A method for evaluating the length of the uncemented section in selective cementing to prevent casing deformation

Abstract

The present invention discloses a method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation, comprising: obtaining basic parameters and establishing a formation-selective cementing section cement sheath-casing force analysis model; obtaining the location where maximum stress is generated through the force analysis model and extracting the casing deformation parameters at the maximum stress location; establishing a geometric relationship model between the parameters of the running tool at the maximum stress location and the casing deformation section; obtaining a running tool length-outer diameter chart of the casing deformation section through the geometric relationship model; and evaluating the length of the uncemented section based on the running tool length-outer diameter chart and the size of a subsequently pre-run downhole tool. The present invention provides a method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation, which can obtain a corresponding running tool length-outer diameter chart based on a preset uncemented section length, thereby quickly predicting whether the running tool is at risk of getting stuck in the casing deformation section and evaluating the length of the uncemented section.

Description

Technical Field

[0001] The present invention relates to the technical field of shale development, and in particular to a method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation. Background Art

[0002] Shale gas exploration and development in the Sichuan Basin has entered a large-scale production phase. However, due to the prevalence of natural fractures and faults in shale reservoirs, and the extensive access to reservoirs during large-scale hydraulic fracturing, casing deformation is a common problem in the Weiyuan, Changning, and Zhaotong blocks. This can affect wellbore integrity, hinder downhole tooling, and even lead to the abandonment of some fracturing stages, significantly impacting field operation efficiency.

[0003] Experts and scholars have proposed mitigating the effects of formation shear slip on casing deformation through temporary plugging of natural fractures, improved cementing materials, and selective cementing. Selective cementing, due to its low process complexity, low cost, and effective reduction in casing deformation, is gradually being applied to wells at high risk of casing deformation. Selective cementing involves using a special process to cement sections at risk of natural fracture or fault slip without injecting cement, leaving no medium between the casing and the formation. This "gap" can offset casing displacement caused by formation slip within a certain range, allowing the casing at the formation section to exhibit progressive deformation in the uncemented section, rather than the direct shear deformation of the casing that occurs with full cementing. This mitigates the effects of formation slip on the casing structure.

[0004] However, for selective cementing, current research has shortcomings such as the unclear degree to which the uncemented section can mitigate casing deformation and the inability to effectively evaluate the length of the uncemented section. Therefore, a method is urgently needed to evaluate the length of the uncemented section for selective cementing to prevent casing deformation. Summary of the Invention

[0005] In view of this, the present invention provides a method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation, so as to overcome the defect in the prior art that the length of the uncemented section cannot be effectively evaluated.

[0006] The specific technical solution of the embodiment of the present invention is:

[0007] A method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation, characterized in that it comprises the following steps:

[0008] (1) Obtain basic parameters and establish a cement sheath-casing stress analysis model for the formation-selective cementing section;

[0009] (2) obtaining the position where the maximum stress is generated through the formation-selective cementing section cement sheath-casing force analysis model, and extracting the casing deformation parameters at the maximum stress position;

[0010] (3) Establish a geometric relationship model between the running tool parameters at the maximum stress position and the casing deformation section;

[0011] (4) Obtaining a length-outer diameter diagram of the running tool of the casing deformation section through the geometric relationship model in step (3);

[0012] (5) Evaluate the length of the uncemented well section based on the downhole tool length-outer diameter chart and the size of the subsequent pre-downhole tools.

[0013] Furthermore, the basic parameters in step (1) include ground stress parameters, rock mechanics parameters, natural fracture geometry parameters, well logging data, and length of the uncemented section.

[0014] Furthermore, the geostress parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress; the rock mechanics parameters include the Young's modulus of the rock and the Poisson's ratio of the rock; the natural fracture geometric parameters include the azimuth, inclination, and length of the natural fracture; and the logging data include the horizontal wellbore azimuth, horizontal open hole diameter, cement sheath outer diameter and wall thickness, casing outer diameter and wall thickness, and the distance between the wellbore and the center of the fracture length.

[0015] Furthermore, the step (1) also includes: calculating the shear stress of the ground stress along the natural fracture surface, the relative displacement of the natural fracture, and the slip displacement of the formation based on the formation-selective cementing section cement sheath-casing force analysis model.

[0016] Furthermore, the shear stress calculation method of the ground stress along the natural fracture surface is:

[0017] τ y =0.5(σ H +σ h )sin2θ

[0018] Where: τ y is the shear stress along the natural fracture surface, MPa; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum horizontal principal stress, MPa; θ is the angle between the maximum horizontal principal stress direction and the crack surface in the clockwise direction, degrees.

[0019] Furthermore, the relative displacement of natural fractures is calculated as follows:

[0020]

[0021] Where: Δu f is the relative displacement of the natural crack, m; τ yis the shear stress along the natural fracture surface, MPa; E is the Young's modulus of rock, MPa; υ is the Poisson's ratio of rock, dimensionless; a is the half length of the natural fracture, m; x is the distance from the wellbore to the center of the fracture, m.

[0022] Furthermore, the casing deformation parameters in step (2) include displacement data and stress data of the casing section.

[0023] Furthermore, the geometric relationship model in step (3) is:

[0024]

[0025]

[0026] Where: d case is the inner diameter of the casing; l case is the longitudinal height difference between the upper undeformed section and the lower deformed section of the casing; l no-cement is the length of the uncemented section; λ is the angle between the inclined section after casing deformation and the horizontal plane; ψ is the angle between the downhole tool and the inclined section after casing deformation; L is the length of the running tool; d is the outer diameter of the running tool.

[0027] Furthermore, the step (5) of evaluating the length of the uncemented section also includes:

[0028] The allowed downhole tool geometric dimensions in the running tool length-outer diameter chart are compared with the dimensions of the subsequent pre-running tool. If they are within the range of the chart curve, it indicates that the running tool can smoothly pass through the casing change section and the length of the uncemented section is reasonably set; if they are outside the range of the chart curve, it indicates that the running tool cannot pass through the casing change section and the length of the uncemented section is unreasonably set.

[0029] Furthermore, in step (1), ABAQUS software is used to establish a finite element model for stress analysis of the formation-selective cementing section cement sheath-casing.

[0030] Compared with the prior art, the beneficial effects of the present invention are as follows: the present invention provides a method for evaluating the length of the uncemented section for selective cementing to prevent casing change, which can establish the corresponding casing change displacement according to the relative displacement of the natural fractures in the formation and the actual situation of the wellbore, and then obtain the corresponding length-outer diameter plate of the downhole tool according to the preset uncemented section length, so as to quickly predict whether the downhole tool has the risk of getting stuck in the casing change section. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following is a brief introduction to the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0032] Figure 1 Schematic diagram of the natural fracture azimuth, dip, position in the formation model, and geostress orientation in one embodiment of the present invention.

[0033] Figure 2 Schematic diagram of a finite element model of a formation-selective cementing section cement sheath-casing established in one embodiment of the present invention.

[0034] Figure 3 Schematic diagram of a downhole tool getting stuck in a casing change section in one embodiment of the present invention.

[0035] Figure 4 This is a Mises stress cloud diagram of the casing after the casing is simulated in one embodiment of the present invention.

[0036] Figure 5 This is a displacement cloud diagram of the cross section of the casing where the maximum stress is generated after simulating casing deformation in one embodiment of the present invention.

[0037] Figure 6 This is a diagram showing the relationship between the length and outer diameter of a downhole tool when the sleeve variable section is stuck in one embodiment of the present invention. DETAILED DESCRIPTION

[0038] The details of the present invention can be more clearly understood by referring to the accompanying drawings and the description of the specific embodiments of the present invention. However, the specific embodiments of the present invention described herein are only for the purpose of explaining the present invention and are not to be construed as limiting the present invention in any way. Based on the teachings of the present invention, a skilled person can conceive of any possible variations based on the present invention, and such variations should be considered to fall within the scope of the present invention.

[0039] A method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation comprises the following steps:

[0040] (1) Obtain basic parameters and establish a cement sheath-casing stress analysis model for the formation-selective cementing section;

[0041] (2) obtaining the position where the maximum stress is generated through the formation-selective cementing section cement sheath-casing force analysis model, and extracting the casing deformation parameters at the maximum stress position;

[0042] (3) Establish a geometric relationship model between the running tool parameters at the maximum stress position and the casing deformation section;

[0043] (4) Obtaining a length-outer diameter diagram of the running tool of the casing deformation section through the geometric relationship model in step (3);

[0044] (5) Evaluate the length of the uncemented well section based on the downhole tool length-outer diameter chart and the size of the subsequent pre-downhole tools.

[0045] It is understandable that in the above steps, those skilled in the art can select corresponding basic parameters according to the factors that need to be considered in establishing the model. In one embodiment, the ground stress, maximum horizontal principal stress, minimum horizontal principal stress, vertical stress, Young's modulus of rock, Poisson's ratio of rock, natural fracture azimuth and inclination, natural fracture length, horizontal wellbore azimuth, horizontal open hole diameter, cement sheath outer diameter and wall thickness, and casing outer diameter and wall thickness of the shale formation in the target work area can be obtained as basic parameters for modeling.

[0046] After obtaining the basic parameters, the shear stress in the y direction (i.e., along the crack surface) and the relative displacement of the natural crack can be calculated.

[0047] Shear stress in the y direction of the ground stress:

[0048] τ y =0.5(σ H +σ h )sin2θ

[0049] Where: σ H is the maximum horizontal principal stress, MPa; σ h is the minimum horizontal principal stress, MPa; θ is the angle between the maximum horizontal principal stress direction and the crack surface in the clockwise direction, °;

[0050] The relative displacement of natural cracks (slip cracks) can be calculated as:

[0051]

[0052] Where: Δu f is the relative displacement of the natural fracture, m; E is the Young's modulus of the rock, MPa; υ is the Poisson's ratio of the rock, dimensionless; a is the half length of the natural fracture, m; x is the distance from the wellbore to the center of the fracture, m;

[0053] Based on the horizontal well cementing structural parameters of the target well area, a finite element model of the formation-selective cementing section cement sheath-casing is established. Those skilled in the art can select software in this field for modeling without any creative effort. In one embodiment, ABAQUS software can be selected for modeling, wherein the relative displacement of the natural fracture is set as the slip displacement of the formation, and numerical simulation is carried out. From the simulation results of ABAQUS software, displacement cloud maps, stress cloud maps, displacement data, and stress data of the casing section can be extracted.

[0054] According to Saint-Venant's principle, the size of the formation rock model should be no less than five to six times the wellbore size. In one embodiment, the formation rock model can be sized at 6m x 2m x 2m, divided into two sections, each configured as fixed and sliding. The horizontal wellbore is parallel to the direction of minimum horizontal principal stress. The outer wall of the cement sheath contacts the open hole wall at the left and right extremes of the model. The length of the uncemented section is set in the middle, without a cement sheath to simulate the uncemented section, and the inner wall of the cement sheath contacts the outer wall of the casing.

[0055] In the established finite element model, the azimuth and dip of the natural fracture are set as the cross-section angle between the two parts of the formation rock model; the azimuth of the natural fracture is β, and the dip is γ (such as Figure 1 shown);

[0056] In the established finite element model, the minimum horizontal principal stress direction is along the long side of the formation rock model, the maximum horizontal principal stress direction is along the short side of the formation rock model, and the vertical stress direction is along the perpendicular direction of the formation rock model. Pressure is applied to the inner wall of the casing to simulate the surface forces generated on the casing during construction. The contact surfaces between the formation rocks are normal rigid contact with a tangential friction coefficient of 0.4 and a tensile strength of 0. The contact surfaces between the formation rock and the cement sheath, and between the cement sheath and the casing, are normal rigid contact with a tangential friction coefficient of 0.7 and a tensile strength of 0. A predefined force field is set to apply in-situ stress conditions to the formation rock model. The left rock is set to be completely fixed, while the right rock is set to move in various directions based on the previously calculated fracture surface displacement values. The left cement sheath is set to be completely fixed, while the right cement sheath is unconstrained to simulate displacement caused by sliding along the fracture surface. The middle of the cement sheaths on both sides is free of cement, representing the annulus between the casing and the wellbore wall, simulating an uncemented section. No constraints are set on the casing.

[0057] Calculate the tool length-outer diameter chart when the downhole tool is stuck in the casing change section;

[0058] Find the location where the maximum stress is generated, extract the casing cross-sectional morphology at that location, and extract the coordinates and displacement of the casing inner wall at that location from the calculation results;

[0059] At the same time, according to the size and displacement of the sleeve change section, the downhole tool length-outer diameter chart allowed for the downhole tool to pass through the sleeve change section is calculated;

[0060] Among them, in the above steps, the downhole tool encounters a stuck schematic diagram in the casing deformation section as shown in the following figure: Figure 3 As shown in the figure, the angle between the deformed inclined section of the casing and the horizontal plane is λ, and the angle between the downhole tool and the deformed inclined section of the casing is ψ. Then the angle between the downhole tool and the horizontal plane is λ-ψ, the length of the downhole tool is L, and the outer diameter is d; the contact point between the left side of the downhole tool and the undeformed section of the upper part of the casing is s1, and the lower left part of the downhole tool is s2. The perpendicular line through point s1 intersects with the downhole tool at s3, and intersects with the horizontal plane at s4; the intersection point of the downhole tool and the lower bending point of the casing is t1, the intersection point of the downhole tool and the upper bending point of the casing is t2, the contact point of the right side of the downhole tool with the lower horizontal section of the casing is t3, and the lower bending point of the casing is t4. The line connecting points t2 and t4 intersects with the lowest part of the downhole tool at point t5, and the perpendicular line through point t2 intersects with the lower part of the downhole tool at t6.

[0061] According to the casing displacement value and model parameters in the simulation results, the geometric relationship between L and d can be obtained:

[0062]

[0063] Among them, the geometric relationship between d and ψ is:

[0064]

[0065] Where: d case is the inner diameter of the casing, l case is the longitudinal height difference between the upper undeformed section and the lower deformed section of the casing, l no-cement is the length of the uncemented section;

[0066] Based on the above geometric relationship, a chart showing the length L and outer diameter d of the downhole tool allowed to pass through the sleeve change section is obtained;

[0067] Furthermore, the length of the uncemented section is evaluated based on the tool length-outer diameter chart and the size of the subsequently pre-run downhole tool;

[0068] Compare the allowed downhole tool geometry in the drawing with the size of the downhole tool to be subsequently run;

[0069] If it is within the curve on the chart, it can be judged that the sleeve change does not affect the subsequent construction, and the downhole tools can pass through the sleeve change section smoothly, which is reasonable;

[0070] If it is outside the curve on the chart, the length of the uncemented section can be used to determine the impact of casing change on subsequent construction. The downhole tools cannot pass through the casing change section, and the length of the uncemented section is unreasonable.

[0071] It should be noted that the geometric relationship model for the relative displacement of natural fractures, shear stress in the y-direction of geostress, running tool parameters, and casing deformation section described in this specification is merely an example provided to facilitate understanding of the technical solution of the present invention and does not constitute a limitation on the technical concept of the present invention. All other embodiments obtained by persons of ordinary skill in the art without creative effort, such as the use of other calculation methods to establish corresponding force analysis models and casing deformation geometric relationship models, should also fall within the scope of protection of the present invention.

[0072] In addition, in order to facilitate the understanding and application of the present invention by those skilled in the art, the present invention takes a shale gas fracturing well area as an example to illustrate the corresponding steps.

[0073] First, obtain the following basic parameters of the well:

[0074]

[0075] Shear stress τ in the y direction of the ground stress y =6.93MPa;

[0076] The relative displacement of natural cracks Δu can be calculated f =0.084m;

[0077] A finite element model of the formation-selective cementing section cement sheath-casing was established in ABAQUS software. The dimensions of the model components are as follows:

[0078]

[0079] The length of the uncemented section is set to 2m; the ground stress condition is set in the predefined force field, the construction pressure is set on the inner wall of the casing, the contact surface between the formation rock is normal rigid contact, the tangential friction coefficient is 0.4, and the tensile strength is 0; the contact surface between the formation rock and the cement sheath, and between the cement sheath and the casing is normal rigid contact, the tangential friction coefficient is 0.7, and the tensile strength is 0; the left rock is set to be completely fixed, and the vertical displacement value of the right rock is set to 0.09m; the left cement sheath is set to be completely fixed, and no constraint conditions are set for the right cement sheath; no constraint conditions are set for the casing; the surface where the four points ABCD are located is the natural fracture surface ( Figure 2 ). Carry out numerical simulation to calculate stress and displacement;

[0080] In the calculation results, extract the Mises stress cloud diagram of the casing after calculation ( Figure 4 ), intercept the displacement cloud diagram of the casing section at that location ( Figure 5), the coordinate values of the elliptical ring on the casing section are obtained. According to the coordinates of the four vertices of the major axis and minor axis of the ellipse, it can be calculated that the maximum displacement value of the casing at this location is 0.3mm, and the inner diameter of the casing at the minor axis is 0.013964m, indicating that the degree of casing deformation at this location is very small.

[0081] The coordinates of the four vertices of the major axis and minor axis of the elliptical ring on the inner wall of the casing section are as follows:

[0082]

[0083] In the calculation results, the displacement of the casing and the angle of the casing change section are extracted at the same time, and the relationship between the downhole tool length and outer diameter when the casing is stuck in the casing change section is calculated ( Figure 6 );

[0084] Taking the commonly used drillable bridge plug subsequently installed in this block as an example, its outer diameter is 0.114 mm and its length is 0.5 to 0.6 m. The corresponding points of the dimensions on the plate are within the curve. Therefore, it can be judged that the casing change will not affect subsequent construction, and the set uncemented length value of 2 m is reasonable.

[0085] Although the specific embodiments of the present invention are described in detail in conjunction with the accompanying drawings, this should not be construed as limiting the scope of protection of this patent. Within the scope described by the claims, various modifications and variations that can be made by those skilled in the art without creative work still fall within the scope of protection of this patent.

Claims

1. A method for evaluating the length of an uncemented section in selective cementing to prevent casing deformation, comprising: (1) Obtain basic parameters and establish a stress analysis model for the formation-selective cementing section cement sheath-casing; (2) obtaining the position where the maximum stress is generated through the formation-selective cementing section cement sheath-casing force analysis model, and extracting the casing deformation parameters at the maximum stress position; (3) Establish a geometric relationship model between the running tool parameters at the maximum stress position and the casing deformation section; The geometric relationship model is: ； ； Where: d case is the inner diameter of the casing; l case The vertical height difference between the upper undeformed section and the lower deformed section of the casing; l no-cement is the length of the uncemented section; λ is the angle between the inclined section of the casing and the horizontal plane after deformation; ψ is the angle between the downhole tool and the tilted section of the casing after deformation; L is the length of the running tool; d is the outer diameter of the running tool; (4) Obtaining the length-outer diameter diagram of the running tool of the casing deformation section through the geometric relationship model in step (3); (5) Evaluate the length of the uncemented section based on the tool length-outer diameter chart and the size of the subsequent pre-run downhole tool; The step (5) of evaluating the length of the uncemented section also includes: Compare the downhole tool geometry allowed in the tool length-outer diameter chart with the subsequent pre-running tool dimensions. If it is within the range of the curve on the chart, it means that the running tool can pass through the casing change section smoothly, and the length of the non-cementing section is set reasonably; If it is outside the range of the plate curve, it means that the running tool cannot pass through the casing change section and the length of the non-cementing section is set unreasonably.

2. The method for evaluating the length of the uncemented section for selective cementing to prevent casing deformation as claimed in claim 1, wherein the basic parameters in step (1) include ground stress parameters, rock mechanics parameters, natural fracture geometry parameters, logging data, and the length of the uncemented section.

3. The method for evaluating the length of the uncemented section for selective cementing to prevent casing deformation as described in claim 2, wherein the geostress parameters include the maximum horizontal principal stress, the minimum horizontal principal stress, and the vertical stress; the rock mechanics parameters include the Young's modulus of the rock and the Poisson's ratio of the rock; the natural fracture geometric parameters include the azimuth, inclination, and length of the natural fracture; and the logging data include the horizontal wellbore azimuth, horizontal open hole diameter, cement sheath outer diameter and wall thickness, casing outer diameter and wall thickness, and the distance between the wellbore and the center of the fracture length.

4. The method for evaluating the length of the non-cemented section for preventing casing deformation during selective cementing according to claim 1, wherein the step (1) further comprises: Based on the formation-selective cementing section cement sheath-casing stress analysis model, the shear stress along the natural fracture surface, the relative displacement of the natural fracture, and the slip displacement of the formation are calculated.

5. The method for evaluating the length of the uncemented section for selective cementing to prevent casing deformation according to claim 4, wherein the shear stress along the natural fracture plane is calculated as follows: ； Where: τ y is the shear stress along the natural fracture surface, MPa; σ H is the maximum horizontal principal stress, MPa; σ h is the minimum horizontal principal stress, MPa; θ is the angle between the maximum horizontal principal stress direction and the crack surface in the clockwise direction, °.

6. The method for evaluating the length of the non-cemented section for selective cementing to prevent casing deformation according to claim 4, wherein the relative displacement of the natural fracture is calculated as follows: ； Where: Δu f is the relative displacement of the natural fracture, m; τ y is the shear stress along the natural fracture surface, MPa; E is the Young's modulus of rock, MPa; υ is the Poisson's ratio of rock, dimensionless; a is the half length of natural crack, m; x is the distance from the wellbore to the center of the fracture, m.

7. The method for evaluating the length of an uncemented section for selective cementing to prevent casing deformation according to claim 1, wherein the casing deformation parameters in step (2) include displacement data and stress data of the casing section.

8. The method for evaluating the length of the non-cemented section for preventing casing deformation in selective cementing according to claim 1, wherein in step (1), a finite element model for stress analysis of the formation-selective cementing section cement sheath-casing is established using ABAQUS software.

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