Abstract
The pore structure of coal reservoirs is the main factor influencing the adsorption–diffusion rates of coalbed methane. Mercury intrusion porosimetry (MIP), low-pressure nitrogen adsorption (LP-NA), low-pressure carbon dioxide adsorption (LP-CA), and isothermal adsorption experiments with different macerals were performed to characterize the comprehensive pore distribution and methane adsorption–diffusion of coal. On the basis of the fractal theory, the pore structures determined through MIP and LP-NA can be combined at a pore diameter of 100 nm to achieve a comprehensive pore structural splicing of MIP, LP-NA, and LP-CA. Macro–mesopores and micro-transitional pores had average fractal dimensions of 2.48 and 2.18, respectively. The Langmuir volume (VL) and effective diffusion coefficients (De) varied from 31.55 to 38.63 cm3/g and from 1.42 to 2.88 × 10−5 s−1, respectively. The study results showed that for super-micropores, a higher vitrinite content led to a larger specific surface area (SSA) and stronger adsorption capacity but also to a weaker diffusion capacity. The larger the average pore diameter (APD) of micro-transitional pores, the stronger the diffusion capacity. The diffusion capacity may be controlled by the APD of micro-transitional pores.
Highlights
Coal reservoirs have complex pores and fractures
The United States, Canada, and Australia have been successful in developing their coalbed methane (CBM) resources [3], and Poland has been working on it [4]
The maximum reflectivity of vitrinite (Ro,max ) and coal macerals was determined via an optical microscope with an oil immersion reflection light following ASTM Standard D2798-05 and ISO 7404-3 (2009)
Summary
Coal reservoirs have complex pores and fractures. The pore characteristics of coal in these reservoirs, including porosity, pore structure, and specific surface area (SSA), can directly affect adsorption, desorption, diffusion, and seepage. Auxiliary ventilation equipment, and the drainage method have a great influence on the productivity of methane [1]. These all can subsequently influence the commercial development of coalbed methane (CBM) [2]. The production of CBM from coal in China is gradually increasing. CBM recoverable resources of anthracite in China account for approximately 25% of the country’s total CBM
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