Theoretical And Experimental Research On The Rock-breaking Mechanism Of Dual Drill Bit Under Reverse Rotary Condition

As exploration and development of important energy and mineral resources increases,drilling faces challenges in the development of its technology.Existing drilling equipment is mainly driven by a drill string with a "long arm jurisdiction" over the drill bit at a distance from the ground.A large amount of energy is consumed in the drilling equipment and accompanying drilling process,and only a very small amount of energy is used in the drill bit to break up rock.The greater the drilling depth the smaller the percentage of energy used for rock breaking by the drill bit.In view of this,the dual-bit self-balancing drilling technique was proposed as a solution.The technology adopts the method of localized closed torque self-balancing of the near drill bit to drive the dual drill bit,which has the features of high efficiency and stable rock breaking,fast tool up and down,continuous mud circulation and accurate data transmission.It is expected to improve the problems of high energy consumption,small percentage of pure drilling time and complicated equipment maintenance.At present,from the proposal of dual-bit self-balancing drilling technology to the prototype and experimental verification,certain progress has been made,but the theoretical research on the rock-breaking process and mechanism of this technology is not deep enough.Focusing on the above scientific and technical issues,this paper adopts theoretical calculations,numerical analysis and experimental validation to study the two major mechanisms of dual-bit reverse rotary rock breaking and self-balancing drilling bottom disturbance.The specific research and conclusions are summarized below:(1)The theory of rock breaking under reverse rotary mode is proposed,and the evaluation method of rock breaking effect of impregnated diamond drill bit is given.Based on the theories of rock breaking science and fracture mechanics,the near-field stress distribution of rock crack propagation is calculated.Considering the dynamic breaking effect of the drill bit under the rotary action,the shear breaking mechanism at the ring groove of the dual drill bit,the volume breaking mechanism of the step rock when the outer drill bit is over drilling,and the torque self-balancing to reduce the disturbance mechanism are investigated.The results show that the volume breaking effect of the rock at the bottom of the hole can be effectively improved by appropriately setting the ring groove structure on the cutting teeth and the over drilling mode with the outer drill bit.The torque self-balancing effect of the dual drill bit can effectively reduce the disturbance of the drill bit on the formation rock unit.(2)Modeled rock breaking for single and dual bit drilling.The stress distribution and crack propagation law of the rock at the bottom of the hole under the two modes of synchronized drilling and over drilling with dual drill bits were studied and compared with the single drill bit.The results show that reverse rotary drilling not only promotes crack propagation in rocks in the junction broken zone,but also attenuates crack propagation toward the core and stratigraphic direction.The dual bit synchronized drilling mode reduces the cutting force of the drill bit and thus reduces the mechanical specific energy of breaking rock.The mode of over drilling with outer bit can release the stratigraphic stress of the rock at the step.(3)Developed a set of dual-bit self-balancing drilling experiments for single and dual-bit comparative experiments under the same conditions,and carried out drilling experiments in granite,limestone and sandstone.The rock ridge at the bottom of the hole was reconstructed to evaluate the effect of same and opposite rotation on rock breaking at the ring groove from the perspective of the macroscopic breaking morphology of the rock,and finally to verify the volumetric breaking mechanism of dual-bit over drilling.The results show that the ring groove structure of the drill bit will cause the rock surface at the bottom of the hole to form a ring-shaped ridge,which is favorable to rock breaking,and the more developed the rock fissures and the poorer the inter-mineral cementation,the less likely to form a ring-shaped ridge.(4)Laser particle size distribution meter and scanning electron microscope technology were used to analyze the particle size distribution and microscopic morphology of rock chips,and the rock breaking effect of single and dual drills on different types was evaluated from the perspective of microscopic breaking morphology of rocks.The results show that the volumetric breaking of rock under synchronous reverse rotary action is more intense and the rock ridge crack propagation is more extensive.In the traditional rotary mode,diamond particles have grinding and microvolume breaking effect on the rock.In addition to these two types of breakage,the reverse rotary mode has a micro-shear breakage effect on the rock,which produces unique jagged and elongated rock powders.As the rock hardness decreases,the dual drill bit speed up the effect is more obvious.(5)Comparative experiments of single-bit and dual-bit drilling to simulate complex formations were carried out,using tri-axial vibration sensors to determine the vibration of the drill bit,using profilometry and X-ray imaging inspection methods to reconstruct the cross-section and CT scan of the obtained cores,and microscopic analyses of the borehole wall,which then corroborate the conclusions of the numerical simulations with each other.The results show that dual-bit self-balancing drilling can effectively reduce the vibration of the drill bit in the horizontal direction,which in turn reduces the damage to the core and the formation,and it can play a micro-disturbance advantage when drilling in soft rocks and complex formations.Dual drill bits have achieved better results in reducing lateral vibration of the drill bit,minimizing hole wall damage and improving core integrity.