Coal permeability related to matrix-fracture interaction at different temperatures and stresses

Abstract

Heat injection is considered to be an effective technique to increase Coalbed methane (CBM) production, which indicates the important role of temperature in gas flow. In this paper, two different experimental schemes have been designed, and the seepage tests concerning variable effective stress and variable pore pressure have been carried out respectively under different temperature conditions. A permeability model that considered the combined effect of temperature and stress was developed, and further extended so that it could be applied under other boundary conditions. An effective deformation factor fm was introduced to characterize the contribution of the change of coal matrix width to the change of fracture width, and the relationship between fm and temperature, under three boundary conditions, was quantitatively analyzed. Under constant pore pressure, the increase of effective stress will reduce the permeability. Under constant effective stress, an increase of pore pressure will lead to the decrease in permeability. Temperature would affect the permeability characteristics of coal from physical and chemical aspects, which were mainly manifested as inhibition, and this effect significantly weakened during the high stress stage. In terms of model matching, the permeability model results from each boundary condition were in good agreement with the measured results. In addition, the effective deformation factor fm increased exponentially over temperature under different boundary conditions. It is thought that these findings would provide a theoretical basis for the further study of coal matrix-fracture interaction, and CBM exploitation.