Research On Rock Breaking Mechanism Of Drill Bit Under Harmonic Vibration Excitation

Efficient rock breaking technology is the core of oil and gas drilling, which is also the focus of basic theory research of drilling engineering. Especially in deep and ultra-deep wells, it plays a more important role. However, with the increasing of exploration and development in the deep and ultra-deep formation, it is more difficult to explore strata and break rock. In particular, when drilling in complex and hard formation, the effect of rotary drilling is not ideal. The rate of penetration is low and the drilling cost is high. Moreover, the drilling accidents often happen. Therefore, aiming at the drilling difficulties in deep formation and the actual situation of rock breaking technology in our country, harmonic vibration excitation as a new drilling method is proposed. In the paper, means of theoretical research, numerical simulation and laboratory test are adopted to study on rock breaking mechanism of drill bit under harmonic vibration excitation.The main contents and research results of the paper are as follows:1. Mechanics models of vibro-impact of rock are studied. The mechanical models of rock, vibration system and impact system are introduced from the perspective of medium, vibration response and interaction mechanism respectively. Also, the amplitude-frequency characteristic curve of steady state response of rock is given.2. Based on the modal analysis theory, the natural frequency calculation model of rock with a single fracture and the vibration model of rock with multiple fractures are presented to study the resonance characteristics of rock medium. Here, ANASYS software is used to analyze the influence of crack trend, crack size, and the number of cracks on harmonic frequency of rock. It proves that the presence of fractures will reduce the natural frequency of rock medium.3. Based on the least action principle,the micro vibration model of rock by harmonic impact is proposed. The micro vibration equation of one DOF rock is derived from multi-DOF motion equation. Besides, the influence of impact force, impact frequency and mass of rock on vibration response is analyzed by numerical method. The results show that the vibration displacement curve of rock under harmonic excitation is also in the form of harmonic function.4. The energy response models of rock medium with or without damping and the corresponding energy dissipation equations are obtained. In the paper, MATLAB software is used to solve models. Then two-dimensional graphs and three-dimensional trend diagrams are plotted which represents the influence law of model factors on response energy. It also can be concluded that natural frequency of rock and impact frequency will play a decisive role on the vibration period of energy response, while impact force, damping coefficient and mass of rock will mainly decide the vibration amplitude of energy response.5. On the basis of contact mechanics theory,the rock breaking models of conical indenter and spherical indenter in loading and unloading phase are introduced under dynamic and static load. On the one hand, the relationships between model factors and invasion depth of indenter are got by numerical calculation of MATLAB software. On the other hand, ANASYS software is used to analyze the displacement response and stress response of rock. Our investigations confirm that dynamic and static load expands its action area and the scope of rock response, exacerbates rock vibration and promotes rock breaking.6. Considering vibration theory, the physical model, mathematical model and simplified model of high frequency harmonic vibration impact system are established. Similarly, MATLAB software is used to solve models and three stages of motion equation are obtained which includes no contact, contact without progression and progression. It is confirmed that the drilling efficiency of drill bit is affected by rock stiffness, damping, critical force, impact force, excitation frequency and other factors.7. Rock breaking experiments under harmonic vibration excitation are carried out indoor. First, triaxial stress testing machine is applied to test the mechanical parameters. Then, ultrasonic wave transmitting transducer and acoustic emission testing system are used to test resonant frequency of rock. Finally, rock breaking experiments under harmonic vibration excitation are done by rock vibration excitation simulation test system. Experimental results prove that harmonic vibration excitation drilling can improve the vibration amplitude of rock greatly and promote rock breaking, thereby enhance the rate of penetration.Resonance enhanced drilling is an important direction for development of drilling technology in the future. The research on rock breaking mechanism under harmonic vibration excitation is the fundamental problem that must be addressed for achieving resonance enhanced drilling technique. This study can provide scientific guidance and theoretical support on drilling engineering and mining engineering for increasing penetration rate and reducing cost.