Study on the Coal-Breaking Characteristics and Repair Effects of Water Jets in Gas Extraction Boreholes

Water jet repair technology, used for gas extraction boreholes, is an effective way to improve the effect of gas extraction. Nevertheless, the effects water jets have on the repair of gas extraction boreholes are still ambiguous. To investigate the characteristics of coal breaking using water jets in gas extraction boreholes and their repair effectiveness, a mathematical model for water jet coal breaking was established and a method for determining the depth of coal breaking using water jets was proposed. The coal-breaking depth, stress distribution law, and damage distribution law under the influence of water jet pressure, jet target distance, and coal strength were investigated using numerical simulations so as to make clear the repair effects of water jets on gas extraction boreholes. The outcomes indicate the following: the depth of coal breakage increases linearly with the increase in jet pressure: the depth of coal breakage decreases inversely proportional to the increase in coal body strength: and a reverse linear correlation exists between the water jet coal-breaking depth and the jet target distance. The extent of the influence exerted by coal strength, jet pressure, and jet target distance on the depth of coal breakage diminishes successively. The scope of damage caused by water jet flow to the rock surrounding gas extraction boreholes is limited. The coal-breaking depth model, coal body stress distribution model, and damage distribution model under the influence of jet pressure, coal body strength, and jet target distance were obtained using fitting. The research findings can offer a theoretical basis and technical support for enhancing the utilization efficiency of coal seam gas extraction boreholes.