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

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 mechanismof a high-pressure pulsed water jet was studied through theoretical analysis,experimentand field measurement.The results show that high-pressure pulsed water jet has threedynamic properties.What's more,the three dynamic effects can be found in low-permeabilitycoal seams.A new pulsed water jet with 200-1 000 Hz oscillation frequency andpeak pressure 2.5 times than average pressure was introduced.During bubble collapsing,sound vibration and instantaneous high pressures over 100 MPa enhanced the cuttingability 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 seamsrapidly decreased.Therefore,the permeability of coal seams was improved and gas absorptionrate also decreased.Application results show that gas adsorption rate decreasedby 30%-40%and the penetrability coefficient increased 100 times.This proves that high-pressurepulsed water is more efficient than other conventional methods.

Formation Principle and Characteristics of Self‑Supercharging Pulsed Water Jet

High-pressure pulsed water jet technology has considerable development potential in the field of rock fragmentation.To overcome the shortcomings of existing pulsed jets,a self-supercharging pulsed water jet(SSPWJ)generation method is proposed,which is based on the theory of the pulsed water jet and the principle of hydraulic boosting.The proposed method changes the flow direction of the fluid medium through the valve core to make the piston reciprocate in the cylinder and relies on the effective area difference between the front and rear chambers in the stroke stage of the piston to realize the organic combination of“pulse”and“supercharging”of the jet,thus forming an SSPWJ.On the basis of the formation principle of the SSPWJ,a SSPWJ testing platform was constructed,and tests were performed on the jet pressure acquisition,morphology capture,and granite erosion.Both the jet pressure and the jet morphology exhibited periodic changes,and a higher pulse pressure was obtained at lower inlet pressure.The error of the pressure ratio calculated according to the experimental results was<3%relative to the theoretical design value,confirming the feasibility of the method.The pulse pressure and pulse frequency are controllable;that is,as the inlet flow rate increases in the stroke stage of the piston,the pulse pressure and pulse frequency increase,and the pulse duration decreases.As the inlet flow rate increases in the backward-stroke stage of the piston,the pulse frequency increases,and the pulse pressure and pulse duration remain unchanged.Under the combined action of the waterhammer pressure,high-speed lateral flow,and high-frequency dynamic load of the SSPWJ,local flaky exfoliation was observed when the granite surface was eroded.The results of this study lay the foundation for enriching the theory of pulsed jet generation and expanding its application range.