Micro-fractures in coal induced by high pressure CO₂ gas fracturing

To effectively drain gas from coal seam, many technologies have been developed to fracture the coal which in turn to release the formation stress and increase permeability and finally achieve the desired gas drainage. CO₂ gas fracturing is a newly developed technique for coal seam stimulation towards effective gas drainage. This technique can provide radial fracturing from centimeter to meters scales, but the small-scale fracture system that feeds into this radial system, at millimeter to micrometer scale, has not previously been systematically studied for quantifying the matrix damages. This study addresses this deficiency through blast-loading experiments with CO₂ on anthracite specimens at overpressures of 120 MPa, 150 MPa and 185 MPa in the in-house novel experimental system. The Field Emission Scanning Electron Microscope (FESEM) images suggest that the cleat system is destroyed, and the intervening matrix effectively pulverized due to the impact load induced by the CO₂ blasting. Radial and branching fractures are featured with the process, as typical in high velocity fracturing. The center point is a serious damage mark (DM) that always shows as a shallow pit, and number of smaller broken coal scatter surround the mark. These newly produced fractures are open and zigzag. Based on the observations, it is supposed that the matrix fracturing mechanism by CO₂ gas impacting can be described as four consecutive steps: CO₂ gas beam destroyed cleat system firstly, and coal broken to millimeter size particles. Meanwhile, CO₂ gas jet beam impacting on coal matrix induce a DM and resulting in the matrix's tensile deformation. Consequently, tri- fracture initiated from the DM and extending radially to different direction. When numerical tri-fracture connected and formed a complex fracture network in which permeability is expected to be significantly promoted for the effective gas drainage.