Impact of coal matrix strains on the evolution of permeability

The goal of this study is to investigate how coal matrix strains affect the evolution of coal permeability. In previous studies, this impact was quantified through splitting the matrix strain into two parts: one contributes to the internal swelling while the other to the global strain. It was assumed that the difference between the internal swelling strain and the swelling strain of matrix determines the evolution of fracture permeability through a constant splitting factor. This assumption means that the impact of internal swelling strain is always same during the whole gas injection/production process. This study extends this concept through the introduction of a strain splitting function that defines the heterogeneous distribution of internal swelling. The distribution function changes from zero to unity. Zero means that the internal swelling strain has no impact on permeability evolution while unity means 100% of the internal strain contributes to the evolution of coal permeability. Based on this approach, a new permeability model was constructed and a finite element model was built to fully couple the coal deformation and gas transport in coal seam reservoirs. The model was verified against three sets of experimental data under the condition of a constant confining pressure. Model results show that evolution of coal permeability under the condition of a constant confining pressure is primarily controlled by the internal strain at the early stage, by the global strain at the later stage, and by the strain splitting function in-between, and that the impact of the heterogeneous strain distribution on the internal swelling strain vanishes as the swelling capacity of matrix increases.