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Importance of Coal Fabric in Determining Production Profiles and Gas Compositional Shifts During Coalbed Methane Production and Canister Desorption

Bustin, R. Marc1, Xiaojun Cui1 (1) University of British Columbia, Vancouver, BC

The production profile and gas compositional shifts during production is critical to the economics of coalbed methane plays. Carbon dioxide concentration generally but not always, increases steadily during the productive life of a field. In contrast during on onsite coal core desorption experiments, carbon dioixde decreases with time. Experimental and numerical modeling results indicate that gas production at a very early stage is mainly caused by the high pressure gradient near the well, imposed by well pumping, and is mainly affected by differential gas diffusion in the microporous coal matrix close to the well. Consequently, during this early stage of production,carbon dioxide concentration firstly increases with time and then decreases because of the higher apparent carbon dixoide diffusion coefficient (than methane)in the coal. The major stage of gas production on the other hand, is controlled by the equilibrium desorption of carbon dioxide and methane, The preferential sorption of carbon dioxide over methane results in an initially high relative methane content followed by a steady increase in carbon dioxide concentration with time. During field desorption the differential gas diffusion determins the gas partioning due to the limited size of coal cores.

Our numerical modeling results show the importance of cleat spacing and cleat porosity, independent of permeability, in determining the producton profile and compositional shifts during production.