Influence of Nanopore Structure Deformation on Gas Migration in Coal

Abstract

To better understand the influence and control of nanopore characteristics on gas migration, three kinds of coal samples with different metamorphic degrees were selected for the experiments including high-pressure isothermal gas adsorption, low-pressure CO2 adsorption, and low-pressure Ar adsorption. The changes of the pore volume (PV) and specific surface area (SSA) of coal samples before and after adsorption–desorption were compared and analyzed. The adsorption data of all coal samples at a low pressure stage (<8 MPa) conformed to the Langmuir equation, and the adsorption capacity of powdered coal samples was higher than that of columnar coal samples. Some adsorption data deviated from the original fitting curve at a high pressure stage (>8 MPa), and this was the most remarkable in columnar coal samples. There was a positive correlation between the cumulative SSA of pores and adsorption capacity of coal samples. When the adsorption time was more than 10 min, the adsorption efficiency of 200 mesh coal samples from YJL was lower than those of 200 mesh coal samples from CZ and WY, which was due to the good development and connectivity of micro-fissures and nanopores in YJL coal samples. The pore size distribution of coal samples had changed after adsorption–desorption, and the cumulative deformation of the nanopore structure was anisotropic. As a result of the swelling or shrinkage deformation of the coal matrix, the PV and SSA with the same pore size presented many forms, such as almost unchanged, increased, or decreased. There are two types of deformation mechanisms: the whole collaborative deformation and partial deformation. Both gas adsorption and desorption can lead to the shrinkage or swell deformation of nanopores and fissures. In brief, the research provides theoretical and technical support for reservoir evaluation, fine drainage, and efficient development of coalbed methane.

Experimental Study on Methane Diffusion Characteristics of Different Metamorphic Deformed Coals Based on the Counter Diffusion Method

“…As the CH 4 gas pressure in coal increases, the coal adsorption of CH 4 gas molecules strengthens [55,56]. Under external constraint conditions, the adsorption expansion stress increases, leading to a decrease in the effective stress of the coal.…”

“…The change in the effective stress determined the exponential increase in the CH4 diffusion coefficient of the original columnar coal samples with increased gas pressure. As the CH4 gas pressure in coal increases, the coal adsorption of CH4 gas molecules strengthens [55,56]. Under external constraint conditions, the adsorption expansion stress increases, leading to a decrease in the effective stress of the coal.…”