Switching mechanism and optimisation research on a pressure-attitude adaptive adjusting coal seam water jet slotter

To investigate the attitude-switching mechanisms of existing jet slotters,which integrate drilling,punching and slotting operations,and to improve its fracture ability,we used the power bond diagram theory to analyse the dynamic flow pressure,and force of slotters.A mathematical model was developed for the dynamic characteristics of slotter systems.Furthermore,to study the effect of the main characteristic parameters on the ability of the nozzle to erode sandstone,multi-orthogonal experiments were carried out.And the optimised slots were applied in later practical operations.The research results show that the inlet fluid passed through the time-varying orifice to generate pressure differential thrust,which overcame the spring force,pushed the valve core to open the side nozzle,and closed the rear cavity channel thereby realising the switch of the slotter attitude.An optimal plan was established to balance the diameter,depth,and volume of punching,and a rock-breaking plan was developed for the slotter.Subsequently,the optimised water jet slotter was practically used in coal seam gas drainage.Compared with conventional dense drilling,water jet slotting technology significantly improves the ability,efficiency,and effect of increasing the permeability of the coal seam.

Analysis and Numerical Simulation of Hydrofracture Crack Propagation in Coal-Rock Bed

In underground coal mines,hydrofracture can cause the increase of breathability in the fractured coal bed.When the hydrofracture crack propagates to the interface between the coal bed and the roof-floor stratum,the crack may enter roof-floor lithology,thus posing a limit on the scope of breathability increase and making it difficult to support the roof and floor board for subsequent coal mining.In this work,a two-dimensional model of coal rock bed that contains hydrofracture crack was constructed.Then an investigation that combines the fracture mechanics and the system of flow and solid in rock failure process analysis(RFPA2D-Flow)were carried out to study the failure mechanism at the interface between rocks and coals,and critical water pressure that hydrofracture crack propagates.The results indicated that the main factors that affect the direction of hydrofracture crack propagation are the angle of intersection between coal-rock interface and horizontal section,horizontal crustal stress difference,tension-shear mixed crack fracture toughness in coal-rock interface and differences in elasticity modulus of coal-rock bed.The possibility of crack directly entering coal-rock interface would increase with the increase in angle of intersection or horizontal crustal stress difference.The trend that crack propagates along the coal-rock interface will become stronger with the decrease of the fracture toughness at the coal-rock interface and the increase of the elasticity modulus difference between the coal bed and the roof strata.The results of this study was to put forward a method of controlling hydrofracture crack,optimize the fracturing well location provides a certain theoretical basis.

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.

The influences of composition and pore structure on the adsorption behavior of CH_(4) and CO_(2) on shale

CO_(2) enhanced shale gas recovery(CO_(2)-ESGR)has attracted extensive attention as it can improve the shale gas recovery efficiency and sequestrate CO_(2) simultaneously.In this study,the relationship between mineral composition,pore structure,CH_(4) and CO_(2) adsorption behavior as well as selective adsorption coefficient of CO_(2) over CH_(4)(αCO_(2)/CH_(4))in marine and continental shales at different temperatures was investigated.The results illustrated that shale with higher total organic carbon(TOC),higher clay minerals and lower brittle mineral contents has a larger micropores and mesopores volume and specific surface area.TOC content was positively correlated with fractal dimension Df.Both CH_(4) and CO_(2) adsorption capacity in shale have positive correlations with TOC and clay mineral content.CO_(2) adsorption capacity of the all the tested shale samples were greater than CH_(4),and theαCO_(2)/CH_(4) of shale were larger than 1.00,which indicated that using CO_(2)-ESGR technology to improve the gas recovery is feasible in these shale gas reservoirs.A higher TOC content and in shale corresponding to a lowerαCO_(2)/CH_(4) due to the organic matters show stronger affinity on CH_(4) than that on CO_(2).Shale with a higher brittle mineral content corresponding to a higherαCO_(2)/CH_(4),and no obvious correlation betweenαCO_(2)/CH_(4) and clay mineral content in shale was observed due to the complexity of the clay minerals.TheαCO_(2)/CH_(4) of shale were decreased with increasing temperature for most cases,which indicated that a lower temperature is more favorable for the application of CO_(2)-ESGR technique.