A low-pressure easy-leakage formation drilling parameter optimization method

Abstract

The present application relates to a kind of low-pressure easy leakage formation drilling parameter optimization method, first, formation leakage pressure and collapse pressure are predicted;Then design drilling fluid performance parameters, wherein the density of drilling fluid is designed based on the equivalent density of smaller one in it;Circulation discharge and drilling fluid dynamic shear force, plastic viscosity are designed with the circulating resistance during drilling not more than the difference between leakage pressure and collapse pressure;Then calculate the critical annulus return speed when no cuttings bed is formed, if the current designed circulation return speed is less than the critical annulus return speed, then re-optimize circulation discharge and drilling fluid dynamic shear force, plastic viscosity, until the optimized circulation discharge meets the circulation return speed less than the critical annulus return speed;Complete the design of drilling fluid performance parameters for the whole well section;Finally, according to the drilling fluid performance parameters, optimize the drilling or casing speed, control the excitation pressure in the range of the difference between leakage pressure and collapse pressure, complete the drilling parameter optimization. Improve the drilling efficiency, reduce the cost.

Description

Technical Field

[0001] This invention relates to a method for optimizing drilling parameters in low-pressure, easily leaking formations, and belongs to the field of drilling optimization design technology. Background Technology

[0002] In drilling operations, formations with well-developed fractures, good permeability, and poor bedding bonding often encounter low-loss-pressure formations. Drilling fluid loss occurs because the wellbore pressure exceeds the formation loss-pressure. Well leakage not only wastes drilling time and results in significant drilling fluid loss, but improper handling can also lead to complex accidents such as blowouts, well collapses, and stuck pipe, making it one of the most common drilling accidents. Numerous scholars both domestically and internationally have conducted research on well leakage mechanisms and plugging technologies, resulting in a series of highly effective plugging materials and systems, significantly improving the targeted nature and efficiency of well leakage control. Based on the principles of "prevention first" and "pre-emptive pressure bearing," optimizing drilling parameters and precisely controlling the wellbore pressure to be lower than the loss-pressure is a key measure to improve drilling efficiency and reduce costs.

[0003] Wellbore pressure is influenced by numerous factors. Drilling fluid density directly affects hydrostatic pressure, while mechanical drilling rate, circulation flow rate, and drilling fluid properties affect annular circulation friction. Drilling (casing) speed and drilling fluid properties affect fluctuation pressure (fluctuation pressure can be considered a combination of excitation and extraction pressure; in this invention, fluctuation pressure is caused by excitation pressure during drilling or casing). Therefore, to accurately control wellbore pressure, a wellbore pressure prediction method needs to be established. Based on this method, the influence of each factor on wellbore pressure needs to be analyzed, and drilling engineering parameters can be optimized by considering wellbore cleanliness and drilling speed requirements. Summary of the Invention

[0004] The purpose of this invention is to provide a method for optimizing drilling parameters in low-pressure, easily leaking formations, so as to achieve accurate prediction and optimization of drilling engineering parameters in formations with low leakage pressure.

[0005] To achieve the above objectives, the present invention includes:

[0006] The technical solution of a drilling parameter optimization method for low-pressure, easily leaking formations according to the present invention includes the following steps:

[0007] 1) Predict formation leakage pressure and collapse pressure;

[0008] 2) Design drilling fluid performance parameters, wherein the density of the drilling fluid is designed based on the equivalent density of the collapse pressure;

[0009] The circulation displacement, drilling fluid dynamic shear force, and plastic viscosity are designed so that the circulation friction during drilling does not exceed the difference between the leakage pressure and the collapse pressure.

[0010] 3) Calculate the critical annular return velocity when no cuttings bed is formed. If the currently designed circulation return velocity is less than the critical annular return velocity, return to step 2) and re-optimize the circulation displacement, drilling fluid dynamic shear force, and plastic viscosity until the optimized circulation displacement satisfies that the circulation return velocity is less than the critical annular return velocity.

[0011] 4) Repeat steps 2) and 3) to design drilling fluid performance parameters for different formations, well depths and inclinations, and complete the design of drilling fluid performance parameters for the entire well section;

[0012] 5) Based on the drilling fluid performance parameters, optimize the running-in or running-out speed, control the energizing pressure within the range of the difference between the leakage pressure and the collapse pressure, and complete the drilling parameter optimization.

[0013] Studies have shown that wellbore pressure in drilling operations consists of hydrostatic pressure, circulating friction, and fluctuating pressure. This invention, with formation leakage pressure and wellbore cleanliness as primary constraints, establishes predictive models for circulating friction and fluctuating pressure based on a clear understanding of the wellbore pressure composition. It then creates a system optimization design method for drilling parameters in low-pressure, easily leaking formations, targeting normal drilling, running-in drilling, and casing running conditions.

[0014] Further, in step 1), the leakage pressure is predicted by statistically analyzing drilling data and interpolating to establish a lateral distribution profile of the main leakage layers in the target area; or, it is predicted by using logging data to fit a relationship model of leakage velocity, leakage pressure difference and fracture porosity.

[0015] Furthermore, in step 1), the collapse pressure is predicted based on well logging data using the Mohr-Coulomb criterion to establish a collapse pressure profile.

[0016] Furthermore, in step 2), the circulating friction during drilling is composed of the circulating pressure loss of the drilling fluid in the entire annular laminar flow section and the pressure loss generated by the solid phase particles of rock cuttings in the entire annular laminar flow section.

[0017] Furthermore, drilling fluid circulation pressure loss p al1 Calculated using the following formula:

[0018]

[0019] Where: L is the length of the annulus section used for calculating pressure loss; Q is the circulation displacement; D0 is the wellbore diameter; D i This represents the outer diameter of the drill pipe. (μ) p τ0 and τ0 represent the plastic viscosity and dynamic shear force of the drilling fluid, respectively.

[0020] Furthermore, the pressure loss p generated by the solid phase particles of rock debris al2 Calculated using the following formula:

[0021] p al2 =(ρs -ρ)gL·C a

[0022] Where: ρ s ρ and ρ are the densities of rock cuttings and drilling fluid, respectively; g is the acceleration due to gravity; L is the length of the annulus section used to calculate pressure loss; C a This represents the concentration of annular rock fragments.

[0023] Furthermore, the concentration of circumferential rock fragments, Ca, is calculated using the following formula:

[0024]

[0025] Where R is the mechanical drilling rate; K' is the speed correction coefficient; V f D represents the annular return velocity of the drilling fluid. b D is the drill bit diameter; D0 is the wellbore diameter; D i d is the outer diameter of the drill pipe. s The diameter of the rock fragment is denoted as .

[0026] Furthermore, in step 3), the critical annular return velocity Vc is calculated as follows:

[0027]

[0028] Among them, D s ρ is the equivalent diameter of the rock cuttings; K is the consistency coefficient; θ is the well inclination angle; s Density of rock fragments; ρ m Density of drilling fluid; μ e This refers to the effective viscosity of the drilling fluid.

[0029] Furthermore, the effective viscosity μe of the drilling fluid is calculated as follows:

[0030]

[0031] Where K is the consistency coefficient; n is the flow index; D o D i These are the wellbore diameter and drill pipe outer diameter, respectively; V a This is for the return velocity of drilling fluid in the annulus.

[0032] Furthermore, in step 3), if the optimized result still cannot satisfy the requirement that the cyclic return speed is less than the critical annular return speed, then the drill pipe rotation speed is increased to control the mechanical drilling speed, thereby destroying and removing the cuttings bed. Attached Figure Description

[0033] Figure 1 This is a flowchart of the drilling parameter optimization method for low-pressure, easily leaking formations according to the present invention;

[0034] Figure 2This refers to the circulating friction of the JPH-4XY well under different circulating discharge rates in the example.

[0035] Figure 3 This refers to the pressure increment of the Liujiagou Formation in the JPH-4XY well under different mechanical drilling rates in the example.

[0036] Figure 4 The example is the excitation pressure of well JPH-4XY under different drilling fluid properties and drilling speeds;

[0037] Figure 5 It is the excitation pressure of well JPH-4XY in the example under different drilling fluid properties and casing running speeds. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0039] 1. Wellbore pressure prediction method.

[0040] (1) Calculation model of cyclic friction.

[0041] During drilling, changes in circulation rate or drilling fluid density will cause changes in the bottom hole dynamic fluid column pressure. Changes in circulation rate alter the annular friction between the wellbore and drill pipe, affecting the cleanliness of cuttings and consequently the drilling fluid density. The drilling fluid density is significantly influenced by the drilling rate, in addition to the circulation rate. Therefore, annular friction comprises two parts:

[0042] 1) Drilling fluid circulation pressure loss p al1 :

[0043]

[0044] In the formula: L is the length of the annular section for calculating pressure loss, in meters; Q is the circulating displacement, in meters. 3 / s; D0 is the wellbore diameter, in meters; D i μ represents the outer diameter of the drill pipe, in meters (m). p τ0 and τ0 represent the plastic viscosity and dynamic shear force of the drilling fluid, respectively, in Pa·s and Pa.

[0045] 2) Pressure loss p generated by solid particles in rock debris al2 :

[0046] The falling velocity of rock cuttings is calculated using the Moore formula, and the concentration of rock cuttings in the annulus is calculated using the following formula:

[0047]

[0048] p al2 =(ρs -ρ)gL·C a (3)

[0049] In the formula, R is the mechanical drilling rate, in m / h; K` is the speed correction coefficient; V f ρ is the annular return velocity of drilling fluid, in m / s. s ρ and ρ are the densities of rock cuttings and drilling fluid, respectively, in g / cm³. 3 ;D b D0 is the drill bit diameter in cm; D0 is the wellbore diameter in cm. i d represents the outer diameter of the drill pipe, in cm. s The diameter of the rock fragment is in cm; g is the acceleration due to gravity, 980 g / cm. 3 .

[0050] 3) Depending on the inner and outer diameters of the annulus, the annular circulation friction of the drilling fluid in the laminar flow section of the entire annulus is as follows:

[0051]

[0052] In the formula, M i The total number of annular segments divided according to different inner and outer diameters; i is the sequence of segments from the bottom of the well to the wellhead; D 0i D refers to the wellbore diameter corresponding to the i-th annulus segment, in cm; ii L refers to the outer diameter of the drill pipe corresponding to the i-th annulus segment, in cm. i The length of the i-th circular segment is measured in meters (m).

[0053] (2) Fluctuation pressure calculation model.

[0054] During tripping, casing running, and well cleaning operations, the displacement effect of the tubing string causes drilling fluid to flow within the well, generating additional pressure. Due to uneven tripping speeds, the value of this additional pressure will change during the tubing string's lowering process; excessive pressure can easily cause formation leakage.

[0055] 1) Fluctuating pressure model during drilling:

[0056] A check valve is installed at the bottom of the drill pipe or casing. Regardless of whether the pump is operating, the drilling fluid in the wellbore will not flow into the casing during tripping or casing running; this is known as a plugged casing. As the drill string descends, the movement of the drilling fluid in the well is affected by the displacement force at the bottom of the drill string and the adhesion force between the drill string surface and the drilling fluid. Based on Burkhardt's narrow-channel flow principle and considering actual drilling conditions, a drilling fluid velocity equation is proposed for when the pump is shut off using a plugged casing:

[0057]

[0058] In the formula, vp D represents the average lowering speed of the casing or a single drill pipe. i D0 is the outer diameter of the drill pipe or casing, and D0 is the inner diameter of the wellbore.

[0059] 2) Calculation of fluctuating pressure during well cleaning:

[0060] During well cleaning, the mud pump operates, and the drilling fluid circulates within the well. The upward velocity of the drilling fluid in the annulus is composed of the flow velocity caused by displacement flow at the bottom of the drill string, the flow velocity caused by drill string adhesion, and the flow velocity caused by pump circulation.

[0061]

[0062] In the formula, v p D represents the average lowering speed of the casing or a single drill pipe. i D is the outer diameter of the drill pipe or casing. ii D0 is the inner diameter of the drill pipe or casing, and D0 is the inner diameter of the wellbore.

[0063] The fluctuation pressure prediction model is as follows:

[0064]

[0065] 2. Wellbore cleanliness evaluation method.

[0066] The critical velocity is calculated using a critical velocity prediction model and converted into a corresponding critical discharge rate. The design discharge rate is then compared with the critical discharge rate. If the design discharge rate is greater than the critical discharge rate, it indicates that no cuttings bed will form in the well, and the design meets wellbore cleanliness requirements. If the design discharge rate is less than the critical discharge rate, a cuttings bed will form in the wellbore. The thickness of the cuttings bed is calculated using a cuttings transport theory model. If the cuttings bed thickness does not meet wellbore cleanliness requirements, drilling parameters are optimized and adjusted. Cuttings bed thickness and minimum cuttings removal rate:

[0067] T CB =0.015D o (μ e +6.15μ e 0.5 )(1+0.587ε)(V c -V a (8)

[0068]

[0069]

[0070] In the formula: T CB D represents the thickness of the rock debris bed (a negative value indicates no rock debris bed formation), in mm. o D i These are the wellbore diameter and drill pipe outer diameter, respectively, in mm; D sV represents the equivalent diameter of the rock cuttings, in mm. a V represents the annular drilling fluid return velocity, expressed in m / s. c It is the critical annular return velocity that prevents the formation of a cuttings bed, measured in m / s; n is the flowability index; and K is the consistency coefficient, measured in Pa·s. n θ is the well inclination angle, in rad; ε is the eccentricity of the drill string, a dimensionless quantity; ρ s Density of rock fragments, in kg / L; ρ m The drilling fluid density is expressed in kg / L; μ e The effective viscosity of the drilling fluid is expressed in mPa·s.

[0071] Based on the actual requirements of drilling operations, the limit value T for cuttings bed thickness is set. CBL (T CBL (Limited to within 10% of the wellbore diameter). Initially select a V... a T is calculated using formulas (9) to (11). CB If T CB Equal to or slightly greater than T CBL Then V a This is the minimum return velocity required to meet the rock-carrying requirement. If the calculated T... CB Less than T CBL Then it is necessary to try selecting a larger V. a Continue until the requirements are met. Critical rock-carrying capacity Q c :

[0072]

[0073] In the formula, Q c The critical rock-carrying discharge rate is expressed in L / s.

[0074] 3. Drilling parameter design method for low-pressure, easily leaking formations.

[0075] Under fixed wellbore structure, wellbore trajectory, and drill string assembly conditions, the wellbore diameter, drill pipe outer diameter, and well inclination angle are objectively difficult to change. Meeting requirements such as drilling leakage prevention and wellbore cleanliness allows for the optimization of engineering parameters such as mechanical drilling rate, circulation flow rate, drilling fluid rheological properties, and running-in (casing) speed. Based on the above analysis and model establishment, a method for optimizing drilling parameters in low-pressure, easily leaking formations is constructed. Specific steps are detailed in [link to details]. Figure 1 As shown.

[0076] (1) Collection of basic data;

[0077] Collect information on wellbore structure, wellbore trajectory, formation stratification, geological structure, and adjacent well drilling data to lay the foundation for optimizing drilling parameters.

[0078] (2) Predict formation leakage pressure and collapse pressure;

[0079] Loss pressure prediction can be achieved by statistically analyzing drilling data and interpolating to establish a lateral distribution profile of the main loss zones in the target area; or by using logging data to fit a model relating loss velocity, loss pressure differential, and fracture porosity. Collapse pressure prediction is primarily based on logging data, using the Mohr-Coulomb criterion to establish a collapse pressure profile. Based on the wellbore structure and wellbore trajectory, a safe density window is determined, which is the "window" representing the difference between the loss pressure and collapse pressure in the open hole section.

[0080] (3) Design drilling fluid density;

[0081] If the pressure window is "positive," meaning the leakage pressure is greater than the collapse pressure, the drilling fluid density design should be based on the collapse pressure equivalent density (ρ = collapse pressure equivalent density + 0.01~0.02 g / cm³). 3 If the pressure window is "zero" or "negative", meaning the leakage pressure is less than the collapse pressure, the drilling fluid density design is still based on the collapse pressure equivalent density. However, during drilling, it is necessary to optimize the plugging material while drilling for "pre-plugging" to gradually increase the formation leakage pressure.

[0082] (4) Design the circulation displacement, drilling fluid dynamic shear force, and plastic viscosity during drilling;

[0083] Using formula 4, we analyze the circulating displacement Q, drilling fluid dynamic shear force τ0, and plastic viscosity μ. p The influence of cuttings bed on circulating pressure loss was investigated; relevant parameters were optimized to control the circulating friction during drilling within the "window" range of leakage pressure and collapse pressure.

[0084] (5) Check wellbore cleanliness and design drill pipe rotation speed;

[0085] Using Proof 8-10, calculate the critical annular return velocity V when no cuttings bed is formed. c If the current cycle return speed V under the current cycle displacement Q a Less than V c Then, in step (4) of the previous iteration, Q and μ are re-optimized. p and τ0, until the designed cyclic return velocity V c ≥V a If the optimized cycle displacement Q obtained through step (4) still cannot meet the requirement V c ≥V a (That is, the current cycle displacement Q is at least greater than the critical rock-carrying displacement Q) c Then, by increasing the drill pipe rotation speed, the mechanical drilling speed is controlled, and the cuttings bed is destroyed and removed.

[0086] (6) Design of drilling parameters for the entire well section;

[0087] Repeat steps (3)-(5) to design drilling fluid performance parameters for different formations, well depths and well inclinations. Drilling fluid performance parameters include drilling fluid density, drilling fluid circulation flow rate, drilling fluid dynamic shear force and drilling fluid plastic viscosity.

[0088] (7) Design the drilling (casing) speed;

[0089] Using the information provided in Announcement 7, the drilling fluid performance parameters designed in step (6) are used to optimize the running-in (running-out) speed and control the fluctuation pressure (also known as the excitation pressure in this embodiment) within the "window" range of leakage pressure and collapse pressure.

[0090] The following example will be used to analyze the application effect.

[0091] The Dongsheng Gas Field in the northern Ordos Basin employs horizontal well development, typically with a three-stage wellbore structure: the first stage (311.2mm × 244.5mm), the second stage (222.3mm × 177.8mm), and the third stage (152.4mm × 114.3mm). The second stage includes both vertical and inclined sections. The Liujiagou Formation encountered during drilling has low leakage pressure, while the mudstone walls of the Shiqianfeng and Shihezi Formations in the same open-hole section are prone to collapse. Optimizing drilling parameters during the second stage drilling and intermediate completion phases is crucial for safe drilling.

[0092] The JPH-4XY well will be used as an example for explanation.

[0093] Based on drilling and logging data from adjacent wells, the predicted leakage pressure in the Liujiagou Formation of well JPH-4XY is 1.15-1.20 g / cm³. 3 The well depth in the Liujiagou Formation is 2699m; while the collapse pressure at a depth of 3307m near target point A of the large well in the Lower Shihezi Formation is 1.18g / cm³. 3 To meet safe drilling requirements, drilling fluid density was optimized based on well inclination and strata, with a density of 1.05–1.08 g / cm³ in the Liujiagou Formation and above. 3 The density of the Liujiagou Formation from the bottom to the build-up point is 1.08–1.10 g / cm³. 3 The density in the well section with an inclination of 0-30° is 1.10~1.12g / cm³. 3 The density in the well section with an inclination of 30-45° is 1.12-1.14 g / cm³. 3 The density in the well section with an inclination of 45-60° is 1.14-1.16 g / cm³. 3 The density in the well section with an inclination of 60-90° is 1.16~1.18g / cm³. 3 To ensure wellbore stability in the lower formations when drilling fluid density increases, a compound of flake elastic graphite, bamboo fiber, and rigid ultrafine calcium carbonate is used to gradually increase the leakage pressure of the Liujiagou Formation.

[0094] When drilling reached the Liujiagou Formation, the drilling fluid density was 1.08 g / cm³. 3 The on-site drilling fluid had a plastic viscosity of 40 Pa·s, a dynamic shear force of 6 Pa, and a cuttings density of 2.4 g / cm³. 3 The average diameter of the rock cuttings was 0.85 cm. Based on relevant parameters, the engineering parameters for drilling the Liujiagou Formation were optimized. Field data showed that the average drilling speed when drilling to the Liujiagou Formation was no greater than 12 m / h. The circulating friction under different circulation rates during drilling was analyzed. Figure 2 The circulating friction under different mechanical drilling rates was analyzed, see... Figure 3 To control the circulating friction to be no greater than 1.85 MPa, the circulating displacement during drilling in the Liujiagou Formation should be 20-25 L / s, and the mechanical drilling speed should be 6-10 m / h.

[0095] During the intermediate stage of the operation, the equivalent density of the leakage pressure in the Liujiagou Formation was gradually increased to 1.23-1.24 g / cm³. 3 The faster the drilling speed, the greater the excitation pressure; the higher the plastic viscosity and dynamic shear force, the lower the extreme value of the drilling speed. (See...) Figure 4 If the plastic viscosity μ p =50 mPa·s, dynamic shear force τ0 = 7 Pa, drilling speed should be less than 0.4 m / s; if plastic viscosity μ p =60mPa·s, dynamic shear force τ0=8Pa, and the drilling speed should be less than 0.2m / s.

[0096] Well leakage during casing installation is very common and is closely related to the sensitivity of excitation pressure to casing installation speed. For example... Figure 5 As shown, if the plastic viscosity μ p =20 mPa·s, dynamic shear force τ0 = 4 Pa, the casing speed should be less than 0.15 m / s; if the plastic viscosity μ p =30 mPa·s, dynamic shear force τ0 = 4 Pa, the casing speed should be less than 0.1 m / s; if the plastic viscosity μ p =40 mPa·s, dynamic shear force τ0 = 5 Pa, the Liujiagou Formation will be leaked during casing installation. The sealing slurry injected into the well before casing installation should not be higher than the Liujiagou Formation; by segmented circulation, the plastic viscosity and dynamic shear force of the upper section should be appropriately reduced to reduce the excitation pressure.

[0097] Based on this, drilling leakage prevention construction plans and recommended practices were formulated for the Dongsheng Gas Field, and have been applied to a total of 197 wells. The horizontal well leakage rate has been reduced from 57% to 25.7%, the average single-well drilling fluid loss has been reduced by 80.6%, and the drilling cycle has been shortened by 32.3%. This has helped shorten the drilling cycle in the Dongsheng Gas Field and laid the foundation for efficient exploration and profitable development of tight natural gas.

[0098] For well leakage prevention in low-pressure leakage formations, this invention provides a drilling parameter optimization design method applicable to various working conditions such as drilling, running-in, and casing running. It comprehensively addresses the safety drilling needs of well leakage prevention and wellbore cleaning, guiding the optimization design of drilling engineering parameters and realizing the transformation from "post-delay leakage plugging" to "engineering leakage prevention" and "pre-delay leakage plugging". This significantly reduces drilling complexity and provides technical support for improving drilling efficiency and reducing costs.

Claims

1. A method for optimizing drilling parameters in low-pressure, easily leaking formations, characterized in that, Includes the following steps: 1) Predict formation leakage pressure and collapse pressure; 2) Design drilling fluid performance parameters, where the density of the drilling fluid is designed based on the collapse pressure equivalent density; the circulation displacement, dynamic shear force, and plastic viscosity of the drilling fluid are designed so that the circulation friction during drilling does not exceed the difference between the leakage pressure and the collapse pressure. 3) Calculate the critical annular return velocity V when no cuttings bed is formed using the following method. c : Among them, D s ρ is the equivalent diameter of the rock cuttings; θ is the well inclination angle; s Density of rock fragments; ρ m Density of drilling fluid; μ e This refers to the effective viscosity of the drilling fluid. If the current designed circulation return rate is less than the critical annular return rate, then return to step 2) to re-optimize the circulation displacement, drilling fluid dynamic shear force, and plastic viscosity until the optimized circulation displacement satisfies that the circulation return rate is less than the critical annular return rate. 4) Repeat steps 2) and 3) to design drilling fluid performance parameters for different formations, well depths and inclinations, and complete the design of drilling fluid performance parameters for the entire well section; 5) Based on the drilling fluid performance parameters, optimize the running-in or running-out speed, control the energizing pressure within the range of the difference between the leakage pressure and the collapse pressure, and complete the drilling parameter optimization.

2. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 1, characterized in that, In step 1), the leakage pressure is predicted by statistically analyzing drilling data and interpolating to establish a lateral distribution profile of the main leakage layers in the target area; or, it is predicted by fitting a relationship model of leakage velocity, leakage pressure difference and fracture porosity using logging data.

3. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 1, characterized in that, In step 1), the collapse pressure is predicted by establishing a collapse pressure profile based on well logging data using the Mohr-Coulomb criterion.

4. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 1, characterized in that, In step 2), the circulating friction during drilling is composed of the circulating pressure loss of the drilling fluid in the entire annular laminar flow section and the pressure loss generated by the solid phase particles of rock cuttings in the entire annular laminar flow section.

5. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 4, characterized in that, Drilling fluid circulation pressure loss Calculated using the following formula: Where: L is the length of the annulus section used for calculating pressure loss; Q is the circulation displacement; D0 is the wellbore diameter; D i The outer diameter of the drill pipe; μ p τ0 and τ0 represent the plastic viscosity and dynamic shear force of the drilling fluid, respectively.

6. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 4, characterized in that, Pressure loss generated by solid particles of rock cuttings Calculated using the following formula: Where: ρ s ρ and ρ are the densities of rock cuttings and drilling fluid, respectively; g is the acceleration due to gravity; L is the length of the annulus section used to calculate pressure loss; C a This represents the concentration of annular rock fragments.

7. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 6, characterized in that, Annular rock fragment concentration C a Calculated using the following formula: Where R is the mechanical drilling rate; K' is the speed correction coefficient; V f D represents the annular return velocity of the drilling fluid. b D is the drill bit diameter; D0 is the wellbore diameter; D i d is the outer diameter of the drill pipe. s The diameter of the rock fragment is denoted as .

8. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 4, characterized in that, In step 3), if the critical annular return velocity cannot be greater than the currently designed circulation return velocity even after optimization in step 2), the cuttings bed is destroyed and removed by increasing the drill pipe rotation speed and controlling the mechanical drilling speed.

9. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 1, characterized in that, Effective viscosity μ of drilling fluid e Calculated as follows: Where K is the consistency coefficient; n is the flow index; D o D i These are the wellbore diameter and drill pipe outer diameter, respectively; V a This is for the return velocity of drilling fluid in the annulus.

10. The method for optimizing drilling parameters in low-pressure, easily leaking formations according to claim 1, characterized in that, In step 3), if the optimization still fails to meet the requirement that the cyclic return speed is less than the critical annular return speed, then the drill pipe rotation speed is increased to control the mechanical drilling speed, thereby destroying and removing the cuttings bed.

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