Characteristics and formation mechanisms of microstructures in coal treated with CO₂ phase transition fracturing

CO₂ phase transition fracturing (CPTF) is a newly developed technique for coal mine gas control and is characterized by high efficiency, high coal seam permeability, a high gas drainage rate, and outburst elimination. The core of CPTF is reinforced fracturing and the pressure-relief and permeability-enhancement effects of coal seams under high-pressure dynamic loading. Nevertheless, there is a lack of studies on the morphological characteristics and formation mechanisms of newly formed fractures. Current observation and description of the underground fractures on a centimeter-meter scale primarily aim to reveal the fracturing and permeability-enhancement mechanisms of low-permeability coal seams. However, the study of microfractures on a nanometer-micron scale formed by CO₂ fracturing can describe the morphology and occurrence patterns of fractures more systematically and comprehensively and reveal the failure mechanisms of coal seams under the action of CPTF. In this study, anthracite samples were subjected to high-pressure CO₂ impact (120 MPa) using an independently developed large-scale physical test facility. Based on the observations obtained using a field emission scanning electron microscope (FE-SEM), this study investigated the characteristics, occurrence patterns, and formation mechanisms of micron-scale fractures. The results are as follows: (1) The cleat systems in the fractured coal samples were fully interconnected and formed a multi-scale, complex microfracture network; (2) The coal matrix in the samples was fractured and developed numerous new nanometer-scale microstructures, of which three types of typical microstructures were discovered, namely damage marks, Y-shaped fractures, and foliation structures; (3) The CO₂ in the supercritical phase and in the gas phase or in the supercritical phase mixed with the gas phase impacted and fractured the coal samples near the nozzles, and the remote fracturing of the samples was supposed to be primarily induced by shock wave; (4) The microstructures formed and evolved in three steps. First, damage marks formed on the surface of the coal matrix. Then, Y-shaped tensile, dentate branch fractures formed centering around the damage marks. Finally, multiple Y-shaped fractures combined to form a complex microfracture network. The complex reticular fracture system formed by the enhanced fracturing is the underlying reason for the pressure relief, high permeability, high gas drainage rate, and outburst elimination of coal seams.