| As oil and gas exploration continues to expand to deeper depths,efficient rock breaking techniques need further innovation to better support the exploration and development of oil and gas resources in deep and ultra-deep wells.This paper uses a combination of theoretical studies,numerical simulations and indoor experiments to investigate one of the highly efficient rock-breaking techniques,simple harmonic vibration impact drilling,from the perspective of a fine view of internal rock cracking.A mathematical model for the fracture initiation of a single tooth cutting a rock unit containing a single fracture under the action of simple harmonic vibration impact was developed based on fundamental mechanical theory,and discrete element simulations of single tooth cutting a rock unit containing a prefabricated fracture and axial intrusion of a spherical tooth into the formation under the combined action of static and dynamic loads were carried out respectively.The main research contents and research results of the thesis are as follows:1.The parallel bonded intrinsic model in the PFC was used to simulate sandstone,and the sensitivity analysis of the fine parameters in the intrinsic model was used to determine the degree of influence of the main fine parameters on the macroscopic mechanical properties of the sandstone,and the values of the fine parameters of the discrete element model of sandstone were determined by the trial-combination method combined with indoor uniaxial compression experiments.2.Based on the theory of elastodynamics,a model of the stress distribution in the ground during single tooth cutting under simple harmonic dynamic loading was developed and used as the far-field stress acting at the defect surface to derive the stress intensity factor KII,T-stress and crack initiation angle models for the defect tip in two cases.3.Through numerical calculation of the theoretical model and numerical simulation of discrete elements,it can be obtained that:under the condition of simple harmonic dynamic load,the far-field stress scale factor?acting at the defect will fluctuate in a parabolic pattern with the change of?+?;when the rotation pole angle?is determined,the stress intensity factor KII and T stress at the crack tip will increase and decrease periodically with time under the action of simple harmonic force;under the condition that the crack inclination angle is 30°,45°and60°respectively,the combined action of dynamic and static load will reach the fracture toughness of the crack more easily and thus make the crack crack cracking faster than under static load.4.By carrying out discrete element simulation of the spherical tooth intrusion formation,we can compare the static load alone and the combined static and dynamic load from a fine viewpoint and obtain the following results:in both cases,the rock is mainly shear ruptured;under the same feed,the number of cracks formed in the rock under the combined static and dynamic load is more than that under the single static load,and there are more micro cracks and new cracks in the fracture area of the formation,and the connectivity of the fracture area is better.Under the same loading time,the displacement response of the spherical tooth intruding the formation is greater under the combined dynamic and static load,and the drilling efficiency is significantly better at a simple harmonic dynamic load frequency of 200Hz than at 100Hz,and the displacement response of the particles in the formation is wider under the simple harmonic dynamic load than under the single static load.Under the same loading time conditions,the loading frequency is in the range of 100Hz-300Hz,and the highest breaking efficiency is achieved when the loading frequency is 280Hz.The contact forces and stresses between the particles beneath the spherical teeth are better in magnitude and distribution than those under the single static load,which proves that the rock response is stronger and more likely to fracture and break under the combined dynamic and static load. |
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