Research progress and strategic thinking of coal mine water jet technology to enhance coal permeability in China

The safe and efficient development of coal is the key factor of energy safety in China，which has the significant security effect. With the gradual increase of coal mining depth in China，the situation of complex geological structure，high gas pressure and low seam permeability in deep coal mines is becoming more serious. The difficulty of coal mine disaster prevention and control represented by coal and gas outburst is gradually increasing，which seriously restricts the safe and efficient coal mining in China. To improve the permeability of coal seam is the key to eliminate the coal and gas outburst risk and improve the efficiency of coalbed methane extraction. Traditional coal seam stimulation technology is confronted with problems such as limited enhancing range and low extraction efficiency. Moreover，subjected to the effect of insitu stress，the conventional fracturing technology often has shortcomings such as single shape of hydraulic fracture and the risk of coal seam roof and floor damage. Moreover，the gas left in the “blank zone” on both sides of hydrofracture is prone to induce disaster in the subsequent mining process. In recent decades，many hydraulic pressure relief technologies have been widely used for coal reservoir permeability improvement. In this paper，the research history and stateoftheart progress of water jet technology in coal seam permeability enhancement in China are reviewed. The theory，technology，and research status of hydraulic punching，water jet slotting，hydraulic slot directional fracturing，and treetype borehole technology are systematically summarized. Because of the complexity，particularity and suddenness of deep coal seam gas dynamic disaster，there are still some scientific problems to be solved. This article analyzes the shortcomings and limitations of water jet technology to enhance gas extraction in coal mine and puts forward the development trends and prospects in the future.