Influence of coal seam bedding on the effect of fracturing coal and enhancing permeability by microwave

Abstract

As a typical structural weak surface, bedding is ubiquitous in coal seams. The relative mechanical properties between coal seam bedding and coal matrix are the key to determining the effect of enhancing permeability by microwave in coal seams with bedding development. To this end, a self-developed microwave irradiation (MI) test device was used to carry out microwave fracturing coal experiments on coal samples with an angle of 0°, 30°, 45°, 60° and 90° between the bedding plane and the loading direction. The evolution characteristics of microscopic pore structure and microscopic cracks of coal samples with different bedding directions were explored, and the influence of bedding structure on the number, scale, and connectivity of coal pores and fractures under MI was revealed. The gas permeability characteristics of coal in different bedding directions before and after MI were compared and analyzed. The variation law of coal permeability under the influence of coal seam bedding and fracturing coal by microwave was clarified, and the influence mechanism of coal seam bedding on fracturing coal and enhancing permeability by microwave was revealed. The results show that the coal seam bedding has a significant effect on the effect of fracturing coal and enhancing permeability by microwave. The larger the angle between the bedding plane and the loading direction, the greater the reduction of the bound pores and the growth of the connected fractures of the coal under MI. Compared with the original coal samples, the order of magnitude difference between the permeability of coal samples in different bedding directions after MI is effectively reduced, and the permeability anisotropy is obviously weakened. The development of meso-cracks in coal under MI has a significant bedding effect. The specific evolution process is first to expand the original cracks along the bedding plane, followed by the initiation of new cracks along the bedding plane, and finally to expand the cracks intersecting with the bedding plane. The difference of dielectric loss and heat transfer properties between coal matrix and bedding plane makes the thermal stress distribution of coal subject to the bedding plane. When the bedding plane is perpendicular or parallel to the loading direction, the coal body is dominated by tensile failure. When the bedding plane is oblique to the loading direction, the coal body is dominated by shear slip failure.

Keywords

• fracturing coal and enhancing permeability by microwave
• coal seam bedding
• structural damage
• permeability characteristics
• bedding direction