosive emplacement of volatile-rich magmas and diatremes. Rapid brittle fracture propagation, rather than viscous diapiric uprise, is more likely to result when the pressure of the partially melted mush with increasing volatile content at the roof approaches or exceeds the lithostatic load. Rates of uprise of such partially melted materials from the low velocity layer of the upper mantle or from the potentially unstable hotter boundary zone comprising both mantle and lower lithosphere are controlled primarily by compressive and differential stresses in the upper mantle, fluid pressure, effective kinematic viscosity and rate and volume supply of the fluid. Island arc, subduction zone and plate collision boundary are most suited for the development of such conditions. The effects of the diapiric and forceful uprise of partially melted mantle material on the elastic lithospheric plate and the development of fracture geometry in both oceanic and continental- crust due to diapirism have been discussed earlier.

The present invention provides a system for treating carbon
dioxide, and a method by Which such treated carbon dioxide
can be stored underground at loW cost and With high e?i
ciency. The present invention’ method for storing carbon
dioxide underground includes: a step for pumping up to the
ground groundwater from a pumping Well in a deep aquifer,
and then producing injection Water. Carbon dioxide that has
been separated and recovered from exhaust gas from a plant
facility is changed into the bubbles. The bubbles are mixed
With the injection Water, and hereby produces a gas-liquid
mixture a step for injecting into. The deep aquifer is injected
into the gas-liquid mixture from the injection Well. The
method preferably further includes a step for dissolving a
cation-forming material in the injection Water, and a step for
injecting the injection Water, in Which the cation-forming
material is dissolved, into the deep acquifer at its top and
above the place at Which injection Water has already been
injected.

From a commonsense standpoint, methane gas is a kind of fossil fuel that is a limited energy resource. It is believed that the underground methane was generated mostly by thermal decomposition of oil, coal or other organic matter - fossil. However, large amounts of habitat of methanogens (archaea) have been found recently in wide range of underground environments. Many types of methanogens produce methane from organic acids which are also fossil origin. However, CO2-reducer methanogens produce methane from hydrogen and CO2 which may be derived from the mantle or hydrothermal reaction in deep rocks instead of the direct seepage of deep methane in the unproven Thomas Gold hypothesis (abiogenic petroleum origin). Methane produced from abiogenic deep CO2 and hydrogen predominant in deep igneous rocks and in basaltic oceanic crust is not a fossil energy resource but a renewable energy resource. In particular, the methane hydrate in deep oceanic crust is a renewable energy resource.The stored CO2 in underground reservoirs by the CO2 capture and storage (CCS) will be converted into methane by the underground CO2-reducer methanogens. Then, the converted methane will be available as a renewable energy while CO2 emission into the atmosphere is suppressed. The sustainable hydrocarbon energy development becomes reality by the worldwide systematic deployment of this CO2-recycling CCS.

A new counter-geoengineering scheme: leak-free air-CCS under hydrate sealing is proposed here to slash the artificially increased atmospheric CO2 level directly. The naturally most secure and extensive storages for CO2 are the deep aquifers in cold regions and beneath ocean floors. The high-pressure experiments suggest the precipitation of CO2-hydrate occurs in CO2-saturated aquifers at the pressures higher than about 3MPa. Huge volumes of secure CO2-storages under the CO2-hydrate autogenous sealing are expected in deep aquifers in the sub-permafrost regions throughout the world. Virtually limitless secure CO2-storages under the CO2-hydrate autogenous sealing are possible in worldwide marine sediments and oceanic basalts under sea floors deeper than about 300m. The conventional carbon capture and sequestration (CCS) scheme is not viable in the remote areas due to the large infrastructure investment and energy loss for long-distance transportation of huge amount of CO2. However, the air-CCS may be viable while CO2 is extracted directly from the atmosphere instead of the flue gas of fossil fuels. As the atmospheric CO2 concentration is very thin (about 390ppm), the excess energy is required to extract the CO2 from the atmosphere. CO2 is selectively injected into deep aquifers by the air microbubbles sequestration with the pre-concentration by micro-porous membranes. The unused natural energy (wind, solar, geothermal and natural gas) is used for the recovery of CO2 from the atmosphere and for the underground injection of CO2-rich gas. Energy penalty of air-CCS can be compensated by use of unused natural energy in the remote regions.Carbon dioxide injection under gas-hydrate-filled layers or under permafrost layers may realize the greenhouse gas mitigation and recovery of unused natural gas. Autogenous sealing of carbon dioxide in deep and cool aquifers assures virtually complete and practically unlimited subsurface containment of carbon dioxide. Chemoautotrophs fix carbon dioxide in deep aquifers even in the absence of sunlight. Thermophile methanogens can convert the carbon dioxide into methane in anoxic aquifers. Biogenic restoration of subsurface hydrocarbon deposits is possible in CO2-injected aquifers probably after some ten of years. Microbiological recycling of carbon dioxide in aquifers brings the renewable hydrocarbon gas energy resources into reality.

The climate geoengineering is already inevitable to avoid imminent global climate disasters. Even extremely cautious approach is not enough for any grand-scale practices of climate geoengineering as we never understand the complex global climate system exactly.  However, modern human beings have unintentionally done the global-scale climate geoengineering that increases the  atmospheric CO2 level with the wide-spread massive burning of fossil fuels.     
The CCS-geoengineering in the broad sense includes the CO2 capture and sequestration, CO2 capture and CO2-EOR/EGR, air capture and sequestration, air capture and CO2-EOR/EGR and also underground microbial CO2 recycling. The CCS-geoengineering only reduces the artificial emission of CO2 into the atmosphere or suck the excessive CO2 from the atmosphere. The CCS-geoengineering is a naturally safe geoengineering technology as the CCS-geoengineering restore the atmosphere only toward the natural CO2 level minimizing untoward effects.