Gas Sorption and the Consequent Volumetric and Permeability Change of Coal


Wenjuan Lin







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Coalbed methane has grown in importance as an energy resource since the 1980s. Nevertheless, effective means to release methane from the tight, fractured reservoirs have yet to be developed. Primary recovery by reservoir depressurization is successful, but generally produces only about half of the gas in place. Gas (carbon dioxide, nitrogen, or mixtures of these components) injection is potentially an efficient technique both to enhance coalbed methane recovery as well as sequester greenhouse gases (mainly carbon dioxide) in subsurface geological sites. Due to the special features of coalbed reservoirs and the nature of gas retention in the reservoirs, there are unique issues that need to be taken into account when designing field operations and conducting numerical simulations of gas production and injection in coalbed methane reservoirs. One issue of particular interest is the permeability evolution of the reservoirs as gas is produced or injected. Two mechanisms are believed to change permeability: (1) changing effective stress due to the change of reservoir pressure caused by production or injection activities, and (2) strain caused by gas adsorption/desorption on the internal surfaces of coal. In spite of the conceptual convenience of these statements, better understanding of the physics and sound mathematical representations of the mechanisms have yet to be developed.

Experimental and numerical investigations of gas sorption on coal, and the subsequent volumetric and permeability changes of the coal were conducted. The goals of the study were to investigate the magnitude of permeability change caused by gas sorption, and develop an algorithm to simulate numerically gas sorption and sorption-induced permeability change. The amount of gas sorption and the subsequent volumetric and permeability change of coal samples as a function of pore pressure and injection gas composition were measured in the laboratory. A constant effective confining pressure (difference between the confining pressure and pore pressure) was maintained in the process of the experiments, therefore, the role of effective stress on permeability was eliminated. Several gases, including pure CO2, pure N2, and binary mixtures of CO2 and N2 of various composition were used as the injection gas. The coal sample was first allowed to adsorb an injection gas fully at a particular pressure. The total amount (moles) of adsorption was calculated based on a volumetric method. After adsorption equilibrium was reached, gas samples were taken from the equilibrium gaseous phase and analyzed afterwards. The composition of the gaseous phase prior to and after the adsorption was used to calculate the composition of the adsorbed phase based on material balance. Permeability of the sample was then measured by flowing the injection gas through the core at varying pressure gradient or varying flow rate, and an average permeability was obtained based on Darcy's law for compressible systems. The change of the total volume of the core was monitored and recorded in the whole process of the experiment. Volumetric strain was thereby calculated. Experimental results showed that the greater the pressure the greater the amount of adsorption for all tested gases. At the same pressure, the amount of adsorption was greater for CO2 than N2. For the binary mixtures, the greater the fraction of CO2 in the injection gas, the greater the amount of total adsorption. Volumetric strain followed the same trend as the amount of adsorption with pressure and injection gas composition. Permeability showed opposite behaviors, decreasing with the increase of pressure and the percentage of CO2 in the injection gas.

The experimental adsorption, volumetric strain, and permeability data were analyzed to investigate the numerical correlations between gas sorption, sorption-induced volumetric strain and permeability, and pressure and injection gas composition. The relationship between the amount of adsorption and pressure for pure gases (CO2 and N2) were readily represented by parametric isotherm models, such as Langmuir and the N-layer BET equations. Modeling efforts of multicomponent adsorption included predicting amount of adsorption and adsorbed phase composition based on the extended Langmuir equations and the ideal adsorbed solution model. Activity coefficients of the components in the adsorbed phase were computed based on the real adsorbed solution model and the ABC excess Gibbs free energy model. Algorithms for modeling the CO2/N2-Coal system were developed, and the constraints and strength of each model were discussed. The experimental volumetric strain was found to be linearly proportional to the total amount of adsorption and independent of the injection gas composition. The permeability reduction could not be readily correlated by the models in the literature unless the change of other coal properties (bulk modulus, axial constrained modulus, etc.) due to gas sorption was incorporated.

The sorption, volumetric strain, and permeability data collected in this study can be used for comparison by other researchers conducting similar studies. The algorithms of sorption modeling and the correlations developed in this study are readily incorporated into the simulation of enhanced coalbed methane recovery and CO2 sequestration in coalbeds.

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Copyright 2010, Wenjuan Lin: Please note that the reports and theses are copyright to their original authors. Authors have given written permission for their work to be made available here. Readers who download reports from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the author, Wenjuan Lin.

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