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.

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Equilibrium pressures for the dissociation of propane hydrate confined in silica gel pores of nominal radii 7.5, 5.0, 3.0, and, 2.0 nm were measured across a wide temperature range, and were higher than those for bulk propane hydrate. The upward shift from the bulk pressures, which depended on the pore size, was 200-300% for the smallest pore size. In each of the pressure-temperature profiles, the temperature at which the four phases (hydrate, ice, liquid water, and gas) apparently coexisted was identified, and this temperature was a linear function of the reciprocal pore size. The enthalpy of dissociation for the equilibrium involving hydrate, liquid water, and gas was estimated from the data. The change with pore size in this apparent enthalpy appeared to be analogous to the variation in the heat of melting of pore ice as sometimes reported.

Equilibrium pressures for the dissociation of methane hydrates confined in silica gel

Invasion percolation with trapping (IPT) and diffusion-limited aggregation (DLA) are simple fractal models, which are known to describe two-phase flow in porous media at well defined, but unphysical limits of the fluid properties and flow conditions. A decade ago, Fernandez, Rangel, and Rivero predicted a crossover from IPT (capillary fingering) to DLA (viscous fingering) for the injection of a zero-viscosity fluid as the injection velocity was increased from zero. [Phys. Rev. Lett. 67, 2958 (1991)]]. We have performed experiments in which air is injected into a glass micromodel to displace water. These experiments clearly demonstrate this crossover as the injection velocity of the air is increased. Furthermore, simulations, using our standard pore-level model, also support the predicted IPT-to-DLA crossover, as well as the predicted power-law behavior of the characteristic crossover length.

We show how the modification by Henry et al. of the earlier van der Waals-Plateeuw thermodynamic model can be used to interpret data for hydrates in small pores even when there is a broad distribution of pore sizes. Pore size distribution effects explain the previously reported discrepancy between calculated and measured equilibrium pressures for nominal 7.5 nm radius pores. On the basis of the model, pore volume distributions are reconstructed for the silica gel samples in which the hydrates were formed and compared with distributions constructed from desorption isotherms. The excellent agreement between the calculations and experimental data supports the interpretation of how the pore size distribution present in the silica gel samples affects the observed equilibrium pressures.

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