Structure design on a new type of coupled impactor

Improving drilling efficiency and reducing drilling cost are new challenges for deep drilling. Based on this, a coupling impactor is designed by applying the principle of hydraulic pulse pressurization and axial impact. Based on the impactor compression structure and principle, the fluid flow equation of the drilling fluid flowing through the compression mechanism and the mechanical balance equation of the piston body are established in order to study the pressure pulsation characteristics of the coupled impactor fluid.


Introduction
With the development of economy, the demand for petroleum energy in China is increasing day by day.In the process of oil and gas well exploration and development, the hardness and plasticity of deep formation rock increase with the development ratio of deep and ultra-deep Wells, and the well structure becomes more and more complicated.When the drill bit encounters the non-average formation at the moment of drilling, the drill string torque energy cannot meet the torque demand of rock breaking, the drill string stops rotating instantly, and the excess energy is stored in the drill string.When the stored torque energy is greater than the torque required for rock breaking, the drill string energy will be released instantaneously, causing the drill bit and drill string to vibrate violently, resulting in "stick-slip vibration" [1][2][3][4].This will lead to a large increase in drilling costs, drilling efficiency and other problems.
In view of the above problems, domestic and foreign scholars have studied a variety of rock breaking tools, and carried out a lot of research and field tests to improve drilling efficiency.[5][6][7][8] Xia Chengyu et al. [9] designed a pulsed supercharger, and based on its structure and principle, carried out the influence of screw speed, high-pressure nozzle diameter, drilling fluid density and stroke on the booster frequency, nozzle flow rate and booster pressure, and obtained the relationship between each influencing factor and booster characteristics.Liu Peng et al. [10] analyzed the working principle of the gas-liquid combined impactor, proposed a method based on the valve port flow area to establish its mathematical model, and compared the simulation results with the test to prove the feasibility of the method.Yang Yandong et al. [11] proposed a new type of impact rotary drilling device using the coupling effect of hydraulic energy and drill string vibration.Through the fluid-structure coupling method, factors such as inlet flow rate, motion displacement, vibration frequency and inlet and outlet diameter were obtained to produce load.The results show that the dynamic load produced by the device increases with the increase of influencing factors, while the static load is only related to the change of flow rate.Yiming et al. [12] designed a guide cylinder scheme of a torque impactor, compared the erosion wear model theory verified by the test with the field test results, and analyzed the influence of the change of the cone Angle and the distance of the flow channel on the flow field and the erosion wear performance of the guide cylinder.Ma Ran et al. [13] established a vibration model based on the mechanism of stick-slip vibration, studied its working principle and motion characteristics, and conducted dynamic simulation analysis of the torsional impactor combined with the actual working conditions to study the factors affecting its life.Tian Jialin et al. [14] designed a new torsion impactor for sticking and stick-slip in the drilling process, established a kinematic model based on its actual working conditions, analyzed the relationship between the impact frequency of the impactor, inlet pressure and inlet flow rate, and verified its reliability through experiments.Zha Chunqing et al. [15] put forward a method of bidirectional coupled drilling speed increase, based on which a new type of bidirectional coupled impact drilling tool was designed, and the structural characteristics of the method and tool of bidirectional coupled impact speed increase were analyzed and demonstrated theoretically.Based on the structure and working principle of the pulse type downhole pressurization drilling device, Fu Jiasheng, Huang Zhuang et al. [16][17] established its piston pressurization model, and then carried out numerical simulation research on its pressurization characteristics, which provided a basis for the subsequent prototype design and field test parameter determination.In order to solve the problems of high drilling cost and low drilling efficiency, many studies have been carried out by predecessors and certain results have been achieved.It has also been found that when the impactor is drilling composite rock formation, the bit nozzle is easy to clog and the bit is easy to produce mud bag.Most of the torsional impactors at home and abroad have similar problems in the application process.
In this paper, based on the axial and torsional impactors conducted by the research group in the previous stage, a new coupling impactor is designed and the research on the coupling impactor is carried out.The coupling impactor mainly provides periodic axial and torsional vibration for the drill, and at the same time, the periodic pressure at the front end of the drill can reduce the blockage of the drill nozzle and reduce or avoid the phenomenon of mud wrapping.As shown in Figure 1, the coupling impactor is mainly composed of an upper joint, righting bearing, turbine fixed valve, turbine moving valve, positioning ring, thrust bearing group, upper hammer body, lower hammer body, disc spring, gland, cylinder, lower joint, lower seat, thrust ring, gland, housing, positioning ring, central shaft, valve ball, piston, spring, valve seat, return spring, ball valve, spring, disc spring and other structures.The upper end of the coupling impactor is the driving part, which is connected with the drill string through the upper connector, the lower end of the upper connector is connected with the shell, and the torque of the drill string is transmitted through the threaded connection.A central shaft is arranged inside the housing, and a turbine group is arranged between the upper end of the central shaft and the housing.The turbine group is composed of a dynamic fixed valve, and the upper and lower ends of the turbine group are equipped with centralizing bearings.The central part of the central shaft is provided with a flow channel, and the lower end is successively equipped with a positioning ring, a thrust bearing group, and an upper hammer body.The upper hammer body and the lower hammer body are matched with a special linear structure, and the lower hammer body can move in a small amplitude along the axis.One end of the hammer body is equipped with a disc spring, and the disc spring is installed axially through the gland.The lower hammer body is equipped with a piston group, a spring and two check valves, and the lower end of the lower hammer body is connected with the lower joint through a thread.The housing is simultaneously splined to the lower hammer, thereby transferring the drill string torque to the lower joint.The coupling impactor is mainly combined with the application effect of the axial and torsional impactor designed in the previous stage.Through improvement, on the basis of providing axial impact force and high frequency torsion in the circumference, the effect of pressurizing and pulsating jet near the bit is realized by small piston movement.Thus further improve the hard formation rock breaking efficiency, improve the working state of the bit, in order to improve the drilling efficiency and reduce the drilling cost.

working principle
The driving part of the upper end of the coupling impactor acts on the central shaft through the action of drilling fluid on the turbine dynamic fixed valve.The two ends are equipped with positioning rings to position the turbine set.The righting bearings mainly play the role of righting and thrust.Driven by the turbine stator, the central shaft drives the upper hammer body to rotate.The upper hammer body and the lower hammer body are contacted by a special trajectory line.During the rotation of the upper hammer body, the hammer head rises spirally along the slope trajectory of the lower hammer body.In this process, the upper hammer body generates circumferential force on the lower hammer body, making the lower hammer produce circumferential torque.The axial impact and torsional impact generated by the lower hammer body are transmitted to the drill through the lower joint.The mutual coordination of the axial impact and torsional impact effectively improves the contact stress between the drill and the rock and the crack propagation in the crushing pit, thus improving the rock breaking efficiency of the drill.A special helical linear structure is designed for the tail end of the central shaft corresponding to the upper end of the piston.In the process of rotation, the central shaft achieves axial movement of the piston through linear coordination with the upper end of the piston.In the process of axial periodic movement, the piston compresses the disc spring and the middle cavity, thus achieving the "pressure effect".When the piston is pressed down, the contact between the upper end of the piston and the lower end of the center shaft gradually changes from the high point to the low point.In this process, due to the action of the return spring, the piston moves upward, and the pressure of the fluid discharged by the fluid and impactor is reduced.In one cycle, the periodic reciprocating motion of the upper end of the piston realizes the compression and expansion of the intermediate cavity.

Structural advantage
In order to solve the problem that the bit nozzle is easy to plug and the bit is easy to mud during the application of the impactor, the design of this paper has the following innovations and improvements.
(1) On the basis of re-completing the contact form and structure improvement of the upper and lower hammer body, the "piston" device is installed in the lower part of the center shaft to realize the pressure of drilling fluid before entering the drill bit.
(2) In order to achieve the pressurization effect of the impactor, the tail end of the central shaft is designed as a special spiral linear structure, and the upper end of the piston is a corresponding spiral structure.In the process of rotation, the central axis is linearly coordinated with the upper end of the piston to realize the axial movement of the piston.
(3) During the pressurization process, the output drilling fluid produces periodic pressure fluctuations, which improves the injection effect of the bit nozzle, improves the rock breaking efficiency of the bit, and avoids the phenomenon of the bit nozzle plugging and the bit mud coating.

Turbine fluid flow model
The turbine in the turbine assembly is an axial flow turbine, which is characterized by the flow of liquid along the axial direction.As shown in the figure, the liquid flows out of the stator along the direction of the blades, into the lower rotor, on the one hand as the rotor rotates in a circular motion, on the other hand along the direction of the rotor blades, and then into the next stage of the turbine until the lowest turbine.The resultant motion of the fluid in the turbine between the two coaxial cylinders with diameters D1 and D2 is different due to the different distance between the fluid in each cylindrical layer and the axis, which leads to the different velocity of the fluid particles and the interaction with the blade.Based on this, the fluid movement of any cylindrical layer with diameter Di is selected as the research object, and the fluid movement of the cylindrical layer is equivalent to the average of the fluid movement of all cylindrical layers.This method, which represents the movement of the entire liquid flow by the movement of a certain unit flow, is called the element theory method, and D is called the calculation diameter.The interaction force between liquid particles and turbine blades in each cylindrical layer is the same.
The  are the Prandtl numbers corresponding to the turbulent kinetic energy and dissipated energy respectively.

The Navier-Stokes equation
The special helical linear structure of the upper end of the piston and the end of the central shaft achieves the periodic axial movement of the piston.The piston compresses the disc spring and the intermediate cavity to achieve the effect of pressure in the Fig3.The fluid medium is an incompressible fluid, and its momentum conservation meets the Navier-Stokes equation， Where,  is the mass density of the fluid, V is the velocity vector, u ， v ， w are the velocity component in the x ， y z direction respectively, p is the pressure, f is the external force per unit volume of the fluid, and the constant  is the dynamic viscosity.

Conclusion
Based on the previous research, a new type of coupling impactor is designed in this paper, and the coupling impactor is studied.Through improvement, on the basis of providing axial impact force and high frequency torsion in the circumference, the effect of pressurizing and pulsating jet near the bit is realized by small piston movement.It is of great significance to solve the problem that the bit nozzle is easy to clog and the bit is easy to mud during the application of the impactor.

[ 18 ]Figure 2 .
Figure 2. Turbine diagram.In order to simplify the calculation, it is assumed that the circular velocity of the liquid particle in each cylindrical layer increases linearly with the different distance from the liquid to the axis, and the circular velocity of any cylindrical layer with a diameter of Di is：

M
the torque is calculated for cylindrical layer liquid with arbitrary diameter, N• m; D M is calculated for a cylindrical liquid with an average diameter of D, N• m.The flow of fluid in a turbine can be described by the following equation,

Figure 3 .
Figure 3.The intermediate cavity.In cartesian coordinates, the components are of the following form[19]: is the unit mass force in x,y and z directions; p is the pressure on the fluid element; v is kinematic viscosity;  is the dynamic viscosity; i f k  and 