Abstract
Carbon capture and storage (CCS) is considered a key technology to reduce worldwide emissions of carbon dioxide (CO2). One CCS option is to inject CO2 into subsurface coal beds. When combined with simultaneous, enhanced, production of methane naturally present in the coal, this process is referred to as enhanced coalbed
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methane production (ECBM). However, ECBM is not yet a well-established, mature technology. Demonstration projects are essential for testing the applicability of the ECBM technique under field conditions. The first part of this thesis (Chapters 2 and 3) is therefore concerned with reporting the results of the first field test in Europe. Two important observations were made during the field operations, 1) a reduction of injectivity into the coal seam with time, considered to be caused by permeability reduction due to swelling of the coal upon CO2 exposure, and 2) low production rate of CBM, indicating slow diffusion in the coal matrix. It is concluded that understanding the transport of the gas/fluid through the coal is key to determining the success of future ECBM sites. The second part of the thesis (Chapters 4-6) is therefore dedicated to laboratory investigations of the swelling behaviour of the coal matrix, and its dependency on coal rank, moisture content, and stress state or confinement. The first type of experiment (Chapter 4 and 5) involved measurements of strain development with time at reservoir PT-conditions performed on unconfined block-shaped coal matrix fragments, exposed to CO2, methane and argon, using a high-pressure-temperature, see-through, optical cell located under a microscope. The experimental results confirmed different swelling behavior in coals exposed to different substances: carbon dioxide leads to higher strain than methane, while exposure to argon leads to very little swelling. Another result followed from the development and application of a spherical diffusion model (Chapter 5), characterized by a single valued (uniform) diffusion coefficient. The model provides a satisfactory fit to the experimental data up to the point where equilibrium is reached, and effective diffusion coefficient values were obtained that are within the range of previously published literature values. Rank effects on diffusion were expected but could not be discerned. The present experiments on moisturized samples (Chapters 4 and 5) seem to confirm the role of moisture as a competitor for adsorption sites, showing that the role of water can not be ignored in ECBM operations. The second type of experiment reported here involved measuring stress-strain development in confined, pre-pressed samples of granular bituminous coal, at reservoir PT-conditions. These experiments revealed a previously unrecognized, largely irreversible, swelling effect. We attribute the irreversible swelling phenomenon to a stress-history effect of pre-pressing. This effect may be relevant for coals that have undergone deep burial after formation, or for naturally stressed coal in tectonic zones. The thesis is concluded with a synthesis (Chapter 7) which draws together the results of both the field and laboratory tests, and with recommendations. It is inferred that ECBM offers possibilities for storing CO2 underground, but this will require that several conditions are met.
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