Method for determining optimal gas injection point of gas lift reverse circulation

Abstract

The invention discloses a method for determining an optimal gas injection point of gas lift reverse circulation, which comprises the following steps: acquiring underground parameters, and setting a time node t; reasonable grid division is carried out in the drilling tool, divided grid areas (i, j) are obtained, longitudinal i = 1, 2, 3... m, and radial j = 1, 2, 3... n; calculating a Reynolds number and liquid phase velocity distribution of the grid region (i, j); calculating the initial state of the solid phase in the grid region; calculating the parameters by using a unit parameter calculation module; judging whether the time node t is calculated to reach the longitudinal segment m or not; judging whether the solid-phase velocity is smaller than the settling velocity; recording a data output result; according to the method, the limiting position of the gas injection point capable of ensuring that the rock debris is smoothly discharged out of the wellhead is found, and important reference is provided for determining the gas injection point and the gas injection amount.

Description

Technical Field

[0001] The invention relates to the technical field of oil and gas well control, in particular to a method for determining an optimal gas injection point for gas lift reverse circulation. Background Art

[0002] Gas lift reverse circulation drilling technology is one of the typical "bottom hole negative pressure" drilling technologies. Compared with traditional positive circulation, its advantages are high drilling efficiency, low accident rate, low cost, etc. It is an advanced drilling technology. In the process of gas lift reverse circulation, the core of the technology is to reduce the probability of well leakage on the basis of ensuring the return of cuttings. When the gas injection volume is constant, the closer the gas injection point is to the bottom of the well, the more likely it is that the bottom hole pressure will be too high, thus causing leakage. Therefore, it is necessary to select a position close to the wellhead while ensuring that the cuttings can be discharged smoothly. The purpose of the present invention is to find the extreme position of the gas injection point that can ensure that the cuttings are discharged smoothly from the wellhead. This provides an important reference for determining the gas injection point and the gas injection volume to ensure the discharge of cuttings and prevent drill jamming, which is of great significance to the development of gas lift reverse circulation technology. Summary of the invention

[0003] The purpose of the present invention is to provide a method for determining the best gas injection point for gas lift reverse circulation. The method can be used to analyze the cuttings migration under gas lift reverse circulation during the gas lift reverse circulation process, solving the problem of being unable to use numerical simulation software to simulate long-distance cuttings migration. The method can determine the limit position of the gas injection point that can discharge cuttings to the greatest extent under the premise of ensuring that the bottom hole pressure is not too large under a certain gas injection volume. This is of great significance to the development of gas lift reverse circulation technology.

[0004] To achieve the above object, the technical solution adopted by the present invention is: a method for determining the best gas injection point for gas lift reverse circulation, comprising the following steps:

[0005] 1) Obtain downhole parameters and set time node t;

[0006] 2) Perform reasonable grid division inside the drilling tool to obtain the divided grid area (i, j), where i = 1, 2, 3 ... m in the longitudinal direction and j = 1, 2, 3 ... n in the radial direction;

[0007] 4) Calculate the initial state of the liquid and solid phases in the grid area;

[0008] 5) Calculate the parameters using the unit parameter calculation module;

[0009] 6) Determine whether the time node t has reached the longitudinal segment m;

[0010] 7) Determine whether the solid phase velocity is less than the sedimentation velocity;

[0011] 8) Record the data output results.

[0012] As a further improvement of the present invention, in step 1), the downhole parameters include bottom hole pressure, drilling fluid displacement and drilling fluid flow rate.

[0013] As a further improvement of the present invention, step 2) is specifically as follows:

[0014] The axial division starts from the bottom of the drill bit, and the radial division starts from the center of the drill pipe. The wellbore section is divided into n layers radially. The finite difference method is used to mesh the spatial domain of the liquid-solid two-phase mixed layer at time t. The bottom of the drill bit is taken as the starting point, the z axis is positive upward, and the infinitesimal elements are taken in turn to take m layers. The length ΔL of each axial segment of the drill bit is:

[0015]

[0016] Where: ΔL is the axial segment length, m; Q l is the drilling fluid displacement, m 3 / sΩ is the cross-sectional area of ​​the drill pipe, m 2 ;

[0017] As a further improvement of the present invention, step 4) is specifically as follows:

[0018] Assuming the cuttings mass flow rate, the number of particles, and the particles are evenly distributed in the initial grid:

[0019] Initial solid phase mass:

[0020]

[0021] Where: M ss is the mass flow rate, kg / s; [n] is the number of radial grid layers;

[0022] The number of particles in the initial unit:

[0023]

[0024] Where: s is the solid phase density, kg / m 3 V s is the particle volume, m 3 ;

[0025] Initial void ratio C l(0)(0,n) :

[0026]

[0027] Where: V is the volume of the calculation unit, m 3

[0028] Initial retention rate H (0)(0,n) :

[0029]

[0030] Where: v l(j) is the radial liquid velocity of the jth layer, m / s; C As is the area concentration on the cross section of the solid drilling tool;

[0031] Liquid phase velocity distribution:

[0032]

[0033] Where: v lmax is the maximum velocity of the liquid phase, m / s; r j is the radius of the jth radial layer, m; R is the drill pipe diameter, v l(j-1) is the radial liquid velocity of the j-1th layer, m / s.

[0034] Assume that the initial velocity distribution of the solid phase is the same as that of the liquid phase, that is:

[0035] v s(0)(0,j) =v l(0)(0,j)

[0036] The inlet pressure is the formation pressure, as shown below:

[0037] P (t)(0) =P b .

[0038] As a further improvement of the present invention, step 5) is specifically as follows:

[0039] First, the bottom pressure of the unit is calculated starting from the center of the drill bit. Then, the solid phase velocity and retention rate in the calculation unit are solved according to the motion equation. Then, the solid phase mass in the calculation unit is obtained, and the number of particles and the void ratio in the unit are calculated. After the required parameters are calculated, the next unit is calculated until the boundary layer is calculated to end the cycle.

[0040] As a further improvement of the present invention, the unit parameter calculation module calculates the following parameters:

[0041] Flow calculation:

[0042] ①Mass flow rate:

[0043] M=M ss +M l

[0044]

[0045] Where: M ss Solid phase mass flow rate, kg / s; M l Liquid mass flow rate, kg / s; φ is the drill bit diameter, m; v1 drilling speed, m / s;

[0046] ②Solid phase volume flow rate:

[0047]

[0048] Volume concentration calculation:

[0049] ① Solid phase volume concentration:

[0050]

[0051] Where: Q s is the volume flow rate of the solid phase, m 3 / s; n is the number of particles in the unit; V s is the particle volume, m 3 ; V is the volume of the calculation unit, m 3 ;

[0052] ②Liquid phase volume concentration:

[0053] C l =1-C s

[0054] ③Area average concentration:

[0055]

[0056] Where: Ω s is the area occupied by the solid phase in the drill bit cross section, m 2 ;

[0057] Mixture density calculation:

[0058] Mixed density is the ratio of the mass of the liquid-solid two-phase mixture passing through the drill cross section per unit time to its volume:

[0059] ρ M =ρ s ·C s +ρ l ·C l

[0060] Detention rate calculation:

[0061]

[0062] Mixed viscosity calculation:

[0063] Assuming that the liquid-solid two-phase flow satisfies the homogeneous flow, its mixing property parameters are the same as the homogeneous flow model, and the mixed viscosity adopts the Guth viscosity formula, which is applicable to the case of low solid phase volume fraction:

[0064] μ m =μ l (1+2.5Cs +14.1C s 2 )

[0065] Sedimentation velocity:

[0066] When the concentration S is less than 3%, the settling velocity of the particles in the pipe is close to the free settling velocity:

[0067]

[0068] When the concentration S>3%, the Richardson-Zaki formula is used to calculate the interference sedimentation velocity:

[0069]

[0070] Where: α, β are Swanson shape coefficients, k a is the area index;

[0071] For the pipe wall boundary, in addition to the direct force between the liquid and solid phases, the force between the solid phase and the pipe wall must also be considered. Since the influence of drill pipe rotation is ignored under the assumed conditions, the force on the solid phase from the pipe wall is:

[0072]

[0073] Where: C w is the concentration near the wall; e w is the pipe wall recovery coefficient; v sw is the tube wall slip velocity, m / sC * is the volume concentration in the solid phase accumulation state;

[0074] As a further improvement of the present invention, in step 6), if the time node t is not equal to the longitudinal segment m, it means that the iterative calculation process has not yet ended, and the calculation is returned to continue the loop; if it is equal, the next step is entered.

[0075] As a further improvement of the present invention, in step 7), if the solid phase velocity in all grid cells in the axial segment m is less than the sedimentation velocity, the calculation ends; if there is a solid phase velocity in a grid cell, the solid phase state of the cell at the next moment is continued to be calculated.

[0076] As a further improvement of the present invention, in step 8), the optimal position of the gas injection point is the position where the solid phase of the boundary layer reaches the settling velocity after gas injection, which can ensure the discharge of rock cuttings to the greatest extent.

[0077] The beneficial effects of the present invention are:

[0078] The method for determining the best gas injection point for gas lift reverse circulation of the present invention can select the gas injection point close to the wellhead under the premise of ensuring that the cuttings can be discharged smoothly. The purpose of this study is to find the limit position of the gas injection point that can ensure that the cuttings can be discharged smoothly from the wellhead, which provides an important reference for determining the gas injection point and the gas injection amount to ensure the discharge of the cuttings and prevent the drill from getting stuck. BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Figure 1 This is a schematic diagram of gas lift reverse circulation in an embodiment of the present invention;

[0080] Figure 2 A schematic diagram of grid division in an embodiment of the present invention;

[0081] Figure 3 Schematic diagram of the calculation process of continuous migration of liquid-solid two-phase mixed phase section in gas lift reverse circulation in an embodiment of the present invention;

[0082] Figure 4 Flow chart of the calculation method for estimating cuttings migration in the bottom area of ​​gas lift reverse circulation in the embodiment of the present invention

[0083] Figure 5 4 is a flow chart of a unit parameter calculation module in an embodiment of the present invention. DETAILED DESCRIPTION

[0084] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0085] Example

[0086] A method for determining an optimal gas injection point for gas lift reverse circulation comprises the following steps: (1) obtaining downhole parameter information;

[0087] (2) transmitting the acquired downhole parameters to the analysis system;

[0088] (3) Output the results through the analysis system;

[0089] Finally, the results are output and gas lift operation is performed according to the current calculation results.

[0090] Wherein step (1) obtains downhole parameters including bottom hole pressure, drilling fluid displacement, drilling fluid flow rate, etc.;

[0091] The new method for determining the best gas injection point for gas lift reverse circulation in step (3) includes the following steps: 1) obtaining downhole parameters; 2) setting time nodes; 3) performing reasonable grid division inside the drill pipe (longitudinal i = 1, 2, 3 ... m, radial j = 1, 2, 3 ... n), with the axial division starting from the bottom of the drill tool and the radial division starting from the center of the drill pipe; the axial height of the division unit is determined by the time node and the drilling fluid displacement; 4) calculating the initial state of the liquid and solid phases in the grid area (i, j) within the grid, assuming that the solid phase is uniformly distributed when it enters the drill tool at the beginning; the radial liquid phase velocity distribution is calculated according to the number of radial cells divided; the radial liquid phase velocity distribution is defined as the average value of the boundary velocity of the jth layer and the boundary velocity of the j-1th layer (except the 1st layer) -; 6) calculating the parameter module of the unit (i, j); 7) judging whether the time node has reached the longitudinal segment m; 8) judging whether the solid phase velocity is less than the sedimentation velocity; 9) recording the data output result, and the output result is the point where the boundary layer first reaches the sedimentation velocity.

[0092] The steps of the module for calculating the unit parameters in step (3) are as follows: 1) obtaining downhole parameters and the initial state of the solid phase entering the unit; 2) meshing the mixed phase section in the drill pipe; 3) calculating the bottom pressure of the i-th section; 4) calculating the number of particles, solid phase concentration, and mass flow rate of the (i, j)-th unit; 5) calculating the parameters of the (i, j)-th unit (solid phase velocity, solid phase mass, mixed viscosity, mixed section density, number of particles, solid phase concentration, and retention rate); 6) calculating the flow pressure loss of the (i, j)-th unit; 7) determining whether the calculation process j is equal to n; 8) superimposing each calculation unit; and 9) outputting the result.

[0093] like Figure 1 As shown in the figure, a schematic diagram of gas lift reverse circulation. In the process of gas lift reverse circulation, the research on the migration of gas lift reverse circulation cuttings particles can be divided into three parts, the bottom hole area (liquid-solid two-phase), the gas lift area (gas-liquid-solid three-phase) and the wellhead area. The drilling fluid is injected from the annulus until it flows into the drill pipe from the water hole at the drill bit at the bottom of the well, and mixes with the cuttings to form a liquid-solid two-phase flow. When the bottom hole fluid flow rate reaches a certain value, it continuously carries the bottom hole cuttings into the inner cavity of the drill pipe through the bottom of the drill bit to form a liquid-solid two-phase flow. When the liquid-solid two-phase flow rises along the inner cavity of the drill pipe to the position of the gas-water mixer, the gas phase is injected into the drill pipe through the double-wall drill pipe. When the liquid-solid two-phase flow mixed with the cuttings and the drilling fluid migrates to the gas injection point of the drill pipe, it is mixed with the gas phase to form a gas-liquid-solid three-phase flow, and mixed with the compressed air sprayed into the inner cavity of the drill pipe to form a gas-liquid-solid three-phase flow, thereby discharging the cuttings and entering the ground circulation system to achieve the purpose of gas lift reverse circulation drilling.

[0094] First of all, the cuttings migration process is very complicated. Therefore, in order to simplify the analysis, the following basic assumptions are proposed for the bottom area of ​​the well:

[0095] 1. Assume that the drilling fluid is an incompressible fluid;

[0096] 2. Assume that the solid particles are a continuous medium;

[0097] 3. Assume that the solid phase initially enters the drill bit uniformly;

[0098] 4. Ignore the effect of drill pipe rotation speed on cuttings transport;

[0099] 5. Ignore the acceleration section of the vertical well section;

[0100] 6. Assume that the gas phase only affects the fluid velocity in the bottom hole area and that this effect is uniform.

[0101] The calculation method for determining the optimal gas injection point for gas lift reverse circulation includes the following steps:

[0102] The process of calculating cuttings transport characteristics is as follows Figure 3 First, the drill pipe space is divided radially and axially, and then the liquid-solid parameters in the unit are calculated, and the calculation is repeated from the bottom of the well until the solid phase velocity is lower than the sedimentation velocity.

[0103] (1) Grid division:

[0104] like Figure 2 As shown in the figure, the radial distribution of the liquid phase on the drill cross section is inconsistent. Therefore, in order to ensure more accurate calculations, the wellbore cross section is divided into n layers radially. The finite difference method is used to grid the liquid-solid two-phase mixed layer space domain at time t. The bottom of the drill is taken as the starting point, the z axis is positive, and the microelement is taken in turn to take m layers, m is related to the gas lift reverse circulation operation time t. The length ΔL of each drill axial segment is:

[0105]

[0106] (2) Calculate the Reynolds number and liquid velocity distribution in the grid area:

[0107] Reynolds number calculation:

[0108]

[0109] where v la is the average flow rate:

[0110]

[0111] For gas lift reverse circulation technology, the drilling fluid flows from the wellbore annulus into the drill pipe. The radial distribution of the liquid phase on the drill pipe cross section is inconsistent, and its velocity distribution tends to be truncated cone, that is, the velocity gradient near the pipe wall is large, and the velocity distribution in the center area is relatively gentle. The general velocity distribution empirical formula is used:

[0112]

[0113] (3) Calculation of the initial state of cuttings in the grid:

[0114] Assuming the cuttings mass flow rate, the number of particles, and the particles are evenly distributed in the initial grid:

[0115] Initial solid phase mass:

[0116]

[0117] The number of particles in the initial unit:

[0118]

[0119] Initial void ratio C l(0)(0,n) :

[0120]

[0121] Initial retention rate H (0)(0,n) :

[0122]

[0123] Liquid phase velocity distribution:

[0124]

[0125] For gas lift reverse circulation, the initial velocity distribution of the solid phase is assumed to be the same as that of the liquid phase, that is:

[0126] v s(0)(0,j) =v l(0)(0,j) (10)

[0127] During the gas lift reverse circulation process, the inlet pressure is the formation pressure, which is also a known term, as shown in the formula.

[0128] P (t)(0) =P b (11)

[0129] (4) Unit parameter calculation module:

[0130] For the calculation of unit parameters, such as Figure 4 and Figure 5 First, the bottom pressure of the unit is calculated from the center of the drill bit, and then the solid phase velocity and retention rate in the calculation unit are solved according to the motion equation, and then the solid phase mass in the calculation unit is obtained, so as to calculate the number of particles and void ratio in the unit. After the required parameters are calculated, the next unit is calculated until the boundary layer is calculated to end this cycle.

[0131] The unit parameter calculation module mainly calculates the following parameters:

[0132] Flow calculation

[0133] 1) Mass flow:

[0134] M=M ss +M l (12)

[0135] In gas lift reverse circulation operations, different drill bit diameters and drilling speeds will result in different cuttings volume flow rates, which in turn will result in changes in mass flow rates. The particle mass flow rate can be determined by determining the cuttings volume.

[0136]

[0137] 2) Solid phase volume flow rate:

[0138]

[0139] Volume concentration calculation:

[0140] 1) Solid phase volume concentration:

[0141]

[0142] 2) Liquid phase volume concentration:

[0143] C l =1-C s (16)

[0144] 3)Area average concentration:

[0145]

[0146] Mixture density calculation:

[0147] Mixed density is the ratio of the mass to volume of the liquid-solid two-phase mixture passing through the drill cross section per unit time.

[0148] ρ M =ρ s ·C s +ρ l ·C l (18)

[0149] Detention rate calculation:

[0150]

[0151] Mixed viscosity calculation:

[0152] Assuming that the liquid-solid two-phase flow satisfies the homogeneous flow, its mixing property parameters are the same as the homogeneous flow model. The mixed viscosity adopts the Guth viscosity formula, and its calculation formula is suitable for the case of low solid phase volume fraction.

[0153] μ m =μ l (1+2.5C s +14.1C s 2 ) (20)

[0154] Sedimentation velocity:

[0155] The rising speed of cuttings is related to the upward migration of drilling fluid and the settling speed of cuttings in the drill pipe. In the process of cuttings migration, particles are affected by friction and collision between particles and the wall, and between particles, which consumes the kinetic energy of particles and reduces the settling speed of each particle in the particle group to a certain extent. In addition, in vertical pipes, the influence of particle concentration on settling speed is huge. For low concentration (S<3%), the settling speed of particles in the pipe is close to the free settling speed.

[0156]

[0157] When the concentration S>3%, the mutual interference between particles cannot be ignored, which can be considered as interference sedimentation. The more widely used Richardson-Zaki formula is used to calculate the interference sedimentation velocity.

[0158]

[0159] Where: α, β are Swanson shape coefficients, k a is the area index;

[0160] For the pipe wall boundary, in addition to the direct force between the liquid and solid phases, the force between the solid phase and the pipe wall must also be considered. Since the influence of drill pipe rotation is ignored under the assumed conditions, the force on the solid phase from the pipe wall is:

[0161]

[0162] Where: C w is the concentration near the wall; e w is the pipe wall recovery coefficient; v sw is the tube wall slip velocity, m / sC * is the volume concentration in the solid phase accumulation state;

[0163] (5) Determine whether the current calculation is equal to the axial segment m at time node t:

[0164] If it is not equal, it means that the iterative calculation process has not ended, and the loop calculation is returned. If it is equal, it goes to the next step; the influence of the drill pipe rotation speed on the cuttings migration is ignored, and the acceleration section of the vertical well section is ignored; each iteration is calculated from the bottom of the well, and the end of each iteration is marked by whether the set calculation node is calculated or whether the solid phase is less than the sedimentation velocity.

[0165] (6) Determine whether the solid phase velocity is less than the sedimentation velocity:

[0166] If the solid phase velocity in all grid cells in the axial segment m is less than the sedimentation velocity, the calculation ends. If there is a solid phase velocity in a grid cell, the solid phase state of the cell at the next moment will continue to be calculated.

[0167] (7) Output calculation results:

[0168] Finally, the calculation results are output. The optimal position of the gas injection point is the position where the solid phase in the boundary layer reaches the settling velocity after gas injection, which can ensure the discharge of rock cuttings to the greatest extent.

[0169] The above-mentioned embodiments only express the specific implementation of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention.

Claims

1. A method for determining the optimal gas injection point for gas lift reverse circulation, characterized in that: The following steps are involved: 1) Obtain downhole parameters and set time node t; 2) Perform reasonable grid division inside the drilling tool to obtain the divided grid area (i, j), where i = 1, 2, 3 ... m in the longitudinal direction and j = 1, 2, 3 ... n in the radial direction; 4) Calculate the initial state of the liquid and solid phases in the grid area; 5) Calculate the parameters using the unit parameter calculation module; 6) Determine whether the time node t has reached the longitudinal segment m; 7) Determine whether the solid phase velocity is less than the sedimentation velocity; 8) Record the data output results.

2. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 1, characterized in that: In step 1), the downhole parameters include bottom hole pressure, drilling fluid displacement and drilling fluid flow rate.

3. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 1, characterized in that: Step 2) is as follows: The axial division starts from the bottom of the drill bit, and the radial division starts from the center of the drill pipe. The wellbore section is divided into n layers radially. The finite difference method is used to mesh the spatial domain of the liquid-solid two-phase mixed layer at time t. The bottom of the drill bit is taken as the starting point, the z axis is positive upward, and the infinitesimal elements are taken in turn to take m layers. The length ΔL of each axial segment of the drill bit is: Where: ΔL is the axial segment length; Q l is the drilling fluid displacement, m 3 / sΩ is the cross-sectional area of ​​the drill pipe.

4. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 3, characterized in that: Step 4) is as follows: Assuming the cuttings mass flow rate, the number of particles, and the particles are evenly distributed in the initial grid: Initial solid phase mass: Where: M ss is the mass flow rate; [n] is the number of radial grid layers; The number of particles in the initial unit: Where: s is the solid phase density, kg / m 3 V s is the particle volume, m 3 Initial void ratio C l(0)(0,n) : Where: V is the volume of the calculation unit; Initial retention rate H (0)(0,n) : Where: v l(j) is the liquid phase velocity of the jth layer in the current radial direction; C As is the area concentration on the cross section of the solid drilling tool; Liquid phase velocity distribution: Where: v lmax is the maximum velocity of the liquid phase; r j is the radial radius of the jth layer; R is the drill pipe diameter, v l(j-1) is the liquid velocity of the j-1th radial layer; Assume that the initial velocity distribution of the solid phase is the same as that of the liquid phase, that is: v s(0)(0,j) =v l(0)(0,j) The inlet pressure is the formation pressure, as shown below: P (t)(0) =P b 。 5. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 3, characterized in that: Step 5) is as follows: First, the bottom pressure of the unit is calculated starting from the center of the drill bit. Then, the solid phase velocity and retention rate in the calculation unit are solved according to the motion equation. Then, the solid phase mass in the calculation unit is obtained, and the number of particles and the void ratio in the unit are calculated. After the required parameters are calculated, the next unit is calculated until the boundary layer is calculated to end the cycle.

6. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 5, characterized in that: The unit parameter calculation module calculates the following parameters: Flow calculation: ①Mass flow rate: Where: M ss Solid phase mass flow rate; M l Liquid mass flow rate; φ is the drill bit diameter; v1 drilling speed; ②Solid phase volume flow rate: Volume concentration calculation: ① Solid phase volume concentration: Where: V is the volume of the calculation unit; ②Liquid phase volume concentration: C l =1-C s ③Area average concentration: Where: Ω s is the area occupied by the solid phase in the cross section of the drilling tool; Mixture density calculation: Mixed density is the ratio of the mass of the liquid-solid two-phase mixture passing through the drill cross section per unit time to its volume: r M =ρ s ·C s +r l ·C l Detention rate calculation: Where: C As is the area concentration on the cross section of the solid drilling tool; Mixed viscosity calculation: Assuming that the liquid-solid two-phase flow satisfies the homogeneous flow, its mixing property parameters are the same as the homogeneous flow model, and the mixed viscosity adopts the Guth viscosity formula, which is applicable to the case of low solid phase volume fraction: μ m =μ l (1+2.5C s +14.1C s 2 ) Where: μ l is the liquid phase viscosity; Sedimentation velocity: During the gas lift reverse circulation process, the solid concentration in the drill pipe is low, so the settling velocity of the particles is close to the free settling velocity: Where: g is the acceleration due to gravity; d is the particle diameter; C D is the drag coefficient; When the concentration S>3%, the Richardson-Zaki formula is used to calculate the interference sedimentation velocity: Where: α, β are Swanson shape coefficients, k a is the area index; For the pipe wall boundary, in addition to the direct force between the liquid and solid phases, the force between the solid phase and the pipe wall must also be considered. Since the influence of drill pipe rotation is ignored under the assumed conditions, the force on the solid phase from the pipe wall is: Where: C w is the concentration near the wall; e w is the pipe wall recovery coefficient; v sw is the tube wall slip velocity, m / sC * is the volume concentration in the solid phase accumulation state.

7. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 6, characterized in that: In step 6), if the time node t is not equal to the longitudinal segment m, it means that the iterative calculation process has not yet ended, and the calculation cycle is returned to continue. If it is equal, the next step is entered.

8. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 7, characterized in that: In step 7), if the solid phase velocity in all grid cells in the axial segment m is less than the sedimentation velocity, the calculation ends; if there is a solid phase velocity in a grid cell, the solid phase state of the cell at the next moment is continued to be calculated.

9. The method for determining the optimal gas injection point for gas lift reverse circulation according to claim 8, characterized in that: In step 8), the optimal position of the gas injection point is the position where the solid phase of the boundary layer reaches the settling velocity after gas injection, which can ensure the discharge of rock cuttings to the greatest extent.

Images