This article reviews the storage of captured CO2 in coal seams. Other geologic formations, such
as depleted petroleum reservoirs, deep saline aquifers and others have received considerable
attention as sites for sequestering CO2. This review focuses on geologic sequestration of CO2 in
unmineable coalbeds as the geologic host. Key issues for geologic sequestration include potential
storage capacity, the storage integrity of the geologic host, and the chemical and physical processes
initiated by the deep underground injection of CO2. The review topics include (i) the estimated
CO2 storage capacity of coal, along with the estimated amount and composition of coalbed gas; (ii)
an evaluation of the coal seam properties relevant to CO2 sequestration, such as density, surface
area, porosity, diffusion, permeability, transport, rank, adsorption/desorption, shrinkage/swelling,
and thermochemical reactions; and (iii) a treatment of how coalbed methane (CBM) recovery and
CO2-enhanced coalbed methane (ECBM) recovery are performed (in addition, the use of adsorption/desorption isotherms, injection well characterization, and gas injection are described, as well as
reservoir screening criteria and field tests operating in the United States and abroad); (iv) leak
detection using direct measurements, chemical tracers, and seismic monitoring; (v) economic
considerations using CO2 injection, flue gas injection, and predictive tools for CO2 capture/sequestration decisions; (vi) environmental safety and health (ES&H) aspects of CO2-enhanced
coalbed methane/sequestration, hydrodynamic flow through the coal seam, accurate gas inventory,
ES&H aspects of produced water and practices relative to ECBM recovery/sequestration; (vii) an
initial set of working hypotheses concerning the chemical, physical, and thermodynamic events
initiated when CO2 is injected into a coalbed; and (viii) a discussion of gaps in our knowledge base
that will require further research and development. Further development is clearly required to
improve the technology and economics while decreasing the risks and hazards of sequestration
technology. These concerns include leakage to the surface, induced seismic activity, and long-term
monitoring to verify the storage integrity. However, these concerns should not overshadow the
major advances of an emerging greenhouse gas control technology that are reviewed in this paper.
