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 (Impact Factor: 3.31). 12/2004; 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Carbon dioxide (CO2) and methane (CH4) vary in their diffusion behaviour in coal, with CO2 being sorbed more extensively and diffusing faster than CH4. Several different mechanisms have been proposed to explain this behaviour. To test these mechanisms, we describe here experiments investigating the rates of gas sorption in an Australian, high-volatile, bituminous coal as a function of particle size, temperature and gas type on sorption kinetics. One explanation for greater CO2 uptake proposes that CO2 can penetrate finer pore throats than CH4. If this were true, we would expect that the CH4 sorption capacity in crushed coal would approach that of CO2. However, this was not found: crushing slightly increased both CO2 and CH4 sorption capacity, as well as the helium density. Another theory proposed that methane in coal has a greater activation energy associated with its diffusion; however, we found that the temperature dependence of both CO2 and CH4 sorption rates were similar. Experiments measuring the diffusion of other gases (ethane, argon, nitrogen and krypton) showed a relationship between the critical temperature and molecular diameter of the gas and its diffusion rate in coal. We suggest that absorption occurs once gas condenses on the coal surface, with the absorption rate depending on the size and shape of the molecule. The experiments discussed in this paper confirm the dispersive nature of gas diffusion in coal, and provide evidence to explain much of the variation in diffusion rates.
    Fuel 03/2015; 143:620-629. DOI:10.1016/j.fuel.2014.11.087 · 3.41 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A gravimetric method with in situ density measurement is used to determine the adsorption isotherms and kinetic characteristics of CO2 on Chinese dry coal plug at 293.29, 311.11, 332.79 and 352.57 K and pressures up to 19 MPa. The adsorption and desorption process is reversible, which shows that it is a process of physical adsorption for CO2 on coal. The excess adsorption increases with the increasing pressure at low pressures until the CO2 phase transitions pressure is reached. Above this pressure, the excess adsorption decreases with the increasing pressure. The adsorption behaviour is described using the CO2 density instead of pressure in four thermodynamic models such as modified Langmuir, Langmuir + k, DR and DR + k. It is found that the modified Langmuir + k and DR + k models are more suitable for liquid and supercritical CO2 adsorption, respectively. The adsorption kinetics data are also obtained during the measurement of adsorption isotherms. The experimental data are fluctuant at the initial time range due to the temperature variation in the adsorption cell after high-pressure CO2 is injected. The diffusivity is estimated using a modified unipore model. It is observed that the kinetic parameter C, accounting for the effect of gas diffusivity, increases with the increasing pressure at low pressures and has no obvious relations with pressure at high pressures. In this study, C value has no dependencies with temperature for CO2, and the order of magnitude of the effective diffusivity is approximately 10−5 to 10−4 s−1.
    Adsorption 02/2015; 21(1-2):53-65. DOI:10.1007/s10450-015-9649-9 · 1.74 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Coal bed methane (CBM) has received increasing attentions as a significant energy resource. Numerical modeling of the CBM recovery processes entails simulation of the complex coupled mechanisms, including desorption of methane from the coal matrix surface, diffusion of gas to coal cleats, and gas flow from coal cleats to the wellbore. Beyond these complex flowing mechanisms, it is crucially important to represent coal cleats, structural fractures, and/or hydraulic fractures realistically in CBM simulators as they provide flow channels and dominate CBM flow behaviors. Existing CBM simulators are typically extended from oil and gas reservoir simulators with either black-oil or compositional formulation, and sorption and desorption are usually modeled by the Langmuir isotherms. The concept of shape factor is commonly used to characterize the flow between matrix and cleats (or fractures). When the shape factor is treated as only a function of cleat spacing, the detailed characteristics of actual cleats (or fractures) are missing, including the spatial distribution of cleats and interconnectivity of cleats. In this study, we propose a new workflow to perform a 2-D coal bed methane recovery simulation with discrete fracture model (DFM) in consideration of both structural fractures (large-scale fractures) and cleats (small-scale fractures). There are two key steps in our approach. First, we use a detailed network of discrete fractures characterized from core samples to represent the actual distribution of identified cleats, and calculate the shape factor of the realistic cleated coal sample by running a flow simulation to pseudo-steady state. Second, we apply the shape factor to field-scale simulations in which large-scale fractures are modeled as DFM. For this purpose, we treat gas sorption and desorption as a “chemical reaction”, and we developed an extension to an existing geochemical-reservoir simulator. We implemented both a “pseudo-compositional” and a “full-compositional” module to study the effect of mass exchange between gas and aqueous phases. We validated our new formulation and simulator development from a benchmark case in which our simulation results show close agreement with commercial simulators. We also demonstrated the significance of modeling mass exchange between fluid phases on CBM recovery in some cases, which is commonly missing in most commercial simulators. Finally, we presented our workflow in modeling an enhanced CO2-ECBM recovery process to a complex fractured coal bed with both large-scale tectonic fractures and small-scale cleats.
    Journal of Petroleum Science and Engineering 10/2014; DOI:10.1016/j.petrol.2014.09.035 · 1.10 Impact Factor

Full-text (4 Sources)

Available from
May 22, 2014