| Using the micro-radius radial drilling method to exploit low permeability oil and gas, shale gas and coalbed gas, is an important direction for the development of oil and gas resource drilling. The water jet bit, whose performance determines the drilling efficiency and the extension ability, is the core component of radial drilling system. The straight-swirling integrated jet has a simple structure and good drilling capacity, meet the requirements of radial drilling technology. However, there is no further research on the characteristics of the flow field and the mechanism of rock breaking.In this paper, theoretical analysis, numerical simulation, 3DPIV test and rock breaking test are used to study the straight-swirling integrated jet. The characteristics of the jet flow field, the breaking mechanism, the optimal design criterion and the extension ability of the bit are analyzed. The main results are as follows:1 The flow field of straight-swirling integrated jet was researched, and the jet structure was obtained. The jet could be divided to straight jet area, high swirl strength jet area, low swirl strength jet area and periphery jet area along radial, and transition section, uniform mixing section along axial. The boundary line equation of straight jet area and swirling jet area Rz,x=F?k,L? was obtained. The calculation formula of the jet velocity along axial vm F=?df,L,vo? along radialvr F=(r,R,Rz,x,vm) were obtained. With the increase of the mixing ratio of straight and swirling, the low velocity zone of the jet can be eliminated gradually. At this time, the maximum axial velocity is at the center of the jet, and decreases with the increase of radial radius. The radial velocity was close to 0 at the jet axis. With the increase of the radial radius, the radial velocity increases at first and then decreases, and finally to 0. The tangential velocity distribution was similar to radial velocity, but much larger than radial velocity.2 The breaking mechanism of straight-swirling integrated jet was revealed. The erosion experiments and flow field tests of straight jet, swirling jet and straight-swirling integrated jet were carried out. Results show that, the straight-swirling integrated jet could eliminate the low velocity area and the boss at the hole center of swirling jet, and both the jet distribution width and hole diameter are bigger than straight jet. The fracture characteristic of the rock in the different jet area of integrated jet was analyzed. Results show that, in the straight jet area, first the rock is broken by jet dynamic pressure, and the micro fracture is produced, then the micro fracture extends by quasi-static pressure. The dynamic pressure and static pressure are also in the high swirl strength jet area. In addition, the tensile and shear effects of the rock are greatly enhanced, and it is easier to generate a large number of micro cracks on the surface of the rock, and the efficiency of rock breaking is higher than that of the straight jet area. The rock of low swirl strength jet area is damaged by straight jet and high swirl strength jet, the rock can also be effectively broken under the condition of low speed. The grinding effect on the hole wall is mainly carried by periphery jet area.3 The design criterion for the bit of straight-swirling integrated jet was established. Through theoretical calculation, numerical simulation, 3DPIV experiment and drilling experiment, the structure of drill bit was optimized: the design principle of the mixing ratio and the angle of the impeller groove was, making the jet have strong center velocity and swirl strength, hole bottom shape should be relatively flat, the mixing ratio Qz/Qs is 0.8, the impeller groove ?s is 45°. Other parameters should not restrict the diffusion of the jet, mixed cavity length to diameter ratio Lq/Dq is 0.67, mixed cavity angle ratio ?q/2?s is 1.33, front nozzle length to diameter ratio Lf/df is 0.56. The shape size is limited by the structure of the steering gear.4 Through analyzing the force and pressure loss of radial drilling system, the drilling limit depth prediction model was established. The influence law of the hydraulic parameter and the structure parameters?front nozzle diameter, jet diffusion angle, center hole diameter of impeller, back nozzle diameter, back nozzle angle? on the limit depth were studied with the established limit depth prediction model. The drilling limit depth of a certain project was calculated and the pressure loss and force were obtained.The research results of this paper provide a theoretical guidance for the prediction of the limit depth of radial drilling technology, and provide a theoretical guidance for the design of the bit. It enriches the theory of the flow field and the breaking mechanism, which has a certain engineering value and theoretical significance. |
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