Analysis of Gas Migration Characteristics and Controlling Factors in Nanopores of Deep Coal Seams

Abstract

Nanopores of various scales and morphologies in coal are important spaces for gas storage and migration. To deeply analyse the initial gas migration scale and characteristics in coal matrix micropores, this paper constructs an apparent permeability model describing the gas migration scale based on the gas transport mechanism, and analyses the controlling effects of pore structure, temperature, pressure and gas types on gas migration. The research results indicate that gas migration in coal matrix micropores is mainly through surface diffusion, contributing over 90%, while the contributions of Knudsen diffusion and slip flow are several orders of magnitude lower. As the tortuosity increases, the scale of gas migration in the micropores decreases in a negative exponential form. As the porosity increases, the scale of gas migration in micropores increases linearly. Compared with micropores, the forms, scales and levels of gas migration in mesopores and macropores have undergone significant changes. At pressures below 1 MPa, Knudsen diffusion predominates in mesopores and macropores, with the contribution of migration scales ranging from 97.33% to 99.90%. In the high-pressure stage, the contribution of slip flow in mesopores and macropores to the migration scale ranges from 64.55% to 99.86%; the contribution of surface diffusion to the migration scale ranges from 0.14% to 35.45%, without Knudsen diffusion. The research results can provide a theoretical basis for revealing the migration characteristics of deep coalbed methane at the microscopic nanoscale.