Cuttings Cleaning Mechanism Of Pulsed Jet At The Bottom Hole

The rate of penetration(ROP)enhancing technology of pulsed jet vibration drilling can assist the bit to break the rock in the deep formation efficiently.The fluctuation of pressure and flow rate generated by the pulsed jet at the bottom hole encourages the magnitude of the cutting at the bottom hole.While,the mechanism of cuttings cleaning by pulsed jet is not precise,resulting in low efficiency of cuttings cleaning and repeated cutting of drill bit at the bottom hole.Therefore,it is the key to revealing the mechanism of cuttings cleaning at the bottom hole by pulsed jet for bottom hole cleaning.In this thesis,the methods of laboratory experiment,theoretical derivation and numerical simulation are used to analyze the cuttings cleaning at the bottom hole:The experimental visualization system,which considers the condition of the bottom hole,is established.The particle image velocimetry(PIV)is adopted to capture the flow field of the water jet at the bottom hole.The results show that the length of the potential core of the jet at the bottom hole is affected slightly by nozzle diameter.Moreover,the length of the potential core of the jet at the bottom hole is shorter than that of the free jet.The fluid velocity at the bottom hole can be decomposed into the impingement velocity and crossflow velocity,which are proportional to the hydraulic horsepower and kinetic energy of the jet.There are two stable eddies between the jet zone for a dual-nozzle jet at the bottom hole.However,when the included angle of nozzles is over 40°,the eddies become unstable.Finally,the modified power function is used to fit the centerline velocity of the jet at the bottom hole with an accuracy of more than 90% to make the velocity attenuation of the jet at the bottom hole clarified.Based on the model of fluctuation of pressure and flow rate at the bottom hole,the characteristics of pulsed jet fluctuation are obtained.The mechanical model of cuttings at the bottom hole with the pulsed jet is established by combining the fluctuation of pressure and flow rate as well as the attenuation of the jet at the bottom hole.It is found that the pulsed jet can generate pressure fluctuation of 0.23 to 0.53 MPa,corresponding to the flow-rate fluctuation of 6.13 to 14.32 L/s.The fluctuation of pressure can reduce the instantaneous positive pressure force of cuttings,and the fluctuation of flow rate can improve the drag force of cuttings.The results mentioned above can promote the cuttings to break away from the bottom hole.Considering the fluctuation caused by the pulsed jet at the bottom hole,the model of cuttings transport is established.The fluctuation of the cuttings concentration in the drill bit domain is analyzed through the model.It is found that the pulsed jet can reduce the cuttings concentration in the drill bit domain compared to the continuous jet.The pulsed jet of square wave generates the best cuttings cleaning capacity.Sequentially,the pulsed jet of the sine wave has a slightly weaker effect,and the triangular wave generates the worst cuttings cleaning effect.Finally,the optimal pulsed jet parameters in the conditions simulated in the thesis are 16 Hz pulsed jet frequency with 5 m/s pulsed jet amplitude.Based on the optimal parameters of the pulsed jet,the hydraulic structure of the corresponding polycrystalline diamond compact(PDC)bit is optimized.In the conditions in the thesis,it is found that the amplitude of cuttings concentration increases with the increasing blade number.In addition,the increase of the nozzle angle and diameters is unfavorable for cuttings cleaning at the bottom hole.Based on comprehensive analysis,if only the effect of pulse jet bottom hole cleaning is considered,a 5-blade PDC bit with small nozzles is recommended for pulsed jet drilling.This thesis reveals the mechanism of cuttings cleaning at the bottom hole by pulsed jet from the two stages,which includes the cuttings breaking away from the bottom and the cuttings leaving into the annulus from the drill bit domain.The research results are expected to provide a theoretical basis for bottom hole cleaning of pulsed jet vibration drilling.