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Title: Carbon Dioxide Sorption Isotherms and Matrix Transport Rates for Non-Powdered Coal

Abstract

For enhanced coalbed methane/carbon dioxide sequestration field projects, carbon dioxide isotherms and the rate of diffusion of the carbon dioxide from the cleats into the matrix are important parameters for predicting how much carbon dioxide actually will be sequestered under various operating conditions. Manometric (or pressure swing) experiments on powdered coal provide a quick, simple, and relatively inexpensive method for measuring sorption isotherms. However, determination of the rate of transport from cleat into matrix from the rate of gas pressure drop is difficult, if not impossible. (The characteristic time constant for the transport depends on the cleat spacing as well as the rate of diffusion.) Manometric measurements often yield isotherms that are extremely problematic in the region of the carbon dioxide critical point; perhaps even worse, available data seem to indicate that the sorption isotherms measured for powders are much larger than the isotherms of coal cores. Measurements on centimeter-sized samples can take weeks or months to reach equilibrium; for such equilibration times gas leakage rates that would be of no significance in powdered-coal measurements can completely invalidate manometric measurements on coal cores. We have tested and used a simple, inexpensive method for measuring isotherms and carbon dioxide transport ratesmore » in coal cores. One or more cores are placed in a simple pressure vessel, and a constant pressure is maintained in the vessel by connecting it to a gas supply (which contains a very large amount of gas compared to amount that could leak over the course of the experiment). From time to time the gas supply is shut off, the sample is removed, and its weight is recorded at ambient pressure at frequent time intervals for a period of about one hour. The sample is then returned to the pressure vessel, the carbon dioxide pressure restored to its previous value, and the equilibration resumed until the next sample weighing. For a point on the isotherm, the process is repeated until the sample weight reaches a constant value (i.e., typically equilibration times of several weeks). The slope of a plot of sample weight vs. square root of elapsed desorption time gives a measurement for the rate of diffusion. In order to advance all three experimental methods, results from this “ambient-pressure gravimetry” method were compared with data obtained by conventional manometry and by computer tomography. The isotherm and “diffusion” rate measured for the core can be directly used in simulators for reservoir engineering studies of coalseam sequestration and enhanced coalbed methane production.« less

Authors:
; ;
Publication Date:
Research Org.:
National Energy Technology Laboratory (NETL), Pittsburgh, PA, and Morgantown, WV
Sponsoring Org.:
USDOE - Office of Fossil Energy (FE)
OSTI Identifier:
915490
Report Number(s):
DOE/NETL-IR-2007-150
TRN: US200817%%539
DOE Contract Number:
None cited
Resource Type:
Conference
Resource Relation:
Conference: 2007 International Coalbed Methane Symposium, Tuscaloosa, AL, May 21-25, 2007
Country of Publication:
United States
Language:
English
Subject:
01 COAL, LIGNITE, AND PEAT; 02 PETROLEUM; 03 NATURAL GAS; CARBON DIOXIDE; COAL; COAL DEPOSITS; DIFFUSION; ISOTHERMS; METHANE; PRESSURE DROP; PRESSURE VESSELS; RESERVOIR ENGINEERING; SORPTION; TRANSPORT

Citation Formats

Smith, D.H., Jikich, S., and Seshadri, K. Carbon Dioxide Sorption Isotherms and Matrix Transport Rates for Non-Powdered Coal. United States: N. p., 2007. Web.
Smith, D.H., Jikich, S., & Seshadri, K. Carbon Dioxide Sorption Isotherms and Matrix Transport Rates for Non-Powdered Coal. United States.
Smith, D.H., Jikich, S., and Seshadri, K. Tue . "Carbon Dioxide Sorption Isotherms and Matrix Transport Rates for Non-Powdered Coal". United States. doi:.
@article{osti_915490,
title = {Carbon Dioxide Sorption Isotherms and Matrix Transport Rates for Non-Powdered Coal},
author = {Smith, D.H. and Jikich, S. and Seshadri, K.},
abstractNote = {For enhanced coalbed methane/carbon dioxide sequestration field projects, carbon dioxide isotherms and the rate of diffusion of the carbon dioxide from the cleats into the matrix are important parameters for predicting how much carbon dioxide actually will be sequestered under various operating conditions. Manometric (or pressure swing) experiments on powdered coal provide a quick, simple, and relatively inexpensive method for measuring sorption isotherms. However, determination of the rate of transport from cleat into matrix from the rate of gas pressure drop is difficult, if not impossible. (The characteristic time constant for the transport depends on the cleat spacing as well as the rate of diffusion.) Manometric measurements often yield isotherms that are extremely problematic in the region of the carbon dioxide critical point; perhaps even worse, available data seem to indicate that the sorption isotherms measured for powders are much larger than the isotherms of coal cores. Measurements on centimeter-sized samples can take weeks or months to reach equilibrium; for such equilibration times gas leakage rates that would be of no significance in powdered-coal measurements can completely invalidate manometric measurements on coal cores. We have tested and used a simple, inexpensive method for measuring isotherms and carbon dioxide transport rates in coal cores. One or more cores are placed in a simple pressure vessel, and a constant pressure is maintained in the vessel by connecting it to a gas supply (which contains a very large amount of gas compared to amount that could leak over the course of the experiment). From time to time the gas supply is shut off, the sample is removed, and its weight is recorded at ambient pressure at frequent time intervals for a period of about one hour. The sample is then returned to the pressure vessel, the carbon dioxide pressure restored to its previous value, and the equilibration resumed until the next sample weighing. For a point on the isotherm, the process is repeated until the sample weight reaches a constant value (i.e., typically equilibration times of several weeks). The slope of a plot of sample weight vs. square root of elapsed desorption time gives a measurement for the rate of diffusion. In order to advance all three experimental methods, results from this “ambient-pressure gravimetry” method were compared with data obtained by conventional manometry and by computer tomography. The isotherm and “diffusion” rate measured for the core can be directly used in simulators for reservoir engineering studies of coalseam sequestration and enhanced coalbed methane production.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue May 01 00:00:00 EDT 2007},
month = {Tue May 01 00:00:00 EDT 2007}
}

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  • Measurements of sorption isotherms and transport properties of CO2 in coal cores are important for designing enhanced coalbed methane/CO2 sequestration field projects. Sorption isotherms measured in the lab can provide the upper limit on the amount of CO2 that might be sorbed in these projects. Because sequestration sites will most likely be in unmineable coals, many of the coals will be deep and under considerable lithostatic and hydrostatic pressures. These lithostatic pressures may significantly reduce the sorption capacities and/or transport rates. Consequently, we have studied apparent sorption and diffusion in a coal core under confining pressure. A core from themore » important bituminous coal Pittsburgh #8 was kept under a constant, three-dimensional external stress; the sample was scanned by X-ray computer tomography (CT) before, then while it sorbed, CO2. Increases in sample density due to sorption were calculated from the CT images. Moreover, density distributions for small volume elements inside the core were calculated and analyzed. Qualitatively, the computerized tomography showed that gas sorption advanced at different rates in different regions of the core, and that diffusion and sorption progressed slowly. The amounts of CO2 sorbed were plotted vs. position (at fixed times) and vs. time (for various locations in the sample). The resulting sorption isotherms were compared to isotherms obtained from powdered coal from the same Pittsburgh #8 extended sample. The results showed that for this single coal at specified times, the apparent sorption isotherms were dependent on position of the volume element in the core and the distance from the CO2 source. Also, the calculated isotherms showed that less CO2 was sorbed than by a powdered (and unconfined) sample of the coal. Changes in density distributions during the experiment were also observed. After desorption, the density distribution of calculated volume elements differed from the initial distribution, suggesting hysteresis and a possible rearrangement of coal structure due to CO2 sorption.« less
  • Diffusion through macro- and meso-pores with the subsequent filling of open micropores is a relatively fast process and the manometric measurements with fifteen minute pressure stabilization steps provide a good estimate of excess (ad)sorption. This can be followed by a much slower processes of the penetrant diffusion into the macromolecular network, accompanied by its structural relaxation, with or without free volume changes. The “free volume” effect is a change of the sample’s excluded volume because of the penetrant molecules mixing within the formerly excluded volume of the network or because of contraction (either reversible or semi-permanent) due to external pressure.more » If the resulting swelling of the sample leads to the mixture’s volume equal to the sum of the initial volumes of the components, there is no apparent change to the void volume and no pressure relaxation is observed (b). On the other hand, if the external forces (either macroscopic pressure or microscopic molecular interaction forces) change the excluded volume of the network (its density), without exchange of the sorbent molecules between the sample and the free fluid phase, such change causes a corresponding change in the void volume, which constitutes a pure free volume effect (c). In this case, no post-decompression exodus of the sorbent out of the sample is observed. As a special case, we consider an incorporation of the penetrant molecules into the network without any change in the volume of the mixture (d), though mathematically this can be written as a superposition of the previous two cases« less
  • Measurements of sorption isotherms and transport properties of carbon dioxide (CO 2) in coal cores are important for designing enhanced coalbed-methane/CO 2-sequestration field projects. Sorption isotherms measured in the laboratory can provide the upper limit on the amount of CO 2 that might be sorbed in these projects. Because sequestration sites will most likely be in unmineable coals, many of the coals will be deep and under considerable lithostatic and hydrostatic pressures. These lithostatic pressures may reduce the sorption capacities and/or transport rates significantly. Consequently, we have studied apparent sorption and diffusion in a coal core under confining pressure. Amore » core from the important bituminous coal Pittsburgh #8 was kept under a constant, 3D effective stress; the sample was scanned by X-ray computer tomography (CT) before, then while, it sorbed CO2. Increases in sample density because of sorption were calculated from the CT images. Moreover, density distributions for small volume elements inside the core were calculated and analyzed. Qualitatively, the CT showed that gas sorption advanced at different rates in different regions of the core. and that diffusion and sorption progressed slowly. The amounts of CO 2 sorbed were plotted vs. position (at fixed times) and vs. time (for various locations in the sample). The resulting sorption isotherms were compared to isotherms obtained from powdered coal from the same Pittsburgh #8 extended sample. The results showed that for this single coal at specified times, the apparent sorption isotherms were dependent on position of the volume element in the core and the distance from the CO 2 source. Also, the calculated isotherms showed that less CO 2 was sorbed than by a powdered (and unconfined) sample of the coal. Changes in density distributions during the experiment were also observed. After desorption, the density distribution of calculated volume elements differed from the initial distribution, suggesting hysteresis and a possible rearrangement of coal structure because of CO 2 sorption.« less
  • Measurements of sorption isotherms and transport properties of carbon dioxide (CO{sub 2}) in coal cores are important for designing enhanced coalbed-methane/CO{sub 2}-sequestration field projects. Many of the coals will be deep and under considerable lithostatic and hydrostatic pressures. These lithostatic pressures may reduce the sorption capacities and/or transport rates significantly. Consequently, we have studied apparent sorption and diffusion in a coal core under confining pressure. A core from the important bituminous coal Pittsburgh no. 8 was kept under a constant, 3D effective stress; the sample was scanned by X-ray computer tomography (CT) before, then while, it sorbed CO{sub 2}. Increasesmore » in sample density because of sorption were calculated from the CT images. Moreover, density distributions for small volume elements inside the core were calculated and analyzed. Qualitatively, the CT showed that gas sorption advanced at different rates in different regions of the core and that diffusion and sorption progressed slowly. The amounts of CO{sub 2} sorbed were plotted vs. position (at fixed times) and vs. time (for various locations in the sample). The resulting sorption isotherms were compared to isotherms obtained from powdered coal from the same Pittsburgh no. 8 extended sample. The results showed that for this single coal at specified times, the apparent sorption isotherms were dependent on position of the volume element in the core and the distance from the CO{sub 2} source. Also, the calculated isotherms showed that less CO{sub 2} was sorbed than by a powdered (and unconfined) sample of the coal. Changes in density distributions during the experiment were also observed. After desorption, the density distribution of calculated volume elements differed from the initial distribution, suggesting hysteresis and a possible rearrangement of coal structure because of CO{sub 2} sorption.« less
  • Due to increasing atmospheric CO2 concentrations causing the global energy and environmental crises, geological sequestration of carbon dioxide is now being actively considered as an attractive option to mitigate greenhouse gas emissions. One of the important strategies is to use deep unminable coal seams, for those generally contain significant quantities of coal bed methane that can be recovered by CO2 injection through enhanced coal bed natural gas production, as a method to safely store CO2. It has been well known that the adsorbing CO2 molecules introduce structural deformation, such as distortion, shrinkage, or swelling, of the adsorbent of coal organicmore » matrix. The accurate investigations of CO2 sorption capacity as well as of adsorption behavior need to be performed under the conditions that coals deform. The U.S. Department of Energy-National Energy Technology Laboratory and Regional University Alliance are conducting carbon dioxide sorption isotherm experiments by using manometric analysis method for estimation of CO2 sorption capacity of various coal samples and are constructing a gravimetric apparatus which has a visual window cell. The gravimetric apparatus improves the accuracy of carbon dioxide sorption capacity and provides feasibility for the observation of structural deformation of coal sample while carbon dioxide molecules interact with coal organic matrix. The CO2 sorption isotherm measurements have been conducted for moist and dried samples of the Central Appalachian Basin (Russell County, VA) coal seam, received from the SECARB partnership, at the temperature of 55 C.« less