ABSTRACT:

A technique to measure the two most important parameters controlling gas flow in coal - diffusion coefficient and permeability - using the transient approach is discussed. Diffusion coefficient is measured by applying the principle of binary counter diffusion using helium and methane. Permeabilities of samples subjected to triaxial stress are measured by introducing radial flow rather than axial flow. Volumetric strain of coal associated with release of gas is also measured. Preliminary results indicate that the diffusion coefficient, D, is not a constant. Its value varies with decrease in pressure, when accompanied by desorption. The coal matrix shrinks considerably with desorption of gas and this results in a considerable increase in porosity. When coupled with permeability-pressure relations matrix shrinkage also results in a significant increase in permeability.

RESUME:

Une technique pour mesurer les deux plus import ants paramètres controlant le pompage de gaz -le coefficient de diffusion et la permeabilite- en utilisant l"approche de duree de realisation y est discutee. Le coefficient de diffusion des echantillons est_mesure en applicant le principe de la contre diffusion binaire en utilisant de l"helium et du methane. La permeabilite des echantillons soumis a une contrainte triaxiale est mesuree en introduisant un pompage radial mieux qu"un pompage axial. La deformation volumetrique du charbon associee a la liberation du gaz est aussi mesuree. Les resultats mont rent que le coefficient de diffusion, D, n"est pas constant. Sa valeur varle avec la dimunition de la pression accompagnee d"adsorption: La matrice de charbon retrecit considerablement avec l"adsorption de gaz et cela resulte en une augmentatlon considerable en porosite. Couple avec la relatlon permeabilite- pression. le retrecissement"de la mat rice resulte aussi en,une augmentation considerable de la permeabilte.

ZUSAHHENFASSUNG: Erklarung um die zwie wichtigsten Gasstromung in Kohle zu messen - Diffusionkoefficient und Durchlaassighit - durch Appromaktion ist hier erlautert. Diffusionkoefficient wird hiergameuen durch zwei, Einhieten, Hulium und Methane. Durchlaassigkllitvon Exemplaren unter triaxiale spannung wurden gemessen mit Radialfluss eher als Axialfluss. Der Wechsel der Volume in Kohle, venn Gas freigehossen wird, wurde anch gemessen. Das Resultat zeigt dass die Diffusion nicht Konstant ist. Die Grösse hangt ab von dem Verlust des Drucks - wenn begleitet von Desorption. Der Kohlennahrboden schrumpft degeudent dei Desporption von Gas und das ergibt bedeutende poröse Kohend Durchlassigkeit.

1. INTRODUCTION

It is generally agreed that unconventional sources of energy will, play a vital role in meeting the U. S. energy demand in the future. One such resource is the coalbed methane estimated, in U.S. coal reserves alone, to be in excess of 400 trillion cu ft (Tcf) (11.3 trillion cu m) (Rightmire, 1984). When compared with U.S. gas reserves of less than 200 Tcf (5.65 Tcm), and annual consumption of 16 Tcf (.45 Tcm), this resource is of great significance. Production of gas from coal seams in the U.S. increased from less than one million cu ft per day in 1980 to 130 in 1988 and over 300 at the end of 1989. Active development is underway in Australia and interest is growing in Europe. Gas from coalbeds is beginning to represent a significant resource base that, is expected to grow in the future (Kamal and, Six, 1989).

Despite success with existing wells, potential uncertaiaties in production and lack of state-of-the-art techniques represent barriers, to effective recovery of methane from coalbeds. Meaningful description of a coolbed methane reservoir, and its behavior, with continued production of water and gas, requires a good understanding of the, various phenomena involved.

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