Simulation on a new reverse circulation fishing tool: Design and evaluation of the salvage capacity and efficiency

To solve the insufficient energy for reverse circulation fishing tools in complex structure wells, it is necessary to further optimize the design of the tools to improve the impurity salvage efficiency. In this work, a three‐dimensional (3D) flow field simulation model of reverse circulation fishing tool with venturi negative pressure is employed to identify the vortex zone of circulation channel and to investigate the negative pressure and jet velocity distribution around the spray nozzle based on the ANSYS‐CFX. The effects of the spray angle, diameter of the suction nozzle, and the nonstructural parameters on negative pressure and jet velocity are investigated through sensitivity analysis. As revealed from the results, there may be an optimum value for the spray angle, and the nozzle jet direction can indeed affect the propagation and attenuation of the bottom hole flow field greatly. The combination of the inner diameter of the nozzle outlet and that of the nozzle inlet (Φ + Φ′ = 3 mm + 6 mm) is the best design solution when the negative pressure at the nozzle outlet can be the highest. The negative pressure and jet velocity increase with the increasement of the inlet flow rate. The comparative analysis of different bottom hole impurity mass in the cases of the borehole inner diameter 5′ and 8.5″ on the salvage efficiency are conducted numerically. It is observed that the impurity mass salvaged and salvage time are not directly related to its total amount, but only related to the flow rate of the sucked fluid flow. The impurity salvage efficiency can be as high as 95.4%, which meets the engineering requirements in the field. This work improves the working performance of the reverse circulation fishing tool and provides theoretical guidance for well washing.


| INTRODUCTION
There are many difficulties in returning debris, which can easily lead to vicious accidents such as sticking when fishing in various wells, including several stages of fracturing operation in shale gas well, complicating accident handling and increases production cost. [1][2][3][4] Using an effective drilling technology or tool to carry the debris from the drill pipe to better solve the stuck pipe accident caused by the impurity residue in the wellbore is a key problem that needs to be solved urgently.
At present, there are three commonly used cleaning and salvage schemes: blowback, nonnegative pressure salvage tools, and negative pressure salvage tools. 5 Conventional well washing methods are prone to leave debris residue, and the washing medium is susceptible to freezing in winter. [6][7][8] At present, reverse circulation washing tools play important roles in improving the debris salvage efficiency in the well washing operation. The schematic diagram of the reverse circulation drilling system is shown in Figure 1. The working principle is that the working fluid under pressure shrinks through the nozzle mouth of the reverse circulation fishing tool, forming a high-speed fluid injection, and the fluid pressure energy at the nozzle mouth is converted into kinetic energy. A jet nozzle is the most critical actuator of the reverse circulation fishing tools. A local negative pressure area is formed at the nozzle mouth and the suction chamber, causing the suction chamber pressure to be lower than the ambient pressure. Therefore, the working fluid and the sucked fluid are mixed in the suction chamber and exchange their energies, and the fluid at the bottom of the well is sucked upward, forming a complete local reverse circulation channel. Finally, the impurities in the well are sucked into the cavity to salvage the falling objects at the bottom effectively.
However, there is a "circulation short circuit" when the local reverse circulation tool is used. 9 The workover fluid sprayed into the annulus from the water outlet of the local reverse circulation tool does not enter the tool through the bottom hole, but directly returns to the surface from the annulus, so that adequate flushing of debris into the salvage tool cannot be guaranteed. To solve the insufficient energy for reverse circulation, most of the oilfields abroad have studied a partial reverse circulation washing tool. Foreign countries have accumulated rich experience in the study of oil wellbore cleaning and salvage tools. Among them, the three top oil and gas service companies are the most famous: Baker Hughes, Weatherford, and Schlumberger. [10][11][12] They have designed a series of downhole salvage operation tools, but the actual salvage effect still cannot fully meet the on-site salvage requirements. In view of the limitations of the current traditional fishing tools, it is necessary to carry out research on the structure optimization of new reverse circulation fishing tools combined with the characteristics of underground operations, guiding the oilfields application in China. Wang et al. 13 designed a cyclone reverse circulation well washing device and constructed the flow field calculation model considering a two-phase fluid (gas phase and solid phase) by using computational fluid dynamics (CFD) to verify the feasibility of the design. Zhang et al. 14 designed a typical grid model of the internal flow field of the reverse circulation drill bit, analyzed the effects of suction nozzle parameters on the performance of the drill bit, and validated its feasibility using a threedimensional (3D) density-based solver by field trial.
The optimal design of the structural parameters of the jet nozzle is very important for the salvage effect of the tool. To solve the insufficient energy for reverse circulation fishing tools, a 3D flow field simulation model of a new reverse circulation fishing tool considering the principle of venturi negative pressure is employed in this work to identify the vortex zone of washing circulation channel and investigate the negative pressure and jet velocity distribution around the spray nozzle based on ANSYS-CFX. Sensitivity analysis is performed to investigate the effects of the nozzle structure parameters, including spray angle, the diameter of the nozzle, and the nonstructural parameters on negative pressure and spray characteristics of the jet nozzle of the reverse circulation fishing tool. This work provides a theoretical basis for improving the reverse circulation performance of complex structure wells to improve the low debris salvage capacity.

| Optimal efficiency equation of jet pump
An independent nozzle flow channel is assumed in the jet pump. The influence of the nozzle structure parameters and the nozzle inlet and outlet conditions on the working efficiency of the jet pump is determined. Based on the relationship between the pressure drop at the nozzle mouth and the system supply flow, the pressure loss in the nozzle flow channel, and the system supply flow. 15 It is assumed that the energy of the working fluid flowing through the nozzle inlet is E 1 , the energy loss when flowing through the nozzle pipe is E 2 , and the energy flowing out of the nozzle is E 3 . The below expression can be derived based on the energy conservation 16 : According to the Bernoulli equation, a viscous incompressible fluid is as follows: The energy conservation equation of the nozzle runner is established. It is assumed that the pressures at the tool inlet and nozzle inlet are p 0 and p 1 , respectively, the flow rate of the fluid from the tool into the nozzle inlet is Q 1 , and the inlet diameter is d 1 . Then, the flow domain between the tool inlet and the nozzle is simplified as a horizontal flow channel, as written below: where Δp is the pressure difference between the tool inlet and the nozzle inlet, Q is the flow rate, ρ refers to the fluid density, κ is the frictional resistance coefficient of the inner wall of the pipe, and l 1 represents the distance between the tool inlet and the nozzle inlet. Then, expression (4) below can be acquired: According to Q = vs, the velocity of the fluid flowing through the nozzle inlet can be obtained as follows: Because p Q ρgkl Δ = 2 1 , Equation (6) can be obtained: In the above expression, Q 2 refers to the flow rate of the sucked fluid into the tool, p is the bottom hole ambient pressure, p 2 and d 2 refer to the pressure at the nozzle outlet and the outlet diameter, respectively, and l 2 represents the distance between the nozzle outlet end and the tool outlet end.
The fluid velocity at the nozzle outlet can be obtained as follows: The total energy loss generated by the fluid flowing through the entire nozzle channel is represented by h w , which is mainly related to the energy loss h L along the path and the local energy loss h M . The lengths of the large-diameter and small-diameter sections of the nozzle are l 3 and l 4 , respectively.
, the energy loss along the way of the fluid passing through the large-diameter section of the nozzle is given as follows: The energy loss of the fluid along the path of the nozzle constriction is: Substituting (7) into (10), we get: According to the formula h ζ = , we get: Therefore, the total energy loss of the fluid flowing through the nozzle is: Substituting the results into the formula ν + + p ρ , we get:

| Mathematical model of venturi tube
Assume that the diameter of the main section of the venturi tube is D, the diameter of the throat section is d, the inlet pressure and flow rate are P 1 and v 1 , respectively; the throat section pressure and flow rate are P 2 and v 2 , and the outlet pressure and flow rate are P 3 and v 3 , respectively, v 1 = v 3 = v; the local friction coefficients of the shrinking pipe section and the gradually expanding pipe section are k 1 and k 2 . In the case of ignoring the friction along the way, the Bernoulli equation is established and combined with the law of conservation of mass, the following relationship can be derived 17 : For the steady flow in the differential pressure flowmeter, the general calculation formula can be obtained from the fluid continuity equation and Bernoulli equation: | 2125 q m is the mass flow, kg/s; β is the section ratio; D is the inner diameter of the pipe, m; ρ is the density, kg/m 3 ; Δp is the differential pressure, Pa; ε is the fluid expansion coefficient; C is the venturi outflow coefficient; The pressure-head difference can be converted into the water-head difference, the above formula can be further rewritten as the flow calculation formula commonly used in venturi tubes: In the formula, χ is the relation constant term; Δh is the head difference, m.

| Salvage efficiency
Considering the fishing tool and the wellbore as a whole analysis system, the impurities in the wellbore are always uniformly distributed during the cleaning and salvage process, that is, the impurities are evenly sucked into the tool along with the fluid.
Assuming that the length of the entire horizontal wellbore is L, the volume of the horizontal wellbore is V 1 , and the volume of the tool is V 2 . In the initial state, the total amount of impurities in the wellbore is M 0 , the mass in the cavity is m, and the flow rate of the fluid to be sucked is Q. After Δt time, the impurities are sucked into the tool storage. so we have 18 : Differentiating t, we get: Initial conditions: t = 0, m = M 0 , substitute into the above formula, The fishing tool designed in this work is mainly based on the principle of the central jet pump. The tool is mainly composed of three parts: the spray nozzle, the rotating nozzle holder, and the jet shell. The 3D geometry of the fishing tool is shown in Figure 2. In this work, the borehole inner diameter is assumed as φ = 127.0 mm.

| Meshing and boundary
To simplify the model calculations, the following assumptions are made: (1) heat transfer is not considered during the simulation, and the energy equation is in the closed state; (2) the working fluid is incompressible and steady; and (3) gravity is not considered. The diameter of the spray nozzle is smaller than that of other parts, thus the main part of the jet nozzle is partitioned to facilitate the refinement of the mesh accuracy of each part. Figure 3 shows the 3D meshing model of the fishing tool.
The working conditions are set as follows. The wellbore outer diameter is 127 mm, the well depth is 3000 m, the bottom hole environmental pressure is P = 30 MPa, the drilling fluid density is 1.0 g/cm 3 , the relative environment temperature is 25°C, the inlet flow rate is Q = 240-600 L/min, the outlet static pressure is 0 MPa, and the wall of the annulus is free to slip.

| SIMULATION RESULTS AND DISCUSSION
In this simulation, the well depth is 3000 mm, then the bottom confining pressure is P = 30 MPa, the outlet pressure is set to 0 MPa without changing the nozzle structure and throat structure, and the inner diameter of the wellbore is 127 mm, the inlet flow rate Q = 300 L/min.
The basic dimensions of each component of the fishing tool are defined as follows. The inner diameter of the central channel of the rotary nozzle seat is 12.5 mm, the inner diameters of the jet channel on the side of and inside the rotary nozzle seat are 22 and 17.5 mm, respectively; and the number of nozzles is 3. To improve the salvage efficiency of the tool, the negative pressure area of the core structure is first identified, and the negative pressure value of the critical area is further determined. Figure 5 shows the negative pressure zone around the suction nozzle. Figure 6 is the trajectory diagram of flow whirlpool zone identification based on two-dimensional (2D) slice. As observed, the fluid is ejected from the internal nozzle of the tool to form a high-speed water jet, which collides with the annular wall and the bottom hole, generating a rotary motion or a linear motion. There are multiple tangential discontinuities between the jet fluid and its surrounding fluid. Once the discontinuity is disturbed, it will lose its stability and generate multiple whirlpools, which entrains the surrounding fluid into the jet fluid. Finally, the impurities at the bottom of a well are sucked into the cavity and then brought to the ground. Under reasonable conditions, the numerical simulation and optimization of the jet pump for salvage cleaning tools are carried out by changing the structural and nonstructural parameters.

| The spray angle of the suction nozzle
Based on the optimal design of the fishing tool, it is hoped to obtain a relatively large negative pressure value. However, high negative pressure causes high-pressure difference between the rotating nozzle seat and the nozzle, resulting in excessive energy loss through the nozzle flow channel and increasing the risk of deformation at the nozzle. Thus, there is an optimal angle that can generate the target suction ability to complete the reverse circulation cleaning. In this simulation, multiple simulation models were established to investigate the influences of spray angles on the jetting velocity and the negative pressure.   Figure 7 shows the simulation results for the velocity field vector cloud map at different spray angles. It is observed that the maximum jet velocity increases as the spray angle increases within 12-16°. However, the maximum jet velocity tends to decrease slightly as the spray angle increases within 16-18°. With the increase in the spray angle, the average overflow velocity of the bottom hole generally increases in the nozzle inclination direction. When the injection angle θ = 16°, the upward return velocity of the drilling fluid on one side of the inclined annulus of the jet nozzle increases significantly. Therefore, the nozzle jet direction can indeed affect the propagation and attenuation of the bottom hole flow field greatly. Figure 8 shows the simulation results for the negative pressure distribution around the nozzle at different spray angles.  To collect the variation law of the negative pressure value around the nozzle more accurately, the path line AB is drawn along the fluid jetting direction, and 50 collection points are arranged, as shown in Figure 9. Figure 10 presents the negative pressure results of path line AB at different spray angles. As indicated by the results, the relationship between the negative pressure and jet velocity under different spray angles is obtained, as indicated in Figure 11. Obviously, with the changing of the spray angle from 15°to 18°, the extreme value of negative pressure becomes smaller, and then there may be an optimum value for the spray angle. When the spray angle θ = 16°, the negative pressure is the highest. The maximum negative pressure value along the nozzle outlet is −15.27 MPa when the spray angle θ = 16°.

| The diameter of the suction nozzle
In this simulation, multiple simulation models were established to investigate the influences of the diameter of the suction nozzle on the jetting velocity and the negative pressure. The combinations of the inner diameters of the nozzle outlet and inlet are Φ + Φ ′ = (2.5 mm + 6 mm), (3 mm + 6 mm), (4 mm + 6 mm), and (5 mm + 6 mm), respectively.   Figure 12 shows the simulation results for the velocity field streamline distributions with different combination types of the inner diameters of the nozzle outlet and the inlet. Under the constant nozzle structure parameters, system supply flow, and ambient pressure, the smaller the nozzle outlet diameter, the higher the working fluid velocity through the nozzle outlet. When the nozzle outlet diameter is 2.5 mm, the corresponding velocity increases sharply. In this case, the risk of deformation at the nozzle is increased. Figure 13 shows the simulation results for the negative pressure with different combination types of the inner diameters of the nozzle outlet and inlet. The velocity field and the negative pressure value when the inner diameter of the nozzle outlet section is Φ = 2.5 mm are supplemented based on the original design structure and size of the salvage tool. The relationship between the negative pressure versus different combination types of the inner diameters of the nozzle outlet and inlet along the fluid injection direction is obtained based on the above results. Figure 14 presents the negative pressure results of path line AB at different combinations of the inner diameter of the nozzle outlet and the inner diameter of the nozzle inlet. It is found that the negative pressure value of the nozzle outlet increases with the decrease of its inner diameter and increases slightly with the decrease of the inner diameter of the nozzle inlet. Obviously, the greater the negative pressure, the greater F I G U R E 11 Influence of the spray angle of the nozzle on the negative pressure and jet velocity.
F I G U R E 12 The velocity streamline diagram at different combinations of the inner diameter of the nozzle outlet and the inner diameter of the nozzle inlet. the suction force, which is helpful for the salvage of impurities in the wellbore.
Through the structural optimization design of the salvage cleaning tool, it is hoped to obtain a relatively large negative pressure to improve the entrainment capacity of the reverse circulation fishing tool. When the diameter of the nozzle outlet is too small (Φ = 2.5 mm), the negative pressure increases sharply. However, this will lead to excessive pressure difference between the rotating nozzle seat and the nozzle, resulting in excessive pressure loss of the entire negative pressure generating device, increasing the risk of deformation at the nozzle, and affecting the reliability of the tool in downhole operation. When the inner diameter of the nozzle flow channel is twice that of the nozzle outlet (6 mm vs. 3 mm), the negative pressure generated at the nozzle mouth can reach −16.13 MPa, which can effectively suck the impurities in the wellbore into the cavity, cleaning and salvaging the debris in the wellbore. To ensure the safety of the tool, Φ + Φ′ = 3 mm + 6 mm is the best design solution for the nozzle of this structure.

| The nonstructural parameters
In this simulation, the range of the inlet flow rate Q in = 240-600 L/min. Multiple simulation models were established to investigate the influences of the nonstructural parameters on the fishing effect of reverse circulation. Without changing the nozzle structure parameters, numerical simulation is carried out by increasing the flow rate by 60 L/min each time. Figure 15 is the vector diagram for the simulation results of velocity distributions at different inlet flow rates. It demonstrates that the jet velocity gradually increases as the inlet flow rate increases while increasing the tool entrainment effect. Figure 16 shows the simulation results for negative pressure at different inlet flow rates. As indicated from the comprehensive analysis in Figure 17, the   negative pressure increases significantly as the displacement increases. However, in the interval of Q in = 480-600 L/min, increase of the negative pressure is slower, indicating an upper limit for the supply of displacement.
This work provides a theoretical basis for the optimization of structure and hydraulic parameters of the fishing tools.

| COMPARISON
To further verify the reverse circulation of the fishing tools during the well washing, the salvage efficiency is compared and analyzed considering the mass flow rate Q, the bottom hole impurity mass m, and the salvage time t under the actual field conditions according to Equation (23) mentioned above. In the large and small borehole inner diameter of 8.5″ and 5″, respectively, the total impurities in the wellbore are assumed are 20, 30, and 40 kg, the relationships among m, Q, and t are analyzed in computational experiments. The different bottom hole impurity masses in the borehole inner diameter of 5″ and 8.5″ on the salvage efficiency are compared numerically, as shown in Figure 18 and Figure 19, respectively. According to the investigation of field working conditions, the horizontal wellbore volume is 920 L/ 320 L when the horizontal wellbore length is 25 m. It is assumed that impurities are drawn into the tool cavity uniformly and continuously with the fluid. The comprehensive comparison suggests that the salvage impurities increase significantly with the flow rate of the sucked fluid increasing when the salvage time is the same, which can be explained by the higher negative pressure in the tool cavity causes the larger suction force. Then, the greater the flow rate of the fluid to be absorbed, the greater the energy used to entrain the impurities at the bottom of the well, and the higher salvage efficiency per unit time. When the flow rate of the fluid to be sucked is constant, the amount of impurities salvaged increases with the increase of the salvage time. When the total amount of impurity salvage is constant, the time required for salvage decreases with the increase of the flow rate of the absorbed fluid. Compared with fishing tools with small-sized structures (5″), those with large-sized structures (8.5″) have larger annulus volume of the wellbore, and the flow rate of the fluid to be sucked during the salvage is larger. Under the same engineering conditions, the ability to salvage impurities per unit time is better.
The comprehensive comparison reveals that the impurity salvaged and salvage time are not directly related to the total amount of impurities, but only related to the flow rate of the sucked fluid flow. Therefore, it can be concluded that the sucked fluid flow is related to the negative pressure value generated by the nozzle mouth. The larger the negative pressure value, the better the suction effect, and the stronger the salvage capacity.
To further verify the reverse circulation capability of the fishing tools, the relationship between the total impurity mass and the impurity salvaged is investigated in the borehole inner diameter (5″) and (8.5″) considering the total impurities of 20, 30, and 40 kg, respectively, as shown in Figure 18. The salvage time is within 60 min. In addition, the results indicate that the salvage efficiency of large borehole inner diameter (8.5″) is overall higher than that of small diameter (5″). As shown in Figure 20, the impurity salvage efficiency can be as high as 95.4% when the total impurities is 40 kg.
According to the comprehensive analysis, the optimal value of the flow rate of the sucked fluid and that of the negative pressure at nozzle port are determined on the premise of ensuring the fishing efficiency of the tool. Then, the appropriate system supply flow is determined, and the energy loss of the tool is reduced as much as possible to improve the fishing efficiency.

| CONCLUSION
In this work, a series of numerical simulations are conducted to investigate the effects of the nozzle structure and nonstructural parameters on negative pressure and jet velocity distribution around the spray nozzle of fishing tools. Additionally, the salvage efficiency is compared and analyzed numerically, considering the mass flow rate, the bottom hole impurity mass, and the salvage time. The conclusions of this work are summarized as follows: (1) With the increase of the spray angle of the suction nozzle, the average overflow velocity of the bottom hole generally increases in the nozzle inclination direction. When the spray angle changes from 15°to 18°, the extreme value of negative pressure becomes smaller. When the spray angle satisfies θ = 16°, the negative pressure is the highest. Thus, there is an optimal angle that can generate the target reverse circulation capability to complete the well cleaning.
(2) The smaller the nozzle outlet diameter, the higher the working fluid velocity through the nozzle outlet. The negative pressure of the nozzle outlet increases with the decrease of its inner diameter, and increases slightly with the decrease of the inner diameter of the nozzle inlet. The negative pressure increases sharply when the diameter of the nozzle outlet is too small, which will lead to excessive pressure difference between the rotating nozzle seat and the nozzle, thereby increasing the risk of deformation at the nozzle. After comprehensive comparison, it is found that Φ + Φ′ = 3 mm + 6 mm is the best design solution to ensure the safety of the tools. (3) The effects of the nonstructural parameters of the nozzle on the negative pressure and jet velocity were investigated and illustrated. The jet velocity gradually increases as the inlet flow rate increases while increasing the tool entrainment effect. The comprehensive analysis suggests the negative pressure increases significantly as the inlet flow rate increases. But, there is an upper limit. (4) When the salvage time is the same, the salvage impurities increase significantly with the flow rate of the sucked fluid increasing. Then, the higher the flow rate of the fluid to be absorbed, the greater the energy used to entrain the impurities at the bottom of the well. When the flow rate of the fluid to be sucked is constant, the amount of impurities salvaged increases with the increase of the salvage time of impurities. When the total amount of impurities salvaged is constant, the time required for salvaging decreases with the increase of the flow rate of the absorbed fluid. There, the impurity salvaged, and the time are not directly related to the total amount of impurities, but only related to the flow rate of the fluid flow sucked. Under the same field engineering conditions, the salvage efficiency of large borehole inner diameter (8.5″) can be as high as 95.4% than that of small diameter (5″), which fully meets the engineering requirements in field.