Study On The Rock Breaking Mechanism Of Compound Axial And Torsional Impact

The gradual depletion of shallow oil and gas resources forces the exploration target to gradually change to deep strata.With the increase of drilling depth,the rock mechanical strength gradually increases,and the rock breaking efficiency of PDC bit drops significantly,driving up drilling costs and risk.In recent years,on the basis of the conventional impact assisted drilling technology,the compound axial and torsional impact drilling technology has been developed,but its rock-breaking mechanism is relatively backward,which limits the development of this new technology.In this paper,the rock breaking mechanism of PDC bit under compound axial and torsional impact is systematically studied in order to promote the development of practical technology and provide more efficient technical means for deep oil and gas resources exploration.Firstly,based on the explicit finite element method,the rock breaking process,breaking volume and force history of conventional cutting,unidirectional impact,and CATI under different loads are analyzed.The studies showed that the CATI can achieve a larger breakage volume and higher cutting efficiency.The axial impact load can increase the breakage volume by increasing the cutting depth of the PDC cutter and changing the damage mode from ductile to brittle.The torsional impact load can accelerate the rock breaking process by reducing the damage accumulation time.Secondly,on the basis of numerical simulation,the equation of motion for PDC bit under impact loads is established,and the effect of load direction and phase difference on bit trajectory is analyzed.The control of PDC bit motion trajectory can be realized by controlling the amplitude and phase difference of CATI.Increasing the amplitude of the unidirectional impact can deflect the trajectory in the direction,and the existence of the phase difference increases the sweeping space of the trajectory of the PDC cutter.In order to obtain a larger rock breaking volume,the axial impact can be preceded by the torsional impact,and the phase difference is controlled within half a cycle.Then,a drilling rig with an impact module is developed,and the effects of impact direction on mechanical specific energy(MSE)and stick-slip vibration of PDC bit are studied through numerical simulation and experimental tests.The results show that the direction of impact load has a significant effect on MSE and stick-slip vibration.Increasing the impact angle can help to suppress stick-slip vibration of the PDC bit,but it is not conducive to improve the drilling efficiency.There is an optimal range of impact angle of 40° to 60° which takes into account both drilling stability and drilling efficiency.Furthermore,based on the numerical simulations and laboratory experiments of impact load direction,the influences of the amplitude and frequency of CATI on the optimal impact angle are studied.The results show that the optimal impact angle decreases with the increase of impact load amplitude,and the drilling efficiency increases with the increase of impact load amplitude at the same impact angle.The optimal impact angle increases with the increase of impact load frequency,and the drilling efficiency decreases with the increase of impact load frequency at the same impact angle.Finally,a CATI drilling tool is developed,and the impact direction and phase difference are optimized.The tool in the same diameter with different impact directions are designed and have outstanding performance in application.The field test results show that the 10° impact angle tool can effectively improve the drilling efficiency,and the ROP increases by 34.1%-190.5%.The 50° impact angle tool can effectively improve the drilling efficiency and suppress stick-slip vibration,the ROP increases by 34.1%-161.3%,and the bit footage increases by35.4%-49.9%;The 70° impact angle tool can effectively suppress stick-slip vibration,and the bit footage increases by 43.3%-370.9%.Aiming at the rock breaking mechanism of CATI,the rock breaking mechanism and motion characteristics of the PDC bit under CATI are systematically studied by means of theoretical analysis,numerical simulation,and experimental research.The effects of direction,amplitude,and frequency of impact load on MSE and stick-slip vibration are analyzed.The drilling rig with impact module is developed,and a CATI drilling tool is developed and applications are carried out.The research results provide theoretical and technical support for the development of CATI drilling technology.