Method for predicting starting flow velocity of cuttings of extended reach well

Abstract

The invention discloses a method for predicting the starting flow velocity of cuttings of an extended reach well, and relates to the technical field of oil and gas wells. According to design parameters of the extended reach well, an extended reach well rock debris starting flow velocity prediction model is constructed and placed in a three-dimensional coordinate system, target rock debris particles are selected, the rock debris shielding state and stress condition of the target rock debris particles when drilling fluid flows are analyzed, and the rock debris starting flow velocity prediction model is obtained. Determining the exposed area and the static force and the power of the rock debris particles in the extended reach well, simulating to obtain the bottom flow velocity of the rock debris particles when the rock debris particles are started by combining the starting probability of the rock debris particles, and calculating the rock debris particle size according to the particle size of the rock debris particles and the operation state of a drill column in a shaft. The large-displacement well rock debris starting flow velocity prediction model is used for simulating the rolling process of rock debris particles when drilling fluid flows, the influence of rock debris shielding and starting probability on the rock debris starting flow velocity is fully considered, accurate prediction of the rock debris starting flow velocity in a large-displacement well is achieved, and a basis is provided for guaranteeing safe operation of large-displacement well drilling.

Description

Technical Field

[0001] The invention belongs to the technical field of oil and gas wells, and in particular relates to a method for predicting the starting flow velocity of cuttings in an extended-reach well. Background Art

[0002] During the drilling of large-displacement horizontal wells, most downhole accidents are caused by insufficient wellbore cleaning. The cuttings start-up velocity in the Φ215.9mm well section on land and the Φ311mm well section in the sea is an important basic data for the design of the hydraulic scheme for the large-displacement well section.

[0003] At present, there are two main methods for determining the starting flow rate of cuttings in large displacement / high-angle well sections:

[0004] First, the experimental method, through a large number of experimental observations, determines the critical flow rate without the formation of cuttings bed in the experimental well section;

[0005] 2. The rock cuttings particle mechanical analysis method determines the minimum flow rate required for rock cuttings transportation by establishing a moment balance relationship.

[0006] However, the critical state of the experimental data obtained by the experimental method depends on human judgment, and the experimental conclusions are limited by the experimental scale and experimental parameters, which makes it difficult to meet the needs of on-site engineering. In addition, the current rock cutting particle mechanics analysis method regards the rock cuttings as a completely exposed sphere, without considering the shielding between the rock cuttings in the rock cutting bed, which makes the prediction results of the starting flow velocity of rock cuttings migration less accurate, and the prediction results are significantly lower than the engineering practice and experimental results.

[0007] Therefore, it is urgent to propose a method for predicting the starting flow rate of cuttings in extended displacement wells, which fully considers the influence of cuttings shielding and starting probability on the starting flow rate of cuttings in extended displacement wells and improves the accuracy of the prediction of the starting flow rate of cuttings in extended displacement wells. Summary of the invention

[0008] The present invention aims to overcome the deficiencies in the prior art and proposes a method for predicting the starting flow rate of cuttings in extended reach wells. By analyzing the cuttings shielding state and stress conditions of cuttings particles in extended reach wells, the influence of cuttings shielding and starting probability on the starting flow rate of cuttings in extended reach wells is fully considered. The cuttings starting flow rate prediction model for extended reach wells is used for simulation, thereby achieving accurate prediction of the starting flow rate of cuttings in extended reach wells, and providing a basis for guiding safe drilling operations in extended reach wells.

[0009] The present invention specifically adopts the following technical solutions:

[0010] A method for predicting the starting flow rate of cuttings in an extended-reach well, which predicts the starting flow rate of cuttings in an extended-reach well based on cuttings shielding and starting probability, specifically comprises the following steps:

[0011] Step 1: construct a cuttings start-up velocity prediction model for an extended reach well according to the design parameters of the extended reach well, and set the drilling fluid and cuttings in the wellbore annulus in the cuttings start-up velocity prediction model for the extended reach well;

[0012] Step 2: placing the extended-reach well cuttings start-up velocity prediction model in a three-dimensional coordinate system, selecting target cuttings particles in the extended-reach well cuttings start-up velocity prediction model, obtaining the cuttings shielding state of the target cuttings particles, and determining the exposed area of ​​the cuttings particles in the extended-reach well cuttings start-up velocity prediction model;

[0013] Step 3, performing a stress analysis on the target rock cuttings particles, obtaining the stress state of the target rock cuttings particles, and determining the static and dynamic forces on the rock cuttings particles in the prediction model of the starting flow velocity of the extended reach well;

[0014] Step 4, using the cuttings start-up velocity prediction model for extended reach wells to simulate the tumbling process of the target cuttings particles when the drilling fluid flows, and combining the start-up probability of the cuttings particles, determining the bottom velocity of the cuttings particles when the cuttings particles start based on the start-up probability of the cuttings particles;

[0015] Step 5: Based on the cuttings starting flow rate prediction model for extended reach wells, the starting bottom flow rate of cuttings particles and the cuttings particle size obtained by simulation, combined with the drill string rotation state, eccentricity and drilling fluid flow rate, based on the cuttings shielding and starting probability, the starting flow rate prediction model for extended reach wells is used to simulate and determine the starting average flow rate of cuttings particles that meets the rock carrying requirements;

[0016] Step 6: Complete the prediction of the starting velocity of cuttings particles in the extended displacement well.

[0017] Preferably, in step 1, according to the design parameters of the extended reach well, the wellbore parameters, drilling fluid parameters, cuttings parameters and drill string parameters of the extended reach well are obtained, wherein the wellbore parameters of the extended reach well include the well inclination angle α, the wellbore diameter and the annular displacement, the drilling fluid parameters include the components, flow rate, flow direction, dynamic shear force and plastic viscosity of the drilling fluid, the cuttings parameters include the type, density and particle size of the cuttings, and the drill string parameters include the outer diameter, mechanical penetration rate, rotation speed and eccentricity of the drill string;

[0018] A cuttings start-up flow rate prediction model for an extended displacement well is constructed according to the design parameters of the extended displacement well. The cuttings start-up flow rate prediction model for an extended displacement well includes a wellbore model, a drill string model and a drill bit model. The drill string model is eccentrically arranged in the wellbore model and connected to the drill bit model. The space between the drill string model and the wellbore model forms a wellbore annulus. A cuttings bed model is arranged at the bottom of the wellbore annulus. The cuttings bed model is formed by depositing and stacking a number of identical cuttings particles in the form of solid spheres.

[0019] Preferably, in step 2, target cuttings particles are selected in the cuttings bed model, the center of the target cuttings particles is taken as the coordinate origin, the flow direction of the drilling fluid is taken as the positive direction of the z-axis, and the direction perpendicular to the flow direction of the drilling fluid is taken as the positive direction of the y-axis, a three-dimensional coordinate system is constructed, and the cuttings start-up flow velocity prediction model for extended reach wells is placed in the three-dimensional coordinate system;

[0020] Analyze the debris shielding state of the target debris particles in the debris bed model, and determine the mean shielding degree of the debris particles in the debris bed model based on the debris shielding geometric topological relationship;

[0021] It is known that the target rock cuttings particle is located on the surface of the rock cuttings bed model, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the subsurface of the rock cuttings bed model and the target rock cuttings particle is a, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the surface of the rock cuttings bed model and the target rock cuttings particle is b, the distance between the contact point a and the contact point b on the z-axis is Δz, and the distance between the contact point a and the lowest point c of the target rock cutting particle on the y-axis is Δy;

[0022] The rock-cutting shielding state of the target rock-cutting particles is analyzed, and the rock-cutting shielding geometric topological relationship is obtained as follows:

[0023]

[0024] In the formula, Δ′ z is the shielding degree of rock debris particles in the z-axis direction, Among them, d c is the diameter of rock debris particles; Δ′ y is the shielding degree of rock debris particles in the y-axis direction,

[0025] Combined with the probability density calculation formula of rock cuttings shielding degree in engineering practice, the shielding degree of rock cuttings particles in the rock cuttings bed model in the y-axis direction is obtained by integration, and the mean shielding degree of rock cuttings particles in the rock cuttings bed model is determined;

[0026] The probability density calculation formula of the rock debris particle shielding degree is:

[0027]

[0028] In the formula, f(Δ′ y ) is the probability density of rock debris obscuration;

[0029] Then, the target cuttings particles in the cuttings bed model are projected onto the yz plane to obtain a projection circle, and the projection circle is determined to pass through the contact point a along the axis of the wellbore and the intersection point with the projection circle is B. The central angle of the arc is twice the rock cuttings repose angle φ, and the center of the projected circle is located at the chord The area of ​​the above part is the exposed area of ​​rock cuttings particles, and the exposed area of ​​rock cuttings particles is determined as:

[0030]

[0031] in,

[0032]

[0033] In the formula, A e The exposed area of ​​rock debris particles.

[0034] Preferably, in step 4, a force analysis is performed on the target rock cuttings particles to determine the static and dynamic forces on the rock cuttings particles in the rock cuttings bed model, wherein the static force is the plastic force F caused by the buoyancy of the rock cuttings particles in the drilling fluid and the yield stress of the drilling fluid. p The direction of the buoyancy G of the rock cuttings particles is vertically downward through the center of the sphere of the rock cuttings particles, and the plastic force F p The direction is perpendicular to the drilling fluid flow direction and downward; the power includes the drag force F along the drilling fluid flow direction D , the upward lifting force F perpendicular to the flow direction of drilling fluid L , the pressure gradient force F along the drilling fluid flow direction Δp and the support force F of the rock cuttings particles R , because when the target rock cuttings particles are in a critical state, the supporting force F R is the reaction force, and the support force F of the rock cuttings does not need to be considered at this time R ;

[0035] in,

[0036] The buoyant weight G of the rock cuttings particles is:

[0037]

[0038] In the formula, G is the floating weight, the unit is N, G = F g -F b , F g is the gravity of rock cuttings particles, F b is the buoyancy of the rock debris particles; ρ c is the density of rock cuttings particles, in kg / m 3 ρ f is the drilling fluid density, in kg / m 3 ; g is the acceleration due to gravity;

[0039] The drag force F on the rock cuttings particles is D for:

[0040]

[0041] in,

[0042]

[0043] In the formula, F D is the drag force on the cuttings particles, in N; C D is the drag coefficient, which is related to the drilling fluid performance and flow rate; Re is the Reynolds number; v b is the local velocity of the drilling fluid acting on the center of the cuttings particle sphere, in m / s;

[0044] The lifting force F on the rock cuttings particles is L for:

[0045]

[0046] in,

[0047]

[0048] In the formula, F L is the lifting force on the cuttings particles, in N; A L is the lifting force action area; C L is the lifting coefficient; max(·) is the maximum value function; α p is the lift coefficient factor, in, is the velocity gradient between the top and bottom of the rock cuttings particle, r is the radius of the rock cuttings particle; Re p is the particle Reynolds number, dimensionless, Among them, μ a is the apparent viscosity of the drilling fluid at the cuttings particles under the shear rate of the drilling fluid, in Pa·s. τ y is the yield stress of drilling fluid, in Pa, k is the consistency coefficient, in Pa.s n , γ p is the shear rate of rock cuttings particles, n is the flow pattern index, dimensionless;

[0049] Due to the plastic force F p The plastic force F acting on the cuttings particles is determined without being affected by the degree of shielding of the cuttings particles. p for:

[0050]

[0051] In the formula, F p is the plastic force on the rock cuttings particles, in N;

[0052] The pressure gradient force F acting on the cuttings particles along the flow direction of the drilling fluidΔp Affected by the shielding degree of the cuttings particles, the pressure gradient force F exerted on the cuttings particles along the flow direction of the drilling fluid is Δp for:

[0053]

[0054] In the formula, F Δp is the pressure gradient force on the cuttings particles, in N; C d is the pressure gradient force coefficient; G pf is the drilling fluid annular circulation pressure gradient, in Pa / m, p f is the drilling fluid pressure; τ w is the wall shear stress; D hyd is the hydraulic diameter.

[0055] Preferably, in step 4, the drilling fluid parameters, cuttings parameters and environmental control displacement in the extended displacement well cuttings start-up flow rate prediction model are set, and the extended displacement well cuttings start-up flow rate prediction model is used to simulate the tumbling process of the target cuttings particles when the drilling fluid flows. When the drilling fluid flow causes the target cuttings particles to tumble, the target cuttings particles tumble with the contact point a as the support point. At this time, the various moments on which the target cuttings particles are subjected are in a state of equilibrium, and the resultant moment is zero.

[0056] The coordinates of the contact point a of the target rock cuttings particle in the three-dimensional coordinate system are known to be (-y a ,z a ), combined with the cuttings shielding geometric topological relationship and the cuttings repose angle φ, we get:

[0057]

[0058] in,

[0059] cosφ=1-2Δ′ y (12-1)

[0060]

[0061] In the formula, y a is the horizontal coordinate of the point symmetrical to the contact point a on the y-axis, z a is the z-axis coordinate of the contact point a;

[0062] Combined with the stress state of the target rock cuttings particles, determine the moment of the target rock cuttings particles, including the length of the drag force arm l y and the lifting force arm length l z for:

[0063]

[0064] The buoyancy of the target rock cuttings particles is decomposed in the -y and -z directions to obtain the buoyancy component in the -y direction and the buoyancy component in the -z direction. The arm length of the buoyancy component in the -y direction and the arm length of the lifting force are both l z , the length of the force arm of the floating weight component in the -z direction is l G,y ,

[0065] When the power generated by the flow of drilling fluid exceeds the static force, the cuttings particles on the upper part of the cuttings bed model in the cuttings start-up velocity prediction model for extended reach wells roll, and the rolling expression of the cuttings particles is obtained as follows:

[0066] l y (F D +F ΔP )-Gcos(α)l G,y +l z [F L -F P -Gsin(α)]>0 (15)

[0067] Where, α is the well inclination angle;

[0068] Based on the static and dynamic calculation formulas of cuttings particles and the moment expression of cuttings particles, the minimum bottom velocity required for cuttings particles to tumble in the cuttings starting velocity prediction model for extended reach wells is obtained as follows:

[0069]

[0070] In the formula, G pf is the friction pressure drop gradient;

[0071] Since the cuttings particles only reach the starting condition in theory when the force reaches the moment balance, considering the fluctuation characteristics of the bottom flow velocity of the drilling fluid, the randomness of the shielding of the cuttings particles, and the differences in the size and shape of the cuttings particles, the starting probability of the cuttings particles is obtained;

[0072] The starting probability P of the rock cuttings particles is:

[0073]

[0074] Where p is the probability condition of rock debris particle initiation;

[0075] make but:

[0076] P = p[v b 2 >(EF) 2 ]=1-p[v b 2 <(EF)2 ] (18)

[0077] In the formula, E and F are intermediate calculation parameters;

[0078] Since the instantaneous local velocity of drilling fluid obeys normal distribution, then:

[0079]

[0080] In the formula, f(v b ) is the drilling fluid velocity probability distribution function; is the time-averaged local velocity of the drilling fluid when the cuttings particles start, which is used to represent the bottom velocity when the cuttings particles start; σ is the mean square error, the standard deviation σ of the local velocity of the drilling fluid acting on the center of the cuttings particle in the mechanics of sediment movement is proportional to the time-averaged local velocity of the drilling fluid when the cuttings particles start There is a linear relationship between them. In the mechanics of sediment movement, the start of individual debris particles is defined as weak movement, and the corresponding probability of the start of the debris particles is 0.0014. The start of a small number of debris particles is defined as weak movement, and the corresponding probability of the start of the debris particles is 0.0227. The start of most debris particles is defined as strong movement, and the corresponding probability of the start of the debris particles is 0.1585.

[0081] Therefore, the starting probability P of the cuttings particles is expressed as:

[0082]

[0083] In the formula, is the normal distribution function;

[0084] Since the probability of cuttings particles rolling in the reverse direction of drilling fluid is negligible, the reverse rolling of cuttings particles is ignored, and the following is obtained:

[0085]

[0086] Taking the strong movement of rock cuttings as the starting standard of rock cuttings, the starting probability of rock cuttings is determined, and the normal distribution table is queried to obtain Determine the time-averaged local velocity of the drilling fluid when the cuttings particles start to move

[0087] Considering the influence of the size and shape of cuttings on the start-up of cuttings, combined with Han Qi's flattening function f(λ), the time-averaged local velocity of the drilling fluid at the start-up under the strong probability of cuttings movement is obtained. for:

[0088]

[0089] Where λ is the flattening coefficient.

[0090] Preferably, in step 5, the starting bottom flow velocity of the cuttings particles, the cuttings particle size, the rotation state of the drill string, the eccentricity and the drilling fluid flow velocity are set, and the cuttings starting flow velocity prediction model for the extended displacement well is used to simulate the rotary drilling process of the drill bit. At this time, the drill string model in the cuttings starting flow velocity prediction model for the extended displacement well is in a rotating state, and the circumferential flow velocity of the drilling fluid in the area below the drill string model in the wellbore annulus is symmetrically distributed with the center line of the area as the symmetry axis;

[0091] The velocity v of the drilling fluid in the area below the drill string model in the wellbore annulus along the wellbore high side direction line d for:

[0092]

[0093] in,

[0094] R τN =[(0.0003τ y +0.0007)N-0.0223τ y +0.9435]9-0.0082τ y +1.0778) (24)

[0095] In the formula, R τN is the dynamic shear force and rotation speed influencing factor, dimensionless; y is the distance from the lower side of the wellbore in the annulus below the drill string, in meters; is the average flow rate of drilling fluid, in m / s; N is the rotation speed, in rpm;

[0096] Due to the drag force F on the cuttings particles along the flow direction of the drilling fluid D Acting on the diameter of rock cuttings According to get:

[0097]

[0098] The average flow rate of the drilling fluid is thus determined as:

[0099]

[0100] The beneficial technical effects brought by the present invention are:

[0101] The present invention proposes a method for predicting the starting flow rate of cuttings in extended displacement wells, which fully considers the radial shielding and axial shielding of cuttings particles when the drilling fluid flows in the wellbore, combines the influence of the drilling fluid flow rate at different parts of the cuttings particles on the probability of strong movement of the cuttings particles, and uses a cuttings starting flow rate prediction model for extended displacement wells to simulate the starting process of tumbling of cuttings particles in the cuttings bed during the flow of drilling fluid in the wellbore, and realizes accurate prediction of the starting flow rate of cuttings in extended displacement wells based on cuttings shielding and starting probability.

[0102] At the same time, the present invention proposes a method for predicting the starting flow rate of cuttings in large-displacement wells, which not only overcomes the shortcomings of traditional cuttings starting flow rate experimental methods that are limited by the experimental site environment, the critical state needs to rely on human judgment, and it is difficult to meet the needs of on-site engineering, but also solves the problem of low prediction results of traditional cuttings particle mechanical analysis methods. By analyzing the cuttings shielding state and stress state of the deposited cuttings particles in the large-displacement well during the flow of drilling fluid, the influence of the drilling fluid on the stress of the cuttings particles caused by the shielding between adjacent cuttings particles is fully considered, the drilling fluid flow rate at different positions of the cuttings particles is distinguished, the starting conditions of the cuttings particles in the large-displacement well are truly restored, and the prediction accuracy of the cuttings starting flow rate in the large-displacement well is improved based on the cuttings shielding and starting probability, which provides a theoretical basis and technical support for guiding the safe drilling operation of large-displacement wells. BRIEF DESCRIPTION OF THE DRAWINGS

[0103] Figure 1 Schematic diagram of the geometric topological relationship of rock debris masking.

[0104] Figure 2 Schematic diagram of the exposed area of ​​rock debris particles.

[0105] Figure 3 This is the force analysis diagram of rock cuttings particles.

[0106] Figure 4 The figure is a comparison diagram between the average starting flow velocity of rock cuttings predicted by the method of the present invention and the experimental results of rock cuttings transportation. DETAILED DESCRIPTION

[0107] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments:

[0108] Example 1

[0109] The present invention proposes a method for predicting the starting flow rate of cuttings in an extended-reach well, which is characterized in that the starting flow rate of cuttings in an extended-reach well is predicted based on cuttings shielding and starting probability, and specifically comprises the following steps:

[0110] Step 1, according to the design parameters of the extended reach well, obtain the wellbore parameters, drilling fluid parameters, cuttings parameters and drill string parameters of the extended reach well, wherein the wellbore parameters of the extended reach well include the well inclination angle α, the wellbore diameter and the annulus displacement, the drilling fluid parameters include the composition, flow rate, flow direction, dynamic shear force and plastic viscosity of the drilling fluid, the cuttings parameters include the type, density and particle size of the cuttings, and the drill string parameters include the outer diameter, mechanical penetration rate, rotation speed and eccentricity of the drill string.

[0111] A cuttings start-up flow rate prediction model for extended displacement wells is constructed in fluid mechanics simulation software. The cuttings start-up flow rate prediction model for extended displacement wells includes a wellbore model, a drill string model and a drill bit model. The drill string model is eccentrically arranged in the wellbore model and connected to the drill bit model. The space between the drill string model and the wellbore model forms a wellbore annulus. A cuttings bed model is arranged at the bottom of the wellbore annulus. The cuttings bed model is formed by depositing and stacking a number of identical cuttings particles in the form of solid spheres.

[0112] According to the wellbore parameters, drilling fluid parameters, cuttings parameters and drill string parameters of the extended reach well, the drill string in the cuttings start-up velocity prediction model of the extended reach well and the drilling fluid and cuttings in the wellbore annulus are set.

[0113] Step 2: Place the cuttings start-up velocity prediction model for extended-reach wells in a three-dimensional coordinate system, select target cuttings particles in the cuttings start-up velocity prediction model for extended-reach wells, obtain the cuttings shielding state of the target cuttings particles, and determine the exposed area of ​​the cuttings particles in the cuttings start-up velocity prediction model for extended-reach wells. The specific process is as follows:

[0114] The target cuttings particles are selected in the cuttings bed model, the center of the target cuttings particles is taken as the coordinate origin, the flow direction of the drilling fluid is taken as the positive direction of the z-axis, and the direction perpendicular to the flow direction of the drilling fluid is taken as the positive direction of the y-axis. A three-dimensional coordinate system is constructed, and the cuttings start-up flow velocity prediction model for extended reach wells is placed in the three-dimensional coordinate system.

[0115] The debris shielding status of the target debris particles in the debris bed model is analyzed, and the mean shielding degree of the debris particles in the debris bed model is determined by combining the debris shielding geometric topological relationship:

[0116] It is known that the target rock cuttings particle is located on the surface of the rock cuttings bed model, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the subsurface of the rock cuttings bed model and the target rock cuttings particle is a, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the surface of the rock cuttings bed model and the target rock cuttings particle is b, the distance between the contact point a and the contact point b on the z-axis is Δz, and the distance between the contact point a and the lowest point c of the target rock cuttings particle on the y-axis is Δy, as shown in Figure 1 shown.

[0117] The rock-cutting shielding state of the target rock-cutting particles is analyzed, and the rock-cutting shielding geometric topological relationship is obtained as follows:

[0118]

[0119] In the formula, Δ′ z is the shielding degree of rock debris particles in the z-axis direction, Among them, d c is the diameter of rock debris particles; Δ′ y is the shielding degree of rock debris particles in the y-axis direction,

[0120] Combined with the probability density calculation formula of the shielding degree of rock cuttings particles in engineering practice, the shielding degree of the rock cuttings particles in the rock cuttings bed model in the y-axis direction is obtained by integration, and the mean shielding degree of the rock cuttings particles in the rock cuttings bed model is determined.

[0121] The probability density calculation formula of the rock debris particle shielding degree is:

[0122]

[0123] In the formula, f(Δ′ y ) is the probability density of rock debris obscuration.

[0124] Then, the target cuttings particles in the cuttings bed model are projected onto the yz plane to obtain a projection circle, and the projection circle is determined to pass through the contact point a along the axis of the wellbore and the intersection point with the projection circle is B. The central angle of the arc is twice the cuttings repose angle φ, such as Figure 2 As shown, the projection circle is located at the chord The area above is the exposed area of ​​rock debris particles.

[0125] because Figure 2 The area of ​​the triangle is The sector area is The area of ​​the segment is S 弓形acB =S 扇形aoB -S ΔaoB , that is, the exposed area of ​​rock debris particles is 弓形acB , thus determining the exposed area of ​​rock cuttings particles as:

[0126]

[0127] in,

[0128]

[0129] In the formula, A e The exposed area of ​​rock debris particles.

[0130] In this embodiment, the shielding degree of the rock debris particles in the y-axis direction is Δ′ y= 0.215, and φ = 1.5 rad is calculated. Substituting it into formula (3) we get That is, the exposed area of ​​rock debris particles is

[0131] Step 3: Perform force analysis on the target cuttings particles to obtain the force state of the target cuttings particles and determine the static and dynamic forces on the cuttings particles in the large displacement well cuttings starting velocity prediction model, such as Figure 3 As shown, the specific process is:

[0132] The force analysis of the target cuttings particles is carried out to analyze the forces on the cuttings particles at the lower side of the wellbore in the annulus of the large displacement well in the cuttings bed model, including the static and dynamic forces on the cuttings particles. The static force is the plastic force F caused by the buoyancy of the cuttings particles in the drilling fluid and the yield stress of the drilling fluid. p The direction of the buoyancy G of the rock cuttings particles is vertically downward through the center of the sphere of the rock cuttings particles, and the plastic force F p The direction is perpendicular to the drilling fluid flow direction and downward; the power includes the drag force F along the drilling fluid flow direction D , the upward lifting force F perpendicular to the flow direction of drilling fluid L , the pressure gradient force F along the drilling fluid flow direction Δp and the support force F of the rock cuttings particles R , because when the target rock cuttings particles are in a critical state, the supporting force F R is the reaction force, which is taken as zero. At this time, there is no need to consider the support force F of the rock cuttings. R Therefore, we only need to consider the buoyancy G and drag force F of the cuttings particles. D , lifting force F L , plastic force F p , pressure gradient force F Δp .

[0133] in,

[0134] The buoyant weight G of the rock cuttings particles is:

[0135]

[0136] In the formula, G is the floating weight, the unit is N, G = F g -F b , F g is the gravity of rock cuttings particles, F b is the buoyancy of the rock debris particles; ρ c is the density of rock cuttings particles, in kg / m 3 ρ f is the drilling fluid density, in kg / m 3 ; g is the acceleration due to gravity, and its value is 9.81m / s2 .

[0137] During the circulation of drilling fluid, the flow velocity at the upstream and downstream sides of the cuttings particles is different. According to Bernoulli's theorem, a pressure difference will be generated before and after the cuttings particles, and this pressure difference will generate an impact force consistent with the direction of the flow. In addition, the flow flows along the surface of the cuttings particles, generating friction resistance. The combined force of the two is the drag force, which is parallel to the axis of the wellbore and acts above the center of the cuttings particle along the normal direction of the bed surface.

[0138] According to the drag force action position in sediment motion mechanics, the drag force action position of the cuttings particles is The drag force F on the rock cuttings particles is D for:

[0139]

[0140] in,

[0141]

[0142] In the formula, F D is the drag force on the cuttings particles, in N; C D is the drag coefficient, which is related to the drilling fluid performance and flow rate; Re is the Reynolds number; v b It is the local velocity of the drilling fluid acting on the center of the cuttings particle sphere, in m / s.

[0143] During the flow of drilling fluid, the flow velocity at the top and bottom of the cuttings particles is different. The top flow velocity is the drilling fluid flow velocity v, and the bottom flow velocity is the flow velocity v between the cuttings particles. b The flow rate of drilling fluid between cuttings particles is much smaller than that of drilling fluid. According to Bernoulli's principle, the flow rate at the top of cuttings particles is high and the pressure is low, while the flow rate at the bottom is low and the pressure is high. The pressure difference generated is the lifting force F. L , Through the center of gravity of the rock cuttings particles and in an upward direction.

[0144] Since the lifting force action area of ​​the cuttings particles is not affected by the shielding degree of the cuttings particles, The particle Reynolds number based on the apparent viscosity of the drilling fluid and the average shear rate when the power-law fluid flows over the spherical particles are further introduced to determine the lifting force F exerted on the cuttings particles: L for:

[0145]

[0146] in,

[0147]

[0148] In the formula, F L is the lifting force on the cuttings particles, in N; A L is the lifting force action area; C L is the lifting coefficient; max(·) is the maximum value function; α p is the lift coefficient factor, in, is the velocity gradient between the top and bottom of the rock cuttings particle, r is the radius of the rock cuttings particle; Re p is the particle Reynolds number, dimensionless, Among them, μ a is the apparent viscosity of the drilling fluid at the cuttings particles under the shear rate of the drilling fluid, in Pa·s. τ y is the yield stress of drilling fluid, in Pa, k is the consistency coefficient, in Pa.s n , γ p is the shear rate of rock cuttings particles, n is the flow pattern index and is dimensionless.

[0149] Considering the actual calculation of the lift coefficient C L It is difficult to obtain engineering data. According to the mechanics of sediment movement, in this embodiment, C L =0.78C D .

[0150] The cuttings particles on the top surface of the cuttings bed are squeezed by the adjacent cuttings particles and confined to the interstitial space of the cuttings bed, and are in a static state. The drilling fluid flows around the cuttings particles. On the contrary, the drilling fluid in the pores below the cuttings particles is in a static state. If the drilling fluid exhibits Bingham characteristics, the drilling fluid is in a plastic state and exerts a dynamic shear force on the cuttings particles. Plastic force F p It is generated by the yield stress of the static drilling fluid below the cuttings particles and points vertically toward the bottom of the wellbore.

[0151] Slip line field theory provides a method for calculating the resultant forces required to lift cuttings from a stationary layer into the drilling fluid. However, several simplifying assumptions are required to make the calculations tractable. Hill presents a method for calculating the mean compressive and shear stresses acting on the yield surface based on the slip line theory. Since these forces are axisymmetric, the target area can be described using a two-dimensional coordinate system.

[0152] Due to the plastic force F p The plastic force F acting on the cuttings particles is determined without being affected by the degree of shielding of the cuttings particles. p for:

[0153]

[0154] In the formula, F p It is the plastic force on the rock cuttings particles, in N.

[0155] The pressure gradient force F acting on the cuttings particles along the flow direction of the drilling fluid Δp Affected by the shielding degree of the cuttings particles, the pressure gradient force F exerted on the cuttings particles along the flow direction of the drilling fluid is Δp for:

[0156]

[0157] In the formula, F Δp is the pressure gradient force on the cuttings particles, in N; C s is the pressure gradient force coefficient; G pf is the drilling fluid annular circulation pressure gradient, in Pa / m, p f is the drilling fluid pressure; τ w is the wall shear stress; D hyd is the hydraulic diameter.

[0158] Step 4: Use the cuttings start-up velocity prediction model for extended reach wells to simulate the tumbling process of the target cuttings particles when the drilling fluid flows, and determine the bottom velocity of the cuttings particles when the cuttings particles start based on the start-up probability of the cuttings particles. The specific process is as follows:

[0159] The drilling fluid parameters, cuttings parameters and environmental control displacement in the cuttings start-up velocity prediction model for extended reach wells are set. The cuttings start-up velocity prediction model for extended reach wells is used to simulate the tumbling process of target cuttings particles when the drilling fluid flows. When the drilling fluid flow causes the target cuttings particles to tumble, the target cuttings particles tumble with the contact point a as the support point. At this time, the various moments acting on the target cuttings particles are in a state of equilibrium, and the resultant moment is zero.

[0160] The coordinates of the contact point a of the target rock cuttings particle in the three-dimensional coordinate system are known to be (-y a ,z a ), combined with the cuttings shielding geometric topological relationship and the cuttings repose angle φ, we get:

[0161]

[0162] in,

[0163] cosφ=1-2Δ′ y (12-1)

[0164]

[0165] In the formula, y a is the horizontal coordinate of the point symmetrical to the contact point a on the y-axis, z ais the z-axis coordinate of the contact point a.

[0166] Combined with the stress state of the target rock cuttings particles, determine the moment of the target rock cuttings particles, including the length of the drag force arm l y and the lifting force arm length l z for:

[0167]

[0168] The buoyancy of the target rock cuttings particles is decomposed in the -y and -z directions to obtain the buoyancy component in the -y direction and the buoyancy component in the -z direction. The arm length of the buoyancy component in the -y direction and the arm length of the lifting force are both l z , the length of the force arm of the floating weight component in the -z direction is l G,t , In this embodiment, cosφ, sinφ, Δ′ are substituted y , we get l y =0.451d c , l z =0.411d c , l G,y =0.284d c .

[0169] When the power generated by the flow of drilling fluid exceeds the static force, the cuttings particles on the upper part of the cuttings bed model in the extended reach well cuttings start-up velocity prediction model roll. Generally, as the drilling fluid displacement increases, the power exerted on the cuttings particles increases, and the rolling expression of the cuttings particles is obtained as follows:

[0170] l y (F D +F ΔP )-Gcos(α)l G,y +l z [F L -F P -Gsin(α)]>0 (15)

[0171] Where α is the well inclination angle.

[0172] Based on the static and dynamic calculation formulas of cuttings particles and the moment expression of cuttings particles, the minimum bottom velocity required for cuttings particles to tumble in the cuttings starting velocity prediction model for extended reach wells is obtained as follows:

[0173]

[0174] When solving the above equation, it is necessary to determine the friction pressure drop gradient G pf , considering that the pressure gradient force is very small compared with the drag force, lift force and gravity, it can be ignored in the calculation for the sake of simplicity.

[0175] Since the cuttings particles only reach the starting condition in theory when the force reaches the torque balance, considering the fluctuation characteristics of the bottom flow velocity of the drilling fluid, the randomness of the shielding of the cuttings particles, and the differences in the size and shape of the cuttings particles, it is obtained that there is a starting probability for the cuttings particles to start.

[0176] The starting probability P of the rock cuttings particles is:

[0177]

[0178] Where p is the probability condition of rock debris particles starting.

[0179] make but:

[0180] P = p[v b 2 >(EF) 2 ]=1-p[v b 2 <(EF) 2 ] (18)

[0181] Where E and F are intermediate calculation parameters.

[0182] Since the instantaneous local velocity of drilling fluid obeys normal distribution, then:

[0183]

[0184] In the formula, f(v b ) is the drilling fluid velocity probability distribution function; is the time-averaged local velocity of the drilling fluid when the cuttings particles start, which is used to represent the bottom velocity when the cuttings particles start; σ is the mean square error, the standard deviation σ of the local velocity of the drilling fluid acting on the center of the cuttings particle in the mechanics of sediment movement is proportional to the time-averaged local velocity of the drilling fluid when the cuttings particles start There is a linear relationship between them. In the mechanics of sediment movement, the movement of individual rock debris particles is defined as weak movement, and the corresponding movement probability of the rock debris particles is 0.0014. The movement of a small number of rock debris particles is defined as weak movement, and the corresponding movement probability of the rock debris particles is 0.0227. The movement of most rock debris particles is defined as strong movement, and the corresponding movement probability of the rock debris particles is 0.1585.

[0185] Therefore, the starting probability P of the cuttings particles is expressed as:

[0186]

[0187] In the formula, is the normal distribution function;

[0188] Since the probability of cuttings particles rolling in the reverse direction of drilling fluid is negligible, the reverse rolling of cuttings particles is ignored, and the following is obtained:

[0189]

[0190] Taking the strong movement of rock cuttings as the starting standard of rock cuttings, the starting probability of rock cuttings is determined, and the normal distribution table is queried to obtain Determine the time-averaged local velocity of the drilling fluid when the cuttings particles start to move

[0191] Considering the influence of the size and shape of cuttings on the start-up of cuttings, combined with Han Qi's flattening function f(λ), the time-averaged local velocity of the drilling fluid at the start-up under the strong probability of cuttings movement is obtained. for:

[0192]

[0193] Where λ is the flattening coefficient.

[0194] In this embodiment, the flattening function f(λ) takes a value of 2, then That is, when the local flow velocity of the drilling fluid at the lower side of the wellbore in the annulus reaches the value of the above formula, the cuttings particles in the cuttings bed reach the strong motion probability.

[0195] Step 5: Based on the cuttings starting flow rate prediction model for extended reach wells, the starting bottom flow rate of cuttings particles and the cuttings particle size are simulated, combined with the drill string rotation state, eccentricity and drilling fluid flow rate, and based on the cuttings shielding and starting probability, the starting flow rate prediction model for extended reach wells is used to simulate and determine the average starting flow rate of cuttings particles that meets the rock carrying requirements. The specific process is as follows:

[0196] The starting bottom velocity of cuttings particles, cuttings particle size, drill string rotation state, eccentricity and drilling fluid velocity are set, and the cuttings starting velocity prediction model for extended reach wells is used to simulate the rotary drilling process of the drill bit. At this time, the drill string model in the cuttings starting velocity prediction model for extended reach wells is in a rotating state, and the circumferential velocity of the drilling fluid in the area below the drill string model in the wellbore annulus is symmetrically distributed with the center line of the area as the symmetry axis.

[0197] The velocity v of the drilling fluid in the area below the drill string model in the wellbore annulus along the wellbore high side direction line d for:

[0198]

[0199] in,

[0200] R τN =[(0.0003τ y+0.0007)N-0.0223τ y +0.9435](-0.0082τ y +1.0778) (24)

[0201] In the formula, R τN is the dynamic shear force and rotation speed influencing factor, dimensionless; y is the distance from the lower side of the wellbore in the annulus below the drill string, in meters; is the average flow rate of drilling fluid, in m / s; N is the rotation speed, in rpm.

[0202] Due to the drag force F on the cuttings particles along the flow direction of the drilling fluid D Acting on the diameter of rock cuttings According to get:

[0203]

[0204] The average flow rate of the drilling fluid is determined for:

[0205]

[0206] Step 6: Complete the prediction of the starting velocity of cuttings particles in the extended displacement well.

[0207] Example 2

[0208] This embodiment adopts a method for predicting the starting flow rate of cuttings in an extended displacement well proposed in Embodiment 1. According to the design parameters of the extended displacement well, the wellbore parameters, drilling fluid parameters, cuttings parameters and drill string parameters of the extended displacement well are obtained, and a cuttings starting flow rate prediction model for an extended displacement well is constructed. Based on the cuttings shielding and starting probability, the starting flow rate prediction model for an extended displacement well is used to simulate the rolling process of cuttings particles affected by the flow of drilling fluid, and determine the bottom flow rate of cuttings particles when the cuttings particles start and the starting average flow rate of cuttings particles that meets the rock carrying requirements.

[0209] The design parameters of the extended reach well in this embodiment are shown in Table 1.

[0210] Table 1 Setting parameters of the cuttings start-up velocity prediction model for extended reach wells

[0211]

[0212] The cuttings start-up velocity prediction model for extended-reach wells was set according to the parameter values ​​described in Table 1. The simulation was performed using the cuttings start-up velocity prediction model for extended-reach wells. The cuttings transport experiment was performed on the Φ203 mm flow loop according to the design scheme in Table 1. The prediction results of the cuttings start-up velocity prediction model for extended-reach wells were compared with the cuttings transport experimental data. Figure 4 As shown, the prediction accuracy of the method of the present invention is 96.88%, thereby verifying the accuracy of the method for predicting the cuttings start-up flow rate of the extended displacement well based on cuttings shielding and start-up probability proposed by the present invention.

[0213] Of course, the above description is not a limitation of the present invention, and the present invention is not limited to the above examples. Changes, modifications, additions or substitutions made by technicians in this technical field within the essential scope of the present invention should also fall within the protection scope of the present invention.

Claims

1. A method for predicting the starting flow rate of cuttings in an extended reach well, characterized in that: The method predicts the cuttings start-up flow rate of an extended reach well based on cuttings shielding and start-up probability, specifically including the following steps: Step 1: construct a cuttings start-up velocity prediction model for an extended reach well according to the design parameters of the extended reach well, and set the drilling fluid and cuttings in the wellbore annulus in the cuttings start-up velocity prediction model for the extended reach well; Step 2: placing the extended-reach well cuttings start-up velocity prediction model in a three-dimensional coordinate system, selecting target cuttings particles in the extended-reach well cuttings start-up velocity prediction model, obtaining the cuttings shielding state of the target cuttings particles, and determining the exposed area of ​​the cuttings particles in the extended-reach well cuttings start-up velocity prediction model; Step 3, performing a stress analysis on the target rock cuttings particles, obtaining the stress state of the target rock cuttings particles, and determining the static and dynamic forces on the rock cuttings particles in the prediction model of the starting flow velocity of the extended reach well; Step 4, using the cuttings start-up velocity prediction model for extended reach wells to simulate the tumbling process of the target cuttings particles when the drilling fluid flows, and combining the start-up probability of the cuttings particles, determining the bottom velocity of the cuttings particles when the cuttings particles start based on the start-up probability of the cuttings particles; Step 5: Based on the cuttings starting flow rate prediction model for extended reach wells, the starting bottom flow rate of cuttings particles and the cuttings particle size obtained by simulation, combined with the drill string rotation state, eccentricity and drilling fluid flow rate, based on the cuttings shielding and starting probability, the starting flow rate prediction model for extended reach wells is used to simulate and determine the starting average flow rate of cuttings particles that meets the rock carrying requirements; Step 6: Complete the prediction of the starting velocity of cuttings particles in the extended displacement well.

2. The method for predicting the starting flow rate of cuttings in extended reach wells according to claim 1, characterized in that: In the step 1, according to the design parameters of the extended reach well, the wellbore parameters, drilling fluid parameters, cuttings parameters and drill string parameters of the extended reach well are obtained, wherein the wellbore parameters of the extended reach well include the well inclination angle α, the wellbore diameter and the annular displacement, the drilling fluid parameters include the components, flow rate, flow direction, dynamic shear force and plastic viscosity of the drilling fluid, the cuttings parameters include the type, density and particle size of the cuttings, and the drill string parameters include the outer diameter, mechanical penetration rate, rotation speed and eccentricity of the drill string; A cuttings start-up flow rate prediction model for an extended displacement well is constructed according to the design parameters of the extended displacement well. The cuttings start-up flow rate prediction model for an extended displacement well includes a wellbore model, a drill string model and a drill bit model. The drill string model is eccentrically arranged in the wellbore model and connected to the drill bit model. The space between the drill string model and the wellbore model forms a wellbore annulus. A cuttings bed model is arranged at the bottom of the wellbore annulus. The cuttings bed model is formed by depositing and stacking a number of identical cuttings particles in the form of solid spheres.

3. The method for predicting the starting flow rate of cuttings in extended reach wells according to claim 2, characterized in that: In step 2, target cuttings particles are selected in the cuttings bed model, the center of the target cuttings particles is used as the coordinate origin, the flow direction of the drilling fluid is used as the positive direction of the z-axis, and the direction perpendicular to the flow direction of the drilling fluid is used as the positive direction of the y-axis, a three-dimensional coordinate system is constructed, and the cuttings start-up flow velocity prediction model for extended reach wells is placed in the three-dimensional coordinate system; Analyze the debris shielding state of the target debris particles in the debris bed model, and determine the mean shielding degree of the debris particles in the debris bed model based on the debris shielding geometric topological relationship; It is known that the target rock cuttings particle is located on the surface of the rock cuttings bed model, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the subsurface of the rock cuttings bed model and the target rock cuttings particle is a, the contact point between the rock cuttings particle downstream of the target rock cuttings particle and located on the surface of the rock cuttings bed model and the target rock cuttings particle is b, the distance between the contact point a and the contact point b on the z-axis is Δz, and the distance between the contact point a and the lowest point c of the target rock cutting particle on the y-axis is Δy; The rock-cutting shielding state of the target rock-cutting particles is analyzed, and the rock-cutting shielding geometric topological relationship is obtained as follows: In the formula, Δ′ z is the shielding degree of rock debris particles in the z-axis direction, Among them, d c is the diameter of rock debris particles; Δ′ y is the shielding degree of rock debris particles in the y-axis direction, Combined with the probability density calculation formula of rock cuttings shielding degree in engineering practice, the shielding degree of rock cuttings particles in the rock cuttings bed model in the y-axis direction is obtained by integration, and the mean shielding degree of rock cuttings particles in the rock cuttings bed model is determined; The probability density calculation formula of the rock debris particle shielding degree is: In the formula, f(Δ′ y ) is the probability density of rock debris obscuration; Then, the target cuttings particles in the cuttings bed model are projected onto the yz plane to obtain a projection circle, and the projection circle is determined to pass through the contact point a along the axis of the wellbore and the intersection point with the projection circle is B. The central angle of the arc is twice the rock cuttings repose angle φ, and the center of the projected circle is located at the chord The area of ​​the above part is the exposed area of ​​rock cuttings particles, and the exposed area of ​​rock cuttings particles is determined as: in, In the formula, A e The exposed area of ​​rock debris particles.

4. The method for predicting the starting flow rate of cuttings in extended reach wells according to claim 3, characterized in that: In step 4, the target rock cuttings particles are subjected to a force analysis to determine the static and dynamic forces on the rock cuttings particles in the rock cutting bed model, wherein the static force is the plastic force F caused by the buoyancy of the rock cuttings particles in the drilling fluid and the yield stress of the drilling fluid. p The direction of the buoyancy G of the rock cuttings particles is vertically downward through the center of the sphere of the rock cuttings particles, and the plastic force F p The direction is perpendicular to the drilling fluid flow direction and downward; the power includes the drag force F along the drilling fluid flow direction D , the upward lifting force F perpendicular to the flow direction of drilling fluid L , the pressure gradient force F along the drilling fluid flow direction Δp and the support force F of the rock cuttings particles R , because when the target rock cuttings particles are in a critical state, the supporting force F R is the reaction force, and the support force F of the rock cuttings does not need to be considered at this time R ; in, The buoyant weight G of the rock cuttings particles is: In the formula, G is the floating weight, the unit is N, G = F g -F b , F g is the gravity of rock cuttings particles, F b is the buoyancy of the rock debris particles; ρ c is the density of rock cuttings particles, in kg / m 3 ρ f is the drilling fluid density, in kg / m 3 ; g is the acceleration due to gravity; The drag force F on the rock cuttings particles is D for: in, In the formula, F D is the drag force on the cuttings particles, in N; C D is the drag coefficient, which is related to the drilling fluid performance and flow rate; Re is the Reynolds number; v b is the local velocity of the drilling fluid acting on the center of the cuttings particle sphere, in m / s; The lifting force F on the rock cuttings particles is L for: in, In the formula, F L is the lifting force on the cuttings particles, in N; A L is the lifting force action area; C L is the lifting coefficient; max(·) is the maximum value function; α p is the lift coefficient factor, in, is the velocity gradient between the top and bottom of the rock cuttings particle, r is the radius of the rock cuttings particle; Re p is the particle Reynolds number, dimensionless, Among them, μ a is the apparent viscosity of the drilling fluid at the cuttings particles under the shear rate of the drilling fluid, in Pa·s. τ y is the yield stress of drilling fluid, in Pa, k is the consistency coefficient, in Pa.s n , γ p is the shear rate of rock cuttings particles, n is the flow pattern index, dimensionless; Due to the plastic force F p The plastic force F acting on the cuttings particles is determined without being affected by the degree of shielding of the cuttings particles. p for: In the formula, F p is the plastic force on the rock cuttings particles, in N; The pressure gradient force F acting on the cuttings particles along the flow direction of the drilling fluid Δp Affected by the shielding degree of the cuttings particles, the pressure gradient force F exerted on the cuttings particles along the flow direction of the drilling fluid is Δp for: In the formula, F Δp is the pressure gradient force on the cuttings particles, in N; C s is the pressure gradient force coefficient; G pf is the drilling fluid annular circulation pressure gradient, in Pa / m, p f is the drilling fluid pressure; τ w is the wall shear stress; D hyd is the hydraulic diameter.

5. The method for predicting the starting flow rate of cuttings in extended reach wells according to claim 4, characterized in that: In step 4, the drilling fluid parameters, cuttings parameters and environmental control displacement in the extended reach well cuttings start-up velocity prediction model are set, and the extended reach well cuttings start-up velocity prediction model is used to simulate the tumbling process of the target cuttings particles when the drilling fluid flows. When the drilling fluid flows and causes the target cuttings particles to tumble, the target cuttings particles tumble with the contact point a as the support point. At this time, the various moments on which the target cuttings particles are subjected are in a state of equilibrium, and the resultant moment is zero. The coordinates of the contact point a of the target rock cuttings particle in the three-dimensional coordinate system are known to be (-y a ,z a ), combined with the cuttings shielding geometric topological relationship and the cuttings repose angle φ, we get: in, cosφ=1-2Δ′ y (12-1) In the formula, y a is the horizontal coordinate of the point symmetrical to the contact point a on the y-axis, z a is the z-axis coordinate of the contact point a; Combined with the stress state of the target rock cuttings particles, determine the moment of the target rock cuttings particles, including the length of the drag force arm l y and the lifting force arm length l z for: The buoyancy of the target rock cuttings particles is decomposed in the -y and -z directions to obtain the buoyancy component in the -y direction and the buoyancy component in the -z direction. The arm length of the buoyancy component in the -y direction and the arm length of the lifting force are both l z , the length of the force arm of the floating weight component in the -z direction is l G,y , When the power generated by the flow of drilling fluid exceeds the static force, the cuttings particles on the upper part of the cuttings bed model in the cuttings start-up velocity prediction model for extended reach wells roll, and the rolling expression of the cuttings particles is obtained as follows: l y (F d +F ΔP )-Gcos(α)l G,y +l z [F L -F P -Gsin(α)]>0 (15) Where, α is the well inclination angle; Based on the static and dynamic calculation formulas of cuttings particles and the moment expression of cuttings particles, the minimum bottom velocity required for cuttings particles to tumble in the cuttings starting velocity prediction model for extended reach wells is obtained as follows: In the formula, G pf is the friction pressure drop gradient; Since the cuttings particles only reach the starting condition in theory when the force reaches the moment balance, considering the fluctuation characteristics of the bottom flow velocity of the drilling fluid, the randomness of the shielding of the cuttings particles, and the differences in the size and shape of the cuttings particles, the starting probability of the cuttings particles is obtained; The starting probability P of the rock cuttings particles is: Where p is the probability condition of rock debris particle initiation; make but: P=p[v b 2 >(EF) 2 ]=1-p[v b 2 <(EF) 2 ] (18) In the formula, E and F are intermediate calculation parameters; Since the instantaneous local velocity of drilling fluid obeys normal distribution, then: In the formula, f(v b ) is the drilling fluid velocity probability distribution function; is the time-averaged local velocity of the drilling fluid when the cuttings particles start, which is used to represent the bottom velocity when the cuttings particles start; σ is the mean square error, the standard deviation σ of the local velocity of the drilling fluid acting on the center of the cuttings particle in the mechanics of sediment movement is proportional to the time-averaged local velocity of the drilling fluid when the cuttings particles start There is a linear relationship between them. In the mechanics of sediment movement, the start of individual debris particles is defined as weak movement, and the corresponding probability of the start of the debris particles is 0.0014. The start of a small number of debris particles is defined as weak movement, and the corresponding probability of the start of the debris particles is 0.0227. The start of most debris particles is defined as strong movement, and the corresponding probability of the start of the debris particles is 0.1585. Therefore, the starting probability P of the cuttings particles is expressed as: In the formula, is the normal distribution function; Since the probability of cuttings particles rolling in the reverse direction of drilling fluid is negligible, the reverse rolling of cuttings particles is ignored, and the following is obtained: Taking the strong movement of cuttings particles as the starting standard of cuttings particles, the starting probability of cuttings particles is determined, and the normal distribution table is queried to obtain Determine the time-averaged local velocity of the drilling fluid when the cuttings particles start to move Considering the influence of the size and shape of cuttings on the start-up of cuttings, combined with Han Qi's flattening function f(λ), the time-averaged local velocity of the drilling fluid at the start-up under the strong probability of cuttings movement is obtained. for: Where λ is the flattening coefficient.

6. The method for predicting the starting flow rate of cuttings in an extended reach well according to claim 5, characterized in that: In step 5, the starting bottom flow velocity of the cuttings particles, the cuttings particle size, the rotation state of the drill string, the eccentricity and the drilling fluid flow velocity are set, and the cuttings starting flow velocity prediction model for the extended reach well is used to simulate the rotary drilling process of the drill bit. At this time, the drill string model in the cuttings starting flow velocity prediction model for the extended reach well is in a rotating state, and the circumferential flow velocity of the drilling fluid in the area below the drill string model in the wellbore annulus is symmetrically distributed with the center line of the area as the symmetry axis; The velocity v of the drilling fluid in the area below the drill string model in the wellbore annulus along the wellbore high side direction line d for: in, R τN =[(0.0003τ y +0.0007)N-0.0223τ y +0.9435](-0.0082t y +1.0778) (24) In the formula, R τN is the dynamic shear force and rotation speed influencing factor, dimensionless; y is the distance from the lower side of the wellbore in the annulus below the drill string, in meters; is the average flow rate of drilling fluid, in m / s; N is the rotation speed, in rpm; Due to the drag force F on the cuttings particles along the flow direction of the drilling fluid D Acting on the diameter of rock cuttings According to get: The average flow rate of the drilling fluid is thus determined as:

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