Simulation Study On Rock Fragmentation Of PDC Bit Based On Discrete Element Method

The drilling depth of PDC bit has accounted for more than 80%of the total drilling depth.It has characteristics of low cost about unit drilling,high speed,long service life and so on.At present,with the development of deep drilling,PDC bit has been more and more widely used in geological exploration and drilling stratum.In the process of PDC bit drilling rock,the bit drilling efficiency is mainly affected by the bit cutters cutting rock efficiency.The main factors that affect the cutting force of PDC cutter include such as cutting parameters,rock properties and drilling parameters and so on.In the drilling process of bit cutting rock,PDC cutters are directly connected to the rock.The unreasonable cutting parameters will lead to unstable drilling speed and cutting tooth crack phenomenon.At the same time,the bit service life can be reduced.therefore,The study of the interaction between cutting teeth and rock is of great significance to reduce the loss of cutting teeth wear and improve drilling efficiency.Based on the previous research,this paper focuses on The mechanical properties of the cutting teeth in the dynamic drilling of PDC bit are studied by the experiments and the numerical simulation of the discrete element method.In this paper,the following main points are studied:(1)The virtual uniaxial compression test and the virtual Brazil splitting test of rock were established.Based on the uniaxial compression test of rock and the experiment of Brazil splitting,The microscopic parameters of rock is deduced.The constitutive relation of the 2D and 3D discrete element models are determined.Last,the rationality of the virtual experiment results is verified by experiments.(2)On the basis of 2D and 3D discrete element model of rock,the 2D and 3D discrete element model of PDC single tooth cutting rock was established.The effects of cutting tool rake angle,cutting depth are discussed by two models.(3)The model of PDC cutter cutting disc shape rock was established,The effects of different cutting methods on rock breaking effect are analyzed and The drilling parameters of cutting teeth are analyzed by numerical simulation.Finally,the PDC cutter cutting concave and convex rock model are established,and the effect of different rock shape on PDC rock breaking effect is discussed.(4)A single wing of bit breaking rock discrete element model was established.The influence rules of the drilling speed and rotation rate on the cutters are analyzed.At the same time,the "Attack" and crushing specific work of PDC teeth are studied.Under the same drilling speed and the speed of rotation,the torque and axial force of the cutters increased with the increase of the distance from the center of the drill bit,and then increased and then decreased.The cutting force of each cutting tooth increases with the increase of the drilling speed,and the cutting force of cutters near the shoulder is the largest.However,the rotational speed has little influence on cutting force.the drilling speed has little influence on the "attack",and the "attack" of the cutting teeth increases with the increase of the rotation speed.(5)The discrete element model of PDC bit drilling rock was established,and the distribution of the torque and axial force of each bit cutters are predicted.The calculation results of the model are verified by the test of the whole bit drilling rock,and the correctness and accuracy of the model are verified.At the same time,the influence of the load and the crushing work ratio of each of bit cutters on breaking rock effect is discussed.In addition,the cutting torque and axial force of each bit of the cutter are analyzed.The results show that:In the process of PDC bit cutting rock,the torque value of the drill enveloping surface and the circular arc is larger than that of the other cutting teeth,The axial force tends to decrease gradually from the center to the outer side of the drill string.When the torque of each PDC bit wing increases with the distance between the center axis of the drill bit,the torque of the cutting tooth increases gradually and then decreases.The axial force of each PDC bit wing decreases with the increase of the distance between the cutting teeth and the central axis.Through the numerical simulation results of sandstone wob and torque values with the experimental results error were 9.32%and 3.64%respectively.It can be seen that the accuracy and feasibility of the discrete element model of PDC bit drilling rock was established.