Fracture characteristics of coal jointly impacted by multiple jets

Abstract

Drilling technology using multiple jets can achieve an ultra-small hole radius or well, and achieves good engineering results in soft rock such as coal. In this study, multiple jet impingement experiments were conducted to reveal the rock-breaking mechanism for coal. Computed tomography scanning was applied to observe the propagation law of internal rock fractures formed by the impact of multiple jets. By analyzing the relationship between the angle and length of a conical fracture and the arrangement mode of multiple jets, two types of interference fracture propagation laws were obtained. Combined with scanning electron microscope images of the impact debris, the fracture characteristics of coal impact by multiple jets is revealed, and a novel rock-breaking mode for multiple jets containing a center jet based on the combined impingement fracture propagation is proposed. The pilot hole formed by the center jet provides a free boundary for the side jet impact to avoid interference between adjacent jets. A conical fracture on the side near the center jet can also expand on the other side. When the cone length reaches the distance between the side jet and center jet, the erosion pits respectively generated by the adjacent jets will be connected, thus forming a joint erosion pit. Compared with multiple jets without a center jet, the rock-breaking efficiency of multiple jets containing a center jet can be improved by 47.3%. The present research results are a supplement to the rock-breaking theory of water jet impingement, providing theoretical support for the design of a multi-orifice nozzle.