Dynamic effects of high-pressure pulsed water jet in low-permeability coal seams

Abstract

Mine gas extraction in China is difficult due to the characteristics such as micro-porosity, low-permeability and high adsorption of coal seams. The pulsed mechanism of a high-pressure pulsed water jet was studied through theoretical analysis, experiment and field measurement. The results show that high-pressure pulsed water jet has three dynamic properties. What’s more, the three dynamic effects can be found in low-permeability coal seams. A new pulsed water jet with 200∼1 000 Hz oscillation frequency and peak pressure 2.5 times than average pressure was introduced. During bubble collapsing, sound vibration and instantaneous high pressures over 100 MPa enhanced the cutting ability of the high-pressure jet. Through high-pressure pulsed water jet drilling and slotting, the exposure area of coal bodies was greatly enlarged and pressure of the coal seams rapidly decreased. Therefore, the permeability of coal seams was improved and gas absorption rate also decreased. Application results show that gas adsorption rate decreased by 30%∼40% and the penetrability coefficient increased 100 times. This proves that highpressure pulsed water is more efficient than other conventional methods.