Methane and carbon dioxide adsorption–diffusion experiments on coal: upscaling and modeling

Institute of Geology and Geochemistry of Petroleum and Coal, Aachen University (RWTH-Aachen), Aachen, Germany
International Journal of Coal Geology 01/2004; DOI: 10.1016/j.coal.2004.05.002

ABSTRACT Numerical modelling of the processes of CO2 storage in coal and enhanced coalbed methane (ECBM) production requires information on the kinetics of adsorption and desorption processes. In order to address this issue, the sorption kinetics of CO2 and CH4 were studied on a high volatile bituminous Pennsylvanian (Upper Carboniferous) coal (VRr=0.68%) from the Upper Silesian Basin of Poland in the dry and moisture-equilibrated states. The experiments were conducted on six different grain size fractions, ranging from <0.063 to ∼3 mm at temperatures of 45 and 32 °C, using a volumetric experimental setup. CO2 sorption was consistently faster than CH4 sorption under all experimental conditions. For moist coals, sorption rates of both gases were reduced by a factor of more than 2 with respect to dry coals and the sorption rate was found to be positively correlated with temperature. Generally, adsorption rates decreased with increasing grain size for all experimental conditions.Based on the experimental results, simple bidisperse modelling approaches are proposed for the sorption kinetics of CO2 and CH4 that may be readily implemented into reservoir simulators. These approaches consider the combination of two first-order reactions and provide, in contrast to the unipore model, a perfect fit of the experimental pressure decay curves. The results of this modeling approach show that the experimental data can be interpreted in terms of a fast and a slow sorption process. Half-life sorption times as well as the percentage of sorption capacity attributed to each of the two individual steps have been calculated.Further, it was shown that an upscaling of the experimental and modelling results for CO2 and CH4 can be achieved by performing experiments on different grain size fractions under the same experimental conditions.In addition to the sorption kinetics, sorption isotherms of the samples with different grain size fractions have been related to the variations in ash and maceral composition of the different grain size fractions.

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    ABSTRACT: This paper reports a newly developed technique that directly measures total CO2 uptake of mm-sized coal matrix cylinders, without the application of the EoS for CO2 and without the need for swelling corrections or He-pycnometry. The technique makes use of a capsule composed of ductile metals (Au and In), pressure-fitted to the sample, which traps directly both adsorbed and free CO2 taken up by the sample upon exposure to CO2. We applied the method to samples of high volatile bituminous coal (Brzeszcze, Seam 364, Poland), saturated with CO2 at a fixed pressure between 0 and 18 MPa at 40 °C, and, as a function of pressure, yielded a Langmuir-like CO2 uptake curve (within a band less than 0.5mmolgcoal-1) with a maximum CO2 content of 4.08mmolgcoal-1 at 18 MPa CO2 pressure. For comparison, manometric determinations were performed on a combined set of 8 samples, also at 40 °C, which yielded a three-stage uptake curve showing lower uptake than the capsule-derived curve, showing 20–30% lower uptake at pressures above ∼9 MPa. Allowing for worst case errors, the differences in CO2 uptake obtained using the two methods are attributed to (i) random errors and to uncertainties EoS in the manometric data set at low CO2 pressures (3–9 MPa), and (ii) systematic errors, due to erroneous trapping of free CO2, dominating in the capsule data set high CO2 pressures (>9 MPa). The capsule method proved reliable at CO2 pressures of 0–7 MPa, while at pressures higher than 8 or 9 MPa the manometric method was most reliable. Although improvement is needed to prevent erroneous trapping of free CO2 at pressures above 9 MPa, our new encapsulation method has the potential to accurately determine the uptake of any adsorbate by any (swelling) adsorbent, e.g. CO2 uptake by shale and clay caprocks and is suitable for assessment of the effects of small-scale lithological differences in CO2 uptake. Use of the manometric method with sufficiently large samples, or the capsule method at P < 9 MPa, provides a reliable means of measuring the CO2 uptake capacity, yielding errors that are less than the effects of in situ stress on sorption.
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    ABSTRACT: The non-Darcy factor, an indicator for the non-Darcy effect, is dependent on the properties of porous media and pore fluid including permeability, viscosity, density, flow velocity and a coefficient named as β factor. Experimental results show that the β factor can be expressed as a power law of permeability. For conventional gas reservoirs, this β factor can be assumed as a constant as the permeability change is negligible. However, the constant β factor may not be suitable for coal seams with remarkable permeability change and a variable β factor as a function of coal permeability should be an alternative. Moreover, the coal permeability change is complex due to the competing effects of coal cleat compression and sorption induced coal shrinkage/swelling. Few studies have been done previously to incorporate the variable β factor as a function of coal permeability in reservoir simulations. In the present work, both the coal permeability change and the variable β factor are coupled in a dual porosity model to study the non-Darcy flow behavior in coal seams. The simulation results illustrate that the evolution of non-Darcy factor becomes tortuous by using a variable β factor, which differs from the monotonic behavior when constant β factors are applied. Furthermore, increasing the coal cleat compressibility and matrix shrinkage strain tends to intensify the tortuous behavior. The simulation results also indicate that using typical constant β factors, instead of the variable one, may significantly underestimate or overestimate the gas production rate for coalbed methane wells.
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