Abstract
Gas migration in coal is strongly controlled by surface diffusion of adsorbed gas within the coal matrix. Surface diffusion coefficients are obtained by inverse modelling of transient gas desorption data from powdered coals. The diffusion coefficient is frequently considered to be dependent on time and initial pressure. In this article, it is shown that the pressure dependence can be eliminated by performing a joint inversion of both the diffusion coefficient and adsorption isotherm. A study of the log–log slope of desorbed gas production rate against time reveals that diffusion within the individual coal particles is a multi-rate process. The application of a power-law probability density function of diffusion rates enables the determination of a single gas diffusion coefficient that is constant in both time and initial pressure.
Highlights
There is much interest in measurement of gas diffusion coefficients for coal
We develop a new multi-rate, stochastic power-law model by assuming that diffusion rate is characterized by a truncated power-law probability density function (PDF)
Many previous studies claim that the apparent diffusion coefficient for surface diffusion of adsorbed gas in coal is a function of pressure and time
Summary
There is much interest in measurement of gas diffusion coefficients for coal. Such coefficients are required for field-scale coal-bed-methane (CBM) simulators to plan and forecast the performance of CBM production operations. Coal beds generally exhibit an orthogonal set of fractures. Fractures in coal are referred to as cleats. The surrounding blocks of coal are typically referred to as the matrix. Gas adsorption is a pressure-dependent process with adsorption increasing with increasing pressure. CBM production involves reducing pressure in the coal bed by fluid production
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