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NETL Investigates Deep Coal Seam Storage

Thursday, June 28, 2007
Article Type: Cited

Deep coal seams that are not commercially viable for coal production could be used for permanent underground storage of CO2. Researchers at the U.S. Department of Energy's National Energy Technology Laboratory have carried out initial investigations into the potential environmental impacts of CO2 sequestration in unmineable coal seams.

The research team collected 2000 coal samples from 250 coal beds across 17 states.

The researchers found that the depth from which a coal sample is taken reflects the average methane content, with much deeper seams containing less methane. However, the study provides only a preliminary assessment of the possibilities.

An added benefit of storing CO2 in this way is that additional useful methane will be displaced from the coal beds. Worldwide, it is estimated that there are almost 3 trillions tonnes of storage capacity for CO2 in such deep coal seams.

The key question is whether methane can be tapped from the unmineable coal seams and replaced permanently with large quantities of carbon dioxide; if so, such coal seams could represent a vast sink for CO2 produced by industry.

To replicate actual geological conditions, NETL has built a Geological Sequestration Core Flow Laboratory (GSCFL). A wide variety of CO2 injection experiments in coal and other rock cores (e.g. sandstone) are being performed under in situ conditions of triaxial stress, pore pressure, and temperature.

Preliminary results obtained from Pittsburgh No. 8 coal indicate that the permeability decreases with increasing CO2 pressure, with an increase in strain associated with the triaxial confining pressures restricting the ability of the coal to swell.

The already existing low pore volume of the coal is decreased, reducing the flow of CO2, measured as permeability. This is a potential problem that will have to be overcome if coal seam sequestration is to be widely used.

The research team has also investigated some of the possible side-effects of sequestering CO2 in coal mines.

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