A method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures.

Abstract

This invention discloses a method for predicting the initiation and propagation characteristics of fracturing fractures, comprising: designing a fracturing tool assembly that meets the requirements of integrated well construction, wherein the fracturing tool assembly includes a casing assembly and a cement sheath, with an annular gap formed between the outer wall of the cement sheath and the wellbore wall; establishing a single-point fracturing model for the current well section based on the structural parameters of the fracturing tool assembly; using the single-point fracturing model to analyze the stress of the sliding sleeve in the casing assembly at the wellbore wall, and converting the wellbore wall stress into cement sheath stress and formation stress of the corresponding well section, to obtain the first initiation characteristics of the cement sheath fracture and the second initiation characteristics of the formation fracture formed by the extension of the cement sheath fracture, and obtaining the stress distribution characteristics of the annular gap; based on the annular gap stress distribution characteristics, diagnosing the possibility of the cement sheath fracture forming a formation fracture, and predicting the propagation characteristics of the formed formation fracture if it is possible to form one. This method achieves accurate prediction of the initiation and propagation characteristics of fracturing fractures in integrated well construction.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas field development, and particularly relates to a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures. Background Technology

[0002] Currently, hydraulic fracturing is a crucial measure for increasing reserves, reducing costs, and improving efficiency in tight reservoirs. Integrated well construction fracturing schemes utilize a multi-stage fracturing technology with a stationary tubing string, employing a ball-dropped sliding sleeve system. This approach offers unique advantages such as simple construction processes, high fracturing efficiency, and a large tubing diameter after fracturing. Commonly used sliding sleeve sizes for integrated well construction are type 73 (6 20×20mm guide holes), type 89 (6 40×20mm guide holes), and type 101 (8 40×20mm guide holes).

[0003] The fracturing fractures obtained using an integrated well construction scheme have the characteristic of initiating fractures at the cement sheath-formation dual interface, making the fracture trajectory difficult to predict. Simultaneously, during well construction, the superposition of fracture-induced stress and in-situ stress field creates a composite stress field, altering the magnitude and direction of in-situ stress near the cement sheath, thus changing the stress in the cement sheath and forming micro-annular gaps or micro-fractures. Furthermore, perforation causes localized damage to the wellbore, particularly leading to cement sheath destruction, cement sheath separation at the first interface (casing-cement sheath interface) and the second interface (cement sheath-surrounding rock interface), and resulting in micro-annular gaps. In later stages of shale gas horizontal well volumetric fracturing, the injection of high-pressure fluid will exacerbate the impact of interfacial micro-annular gaps on fracture propagation. Specifically, when all three-dimensional stresses (radial, circumferential, and axial) are under tension, the stress failure criterion is cement sheath tensile failure; when the three-dimensional stresses are under a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, the stress failure criterion is cement sheath-formation interface tensile-compressive failure. Summary of the Invention

[0004] To address the aforementioned problems, this invention proposes a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures. The method includes: designing a hydraulic fracturing tool assembly that meets the requirements of integrated well construction, wherein the hydraulic fracturing tool assembly includes a casing assembly and a cement sheath, with an annular gap formed between the outer wall of the cement sheath and the wellbore wall; establishing a single-point fracturing model for the current well section based on the structural parameters of the hydraulic fracturing tool assembly; using the single-point fracturing model to analyze the stress of the sliding sleeve at the wellbore wall in the casing assembly, and converting the wellbore wall stress into cement sheath stress and formation stress in the corresponding well section, to obtain the first initiation characteristics of the cement sheath fracture and the second initiation characteristics of the formation fracture formed by the extension of the cement sheath fracture, and obtaining the stress distribution characteristics of the annular gap; based on the annular gap stress distribution characteristics, diagnosing the likelihood of the cement sheath fracture forming the formation fracture, and predicting the propagation characteristics of the formed formation fracture if it is likely to form.

[0005] Preferably, the step of obtaining the first crack initiation characteristic includes: calculating a first characteristic parameter threshold characterizing cement sheath failure by analyzing the stress state of the cement sheath, thereby obtaining a first crack initiation pressure. If all three axial stresses of the cement sheath are in a tensile stress state, the range of values ​​for the maximum principal stress satisfying the first relationship is taken as the first threshold in the first characteristic parameter threshold. The first relationship characterizes the correlation between the maximum principal stress in the cement sheath stress and the tensile strength of the cement sheath. The first relationship is expressed using the following expression:

[0006] σ1≥σ t

[0007] Where σ1 represents the maximum principal stress of the cement ring, σ t Indicates the tensile strength of the cement ring.

[0008] If the triaxial stress of the cement ring is in a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, then the range of values ​​of the minimum principal stress that satisfies the second relationship is taken as the second threshold in the first characteristic parameter threshold. The second relationship characterizes the correlation between the minimum principal stress in the cement ring stress and the compressive strength of the cement ring, wherein the second relationship is expressed by the following expression:

[0009]

[0010] Where σ3 represents the minimum principal stress of the cement ring, σ c This indicates the compressive strength of the cement ring.

[0011] Preferably, the step of obtaining the second fracturing characteristic includes: calculating a second characteristic parameter threshold characterizing formation fracturing based on the maximum tensile stress criterion, thereby obtaining the second fracturing pressure, wherein the range of values ​​of the minimum principal stress satisfying the third relationship is used as the second characteristic parameter threshold, the third relationship characterizing the correlation between the minimum principal stress in the formation stress and the tensile strength of the formation rock, wherein the third relationship is expressed by the following expression:

[0012] σ3'-α·p p ≤-σ t '

[0013] Where σ3' represents the minimum principal stress of the formation, σ t According to p, which represents the tensile strength of rock p α represents the formation pressure, and α represents the Biot elastic coefficient.

[0014] Preferably, the method further utilizes Mohr's circle stress analysis to obtain the crack initiation angle characteristic parameter in the second crack initiation characteristic.

[0015] Preferably, the step of obtaining the stress distribution characteristics of the annular gap includes: performing a stress analysis on the annular gap when the cement sheath crack extends to the annular gap, thereby determining the stress state corresponding to the annular gap based on the analysis results, calculating a first stress parameter on the wall surface of the annular gap based on this, and converting the first stress parameter into the stress distribution characteristics of the annular gap using the following expression:

[0016]

[0017]

[0018]

[0019] Where β represents the approach angle between the cement annulus crack and the annulus gap, σ βx σ βy K represents the normal stress components along the x-axis and y-axis of the annulus wall in a coordinate system related to the approximation angle, respectively. I Let τ represent the stress intensity factor, r represent the distance from any point on the wall of the annulus to the tip of the cement sheath crack, θ' represent the angle between the line connecting any point on the wall of the annulus to the tip of the cement sheath crack and the direction of the maximum horizontal principal stress of the formation, and τ represent the stress intensity factor. β σ represents the shear stress component of the annular gap wall in a coordinate system related to the approximation angle. H σ represents the maximum horizontal principal stress of the formation. h This represents the minimum horizontal principal stress of the formation.

[0020] Preferably, the step of diagnosing the possibility of the cement sheath crack forming the formation fracture based on the annular stress distribution characteristics includes: determining whether the annular crack has shear slip characteristics based on the shear stress distribution characteristics in the annular stress distribution characteristics, and, if it is determined that it does not have shear slip characteristics, diagnosing the possibility of the cement sheath crack extending to penetrate the annular crack in combination with the tensile strength of the rock in the corresponding formation, wherein, if it can penetrate, determining whether the annular crack has undergone shear failure based on the critical characteristics of the annular crack when shear failure occurs, wherein, if shear failure does not occur, determining that the cement sheath crack can form the formation fracture.

[0021] Preferably, the step of determining whether the annular gap has undergone shear failure includes: determining the shear failure state of the annular gap based on the critical characteristics of the annular gap when it is about to undergo shear failure, so as to determine whether the annular gap has undergone shear failure, wherein the condition satisfied to achieve the critical characteristics is represented by the following expression:

[0022] |τ β |=s0-μσ βy

[0023] Where, τ β σ represents the shear stress component of the annular gap wall in a coordinate system related to the approximation angle, s0 represents the cohesion of the annular gap wall, μ represents the friction coefficient of the annular gap wall, and σ represents the shear stress component of the annular gap wall. βy This represents the normal stress component of the wall of the annulus along the y-axis in a coordinate system related to the approximation angle.

[0024] Preferably, the method further includes: utilizing the distribution characteristics of normal stress in the annular stress distribution characteristics, combined with the pressure parameters of the fluid inside the annular crack, to calculate the opening characteristics of the annular crack, so as to obtain the propagation characteristics of the annular crack formed by the cement annulus crack:

[0025]

[0026] Where w represents the opening width of the annular crack, ν represents Poisson's ratio, and σ h E represents the minimum horizontal principal stress of the formation, and H represents Young's modulus. f p represents the height of the annular gap. net σ represents the original formation pressure. βy This represents the normal stress component of the wall of the annular gap along the y-axis in a coordinate system related to the approximation angle.

[0027] Preferably, in the step of predicting the propagation characteristics of the formed formation fracture, a first propagation characteristic characterizing the width of the formation fracture is obtained using the following expression:

[0028]

[0029] in, E' represents the average width of the fracture in segment k when the perforation fracture at point i in segment j extends to segment n1. K represents the elastic modulus of the rock. IC This indicates the type I fracture toughness of the rock. This represents the length of a single wing when the perforation fracture at the i-th segment of the j-th section of the inclined shaft extends to the n1-th segment. This represents the length of the fracture extension in the preceding m-th segment when the perforation fracture at the i-th segment of the j-th well section extends to the n1-th segment.

[0030] Preferably, the method further utilizes the following expression to obtain a second extended characteristic characterizing the intra-fracture frictional pressure drop of the formation fracture:

[0031]

[0032] in, This represents the total frictional pressure drop of the fracture when the perforation fracture at the i-th point in the j-th section of the deviated well extends to the n1-th section, where n represents the fracturing fluid flow index. This represents the length of segment k when the perforation fracture at point i in segment j of the deviated well extends to segment n1. This represents the average width of the fracture in the k-th segment when the perforation fracture at the i-th location in the j-th segment of the deviated well extends to the n1-th segment. H represents the flow rate of the perforation fracture at the i-th perforation point in the j-th section of the deviated well as it extends to the n1-th section. f Indicates the crack height, K f This represents the consistency coefficient of the power-law type fracturing fluid within the fracture.

[0033] Preferably, the method further utilizes the following expression to obtain a third propagation feature characterizing the total propagation time of the formation fractures:

[0034]

[0035] Where c represents the filtration coefficient, H f Q represents the height of the annular gap. f1 ΔL represents the displacement, ΔL represents the length of the extended fracture, t represents the extension time, n2 represents the well section number, and t(n2) represents the total extension time from the initial fracture in the formation to the n2th section.

[0036] Compared with the prior art, one or more embodiments of the above solutions may have the following advantages or beneficial effects:

[0037] This invention provides a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures. The method designs a fracturing tool assembly that meets the requirements of integrated well construction, wherein the fracturing tool assembly includes a casing assembly and a cement sheath, with an annular gap formed between the outer wall of the cement sheath and the wellbore. Then, based on the structural parameters of the fracturing tool assembly, a single-point fracturing model for the current well section is established. Next, using the single-point fracturing model, combined with the initiation criteria of the cement sheath and the formation, the initiation characteristics of the cement sheath fracture and the formation fractures formed by the extension of the cement sheath fracture are obtained, along with the stress distribution characteristics of the annular gap. Finally, based on the annular gap stress distribution characteristics, the likelihood of the cement sheath fracture forming a formation fracture is diagnosed, and if formation fractures are likely to form, the propagation characteristics of the resulting formation fractures are predicted. This invention achieves accurate prediction of the initiation and propagation characteristics of hydraulic fracturing fractures under integrated well construction methods, and simultaneously reveals the initiation and propagation laws of hydraulic fracturing fractures formed by sliding sleeve single-point fracturing.

[0038] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of the structures particularly pointed out in the description, claims and drawings. Attached Figure Description

[0039] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with the embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0040] Figure 1 This is a step diagram of a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures according to an embodiment of this application.

[0041] Figure 2 This is an example diagram of a single-point fracturing model in a method for predicting the initiation and propagation characteristics of fracturing fractures according to an embodiment of this application.

[0042] Figure 3 This is an example diagram illustrating the pressure variation state of initiation and propagation in single-point hydraulic fracturing and perforation fracturing in the method for predicting the initiation and propagation characteristics of hydraulic fracturing in embodiments of this application.

[0043] Figure 4 This is an example diagram illustrating the changes in the propagation trajectory of sliding sleeve single-point fracturing and perforation fracturing in a method for predicting the initiation and propagation characteristics of fracturing fractures according to embodiments of this application.

[0044] Figure 5 This is an example diagram illustrating the variation of the propagation width in single-point hydraulic fracturing and perforation fracturing in a method for predicting the initiation and propagation characteristics of hydraulic fracturing according to an embodiment of this application.

[0045] Figure 6 This is an example diagram illustrating the influence of cementing quality on the propagation pressure of a single-point fracturing fracture in a sliding sleeve, as described in the method for predicting the initiation and propagation characteristics of fracturing fractures according to an embodiment of this application.

[0046] Figure 7 This is an example diagram illustrating the effect of displacement on the propagation trajectory of a single-point hydraulic fracturing crack in a sliding sleeve, in a method for predicting the initiation and propagation characteristics of hydraulic fracturing cracks according to an embodiment of this application.

[0047] Figure 8 This is an example diagram illustrating the effect of viscosity on the propagation trajectory of a single-point hydraulic fracturing crack in a sliding sleeve, in a method for predicting the initiation and propagation characteristics of hydraulic fracturing cracks according to an embodiment of this application.

[0048] Figure 9 This is an example diagram illustrating the effect of displacement on the width of a single-point hydraulic fracturing crack in a sliding sleeve, in a method for predicting the initiation and propagation characteristics of hydraulic fracturing cracks according to an embodiment of this application. Detailed Implementation

[0049] The embodiments of the present invention will be described in detail below with reference to the accompanying drawings and examples, so that the process of how the present invention uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly. It should be noted that, as long as there is no conflict, the various embodiments and features in the various embodiments of the present invention can be combined with each other, and the resulting technical solutions are all within the protection scope of the present invention.

[0050] Furthermore, the steps shown in the flowchart in the accompanying drawings can be executed in a computer system such as a set of computer-executable instructions, and although a logical order is shown in the flowchart, in some cases the steps shown or described may be executed in a different order than that shown here.

[0051] Currently, hydraulic fracturing is a crucial measure for increasing reserves, reducing costs, and improving efficiency in tight reservoirs. Integrated well construction fracturing schemes utilize a multi-stage fracturing technology with a stationary tubing string, employing a ball-dropped sliding sleeve system. This approach offers unique advantages such as simple construction processes, high fracturing efficiency, and a large tubing diameter after fracturing. Commonly used sliding sleeve sizes for integrated well construction are type 73 (6 20×20mm guide holes), type 89 (6 40×20mm guide holes), and type 101 (8 40×20mm guide holes).

[0052] The fracturing fractures obtained using an integrated well construction scheme have the characteristic of initiating fractures at the cement sheath-formation dual interface, making the fracture trajectory difficult to predict. Simultaneously, during well construction, the superposition of fracture-induced stress and in-situ stress field creates a composite stress field, altering the magnitude and direction of in-situ stress near the cement sheath, thus changing the stress in the cement sheath and forming micro-annular gaps or micro-fractures. Furthermore, perforation causes localized damage to the wellbore, particularly leading to cement sheath destruction, cement sheath separation at the first interface (casing-cement sheath interface) and the second interface (cement sheath-surrounding rock interface), and resulting in micro-annular gaps. In later stages of shale gas horizontal well volumetric fracturing, the injection of high-pressure fluid will exacerbate the impact of interfacial micro-annular gaps on fracture propagation. Specifically, when all three-dimensional stresses (radial, circumferential, and axial) are under tension, the stress failure criterion is cement sheath tensile failure; when the three-dimensional stresses are under a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, the stress failure criterion is cement sheath-formation interface tensile-compressive failure.

[0053] Therefore, to address the problem of inaccurate prediction of the trajectory and morphology of fracturing fractures formed under integrated well construction schemes, this invention proposes a method for predicting the initiation and propagation characteristics of fracturing fractures. This method studies the initiation and propagation characteristics of fracturing fractures under integrated well construction methods. It designs fracturing tool assemblies that meet the requirements of integrated well construction and further establishes a single-point fracturing model to simulate fracturing fractures under integrated well construction based on the structural parameters of the fracturing tool assembly. Then, it combines the initiation criteria of the cement sheath in the fracturing tool assembly and the initiation criteria of the formation to which the current well section belongs with the single-point fracturing model to obtain the initiation characteristics of the cement sheath fracture and the formation fractures formed by the cement sheath fracture. Finally, based on the stress distribution characteristics of the annulus, it diagnoses the possibility of the cement sheath fracture forming formation fractures and predicts the propagation characteristics of the formed formation fractures if they can form. This invention achieves accurate prediction of the initiation and propagation characteristics of fracturing fractures under integrated well construction methods and explores the initiation and propagation laws of fracturing fractures formed by sliding sleeve single-point fracturing based on the prediction results. Meanwhile, the results of the investigation into the initiation and propagation patterns of fracturing fractures provide technical support for the transformation of tight reservoirs using integrated well construction and fracturing technology.

[0054] Figure 1 This is a step diagram illustrating a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures according to an embodiment of this application. The following refers to... Figure 1 This will explain the steps of this method.

[0055] like Figure 1 As shown, in step S110, a fracturing tool assembly that meets the requirements of integrated well construction is designed. The fracturing tool assembly includes a casing assembly and a cement sheath. An annular gap is formed between the outer wall of the cement sheath and the wellbore. The integrated well construction fracturing process is as follows: First, the full-bore continuously variable sliding sleeve is opened by inserting a key; then, fracturing fluid is injected through the sliding sleeve guide hole to penetrate the cement sheath using a pump; finally, the fracturing fractures initiate and propagate in the formation, thereby forming extended fractures that connect the reservoir. Based on this, a fracturing tool assembly that meets the requirements of integrated well construction fracturing process is designed. In the fracturing tool assembly, the outer wall of the casing assembly is adjacent to the inner wall of the cement sheath, and the outer wall of the cement sheath is adjacent to the wellbore and forms an annular gap. In this embodiment, the casing assembly includes tubing and a sliding sleeve, with the sliding sleeve guide hole located at the center of the outer wall of the sliding sleeve.

[0056] Based on the fracturing tool assembly in step S110, in step S120, a single-point fracturing model for the current well section is established according to the structural parameters of the fracturing tool assembly. Figure 2This is an example diagram of a single-point fracturing model in a method for predicting the initiation and propagation characteristics of hydraulic fractures according to an embodiment of this application. Specifically, using the dimensional parameters of the fracturing tools (tubing, sliding sleeve, and cement sheath) in the fracturing tool assembly, as well as the dimensional parameters of the wellbore, a physical model is established to predict the initiation characteristics of single-point fracturing of the sliding sleeve in integrated well construction in the current well section, thereby obtaining the single-point fracturing model for the current well section. (Refer to...) Figure 2 R i R represents the inner diameter of the oil pipe. o R represents the outer diameter of the oil pipe. w This indicates the radius of the well wall.

[0057] In the embodiments of this application, the extended crack is formed by the initiation and extension of the cement sheath crack. The extension path of the extended crack is characterized as follows: First, the cement sheath crack initiates and forms a cement sheath crack, at which point the cement sheath fails; then, when the cement sheath crack penetrates the cement sheath, the inner wall of the annulus cracks and forms an annulus crack; finally, when the cement sheath crack extends to penetrate the annulus, the formation where the wellbore is located initiates at the wellbore and forms a formation fracture in the formation.

[0058] Furthermore, based on the extension path characteristics of the propagating fracture, after obtaining the single-point fracturing model for the current well section, in step S130, the stress of the sliding sleeve at the wellbore wall in the casing assembly is analyzed using the single-point fracturing model. The wellbore wall stress is then converted into cement sheath stress and formation stress in the corresponding well section to obtain the first initiation characteristics of the cement sheath fracture and the second initiation characteristics of the formation fracture formed by the extension of the cement sheath fracture, and to obtain the stress distribution characteristics of the annulus. Specifically, since the distribution of the total stress of the sliding sleeve at the wellbore wall differs before and after the start of fracturing operations, it is necessary to re-analyze the stress distribution characteristics of the sliding sleeve at the wellbore wall in the casing assembly after the start of fracturing operations to obtain the wellbore stress characterizing the total stress distribution characteristics of the sliding sleeve at the wellbore wall after the start of fracturing operations. Then, the wellbore stress is converted into cement sheath stress to obtain the first initiation characteristics of the cement sheath fracture, and the wellbore stress is converted into formation pressure in the corresponding well section to obtain the second initiation characteristics of the formation fracture formed by the extension of the cement sheath fracture. Finally, the stress distribution characteristics of the annulus were obtained when the cement annulus crack extended to interact with the annulus gap.

[0059] First, the process of obtaining wellbore stress in the embodiments of this application is described in detail. Specifically, a single-point fracturing model is used to simulate multiple stress components of the sliding sleeve at the wellbore in the casing assembly, obtaining the distribution characteristics of each stress component of the sliding sleeve at the wellbore. The combination of multiple stress components is the wellbore stress. The following expression is used to calculate each stress component of the sliding sleeve at the wellbore to obtain the distribution characteristics of each stress component of the sliding sleeve at the wellbore:

[0060]

[0061] Where, σ rp p represents the radial stress in the sleeve hole. wf R represents the wellbore pressure, α represents the Biot elastic coefficient, ν represents Poisson's ratio, R(t) represents the excitation radius, t represents the injection time, and r represents the wellbore pressure. w Let r' represent the radius of the sliding sleeve orifice, r' represent the radial distance from any point in the formation to the sliding sleeve orifice, p represent the pore pressure, and σ represent the pore pressure. θp This indicates the circumferential stress in the sleeve hole. σ represents the stress in the x and y directions of the horizontal wellbore coordinate system, respectively, which are transformed from the in-situ stress. z θ represents the stress in the z-direction of the horizontal wellbore coordinate system. * θ represents the angle between the axis of the horizontal wellbore and the radial direction of the sliding sleeve orifice, and θ represents the angle between the circumferential direction of the horizontal wellbore and the axis of the sliding sleeve orifice. τ represents the shear stress in the xy direction of the horizontal wellbore coordinate system, which is the stress transformed from the in-situ stress. θz p represents the shear stress on the cross section formed by the circumferential direction of the horizontal wellbore and the z-direction in the horizontal wellbore coordinate system. w p represents the bottom hole pressure. p σ represents formation pressure. zp This represents the normal stress in the z-direction of the sliding sleeve orifice in the horizontal wellbore coordinate system. σ represents the radial normal stress in a horizontal wellbore. θ τ represents the circumferential normal stress in the horizontal wellbore coordinate system. θzp τ represents the shear stress on the cross section formed by the circumferential direction of the sliding sleeve orifice and the z-direction in the horizontal wellbore coordinate system. Rz τ represents the shear stress on the cross section formed by the radial direction of the horizontal wellbore and the z-direction in the horizontal wellbore coordinate system. Rθ τ represents the shear stress on the cross section formed by the radial and circumferential directions of the horizontal wellbore. rθp τ represents the shear stress on the cross section formed by the radial and circumferential directions of the sleeve hole. rzp This represents the shear stress on the cross section formed by the radial direction of the sliding sleeve orifice and the z-direction in the horizontal wellbore coordinate system.

[0062] Next, using the corresponding stress components in the wellbore stress, the maximum and minimum principal stresses that cause crack initiation in the cement sheath are calculated. Then, the maximum principal stress causing crack initiation is combined with the tensile properties of the cement sheath itself to obtain the corresponding crack initiation characteristics. Similarly, the minimum principal stress causing crack initiation is combined with the compressive properties of the cement sheath itself to obtain the corresponding crack initiation characteristics. The maximum and minimum principal stresses that cause crack initiation in the cement sheath are calculated using the following expressions:

[0063] σ1=σ rp (2)

[0064]

[0065] Where σ1 represents the maximum principal stress of the cement ring, and σ3 represents the minimum principal stress of the cement ring.

[0066] Furthermore, by analyzing the stress state of the cement sheath, a first characteristic parameter threshold characterizing cement sheath failure is calculated, thereby obtaining the first initiation pressure. During fracturing operations, the cement sheath experiences triaxial stresses, including radial, circumferential, and axial stresses. In this embodiment, by analyzing the tensile and compressive states of each of the triaxial stresses, a first characteristic parameter threshold characterizing cement sheath failure is calculated for different stress state combinations: all three stresses are in a tensile state, the three stresses are in a tensile-compressive-compressive stress state, and the three stresses are in a tensile-tensile-compressive stress state. Based on this, the corresponding initiation characteristics of the cement sheath are determined for different stress state combinations, thereby obtaining the first initiation pressure in the first initiation characteristics of the cement sheath crack.

[0067] Next, the calculation of the first characteristic parameter threshold characterizing cement ring failure and the determination of the first crack initiation pressure in the embodiments of this application will be described in detail.

[0068] In a specific embodiment of this application, if the triaxial stress of the cement ring is under tensile stress, the range of values ​​for the maximum principal stress satisfying the first relationship is used as the first threshold value in the first characteristic parameter threshold. The first relationship characterizes the correlation between the maximum principal stress in the cement ring stress and the tensile strength of the cement ring. First, according to the crack initiation rule of the cement ring, under the condition that the triaxial stress of the cement ring is under tensile stress, in order for the cement ring to have a cracking state, it is necessary to ensure that there is a first relationship between the maximum principal stress in the cement ring stress and the tensile strength of the cement ring. Then, the range of values ​​for the maximum principal stress satisfying the first relationship is calculated, and the calculation result is used as the first threshold value in the first characteristic parameter threshold. Specifically, if the triaxial stress of the cement ring is under tensile stress, the tensile strength parameter of the cement ring is substituted into the first relationship to calculate the range of values ​​for the maximum principal stress in the cement ring stress, thereby obtaining the first threshold value in the first characteristic parameter threshold. Further analysis is performed on the two endpoints of the range corresponding to the first threshold value, and the minimum value of the maximum principal stress in the cement ring stress is determined as the first crack initiation pressure. The first relation is represented by the following expression:

[0069] σ1≥σ t (4)

[0070] Where, σ t This indicates the tensile strength of the cement ring.

[0071] In another specific embodiment of this application, if the triaxial stress of the cement ring is in a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, the range of values ​​of the minimum principal stress satisfying the second relationship is used as the second threshold in the first characteristic parameter threshold. The second relationship characterizes the correlation between the minimum principal stress in the cement ring stress and the compressive strength of the cement ring. First, according to the crack initiation rule of the cement ring, under the condition that the triaxial stress of the cement ring is in a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, in order for the cement ring to have a cracking state, it is necessary to ensure that there is a second relationship between the minimum principal stress in the cement ring stress and the compressive strength of the cement ring. Then, the range of values ​​of the minimum principal stress satisfying the second relationship is calculated, and the calculation result is used as the second threshold in the first characteristic parameter threshold. Specifically, if the triaxial stress of the cement ring is in a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, the compressive strength parameter of the cement ring is substituted into the second relationship to calculate the range of values ​​of the minimum principal stress in the cement ring stress, thereby obtaining the second threshold in the first characteristic parameter threshold. Further analysis of the two endpoints of the range corresponding to the second threshold reveals that the maximum value of the minimum principal stress in the cement sheath stress is determined as the first crack initiation pressure. The second relationship is expressed by the following expression:

[0072]

[0073] Where, σ c This indicates the compressive strength of the cement ring.

[0074] In actual construction, the initiation pressure of formation fractures is usually predicted based on the tensile failure state of the formation. That is, in the embodiments of this application, once the maximum principal stress component at any position on the annulus is greater than the tensile strength of the rock (such as dense sandstone) in the formation, the formation will begin to fracture.

[0075] Next, the minimum principal stress required to initiate formation fractures is calculated using the corresponding stress components in the wellbore stress. Then, this minimum principal stress is combined with the tensile properties of the formation rock itself to obtain the corresponding fracture initiation characteristics. It should be noted that, in this embodiment, the calculation method for the minimum principal stress to initiate formation fractures is similar to that for initiating cement sheath fractures, and therefore will not be repeated here.

[0076] Furthermore, based on the maximum tensile stress criterion, a second characteristic parameter threshold characterizing formation cracking is calculated to obtain the second initiation pressure. In this embodiment, firstly, based on the maximum tensile stress criterion, a relationship is established between the minimum principal stress of the formation rock and the characteristic parameters (tensile strength and pressure) of the formation rock. Using the established relationship, the second characteristic parameter threshold characterizing formation cracking is calculated to determine the corresponding initiation characteristics of the formation, thereby obtaining the second initiation pressure of the formation fracture.

[0077] Furthermore, the range of minimum principal stress values ​​satisfying the third relationship is used as the second characteristic parameter threshold. The third relationship characterizes the correlation between the minimum principal stress in formation stress and the tensile strength of the formation rock. First, according to the formation initiation rules, for a formation to exhibit a cracking state, a third relationship must exist between the minimum principal stress in formation stress and the tensile strength of the formation rock. Then, the range of minimum principal stress values ​​satisfying the third relationship is calculated, and the calculation result is used as the second characteristic parameter threshold. Specifically, the tensile strength parameter of the formation rock is substituted into the third relationship to calculate the range of minimum principal stress values ​​in formation stress, thereby obtaining the second characteristic parameter threshold. Further analysis is performed on the two endpoints of the range corresponding to the first characteristic parameter threshold, and the maximum value of the minimum principal stress in formation stress is determined as the second initiation pressure. The third relationship is expressed using the following expression:

[0078] σ3'-α·p p ≤-σ' t (6)

[0079] Where σ3' represents the minimum principal stress of the formation, σt 'Based on the tensile strength of the rock.'

[0080] Furthermore, the method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures described in this invention also utilizes Mohr's circle stress analysis to obtain the initiation angle characteristic parameter in the second initiation characteristic. In practical applications, the direction of formation fracture initiation is not along the wellbore axis, but along the direction of the formation fracture initiation angle. Therefore, in this embodiment, Mohr's circle stress analysis is used to calculate the initiation angle characteristic parameter in the second initiation characteristic. The initiation angle is calculated using the following expression:

[0081]

[0082] Where γ represents the initiation angle of the formation fracture.

[0083] Furthermore, when the cement sheath fracture extends into the annulus, a stress analysis is performed on the annulus to obtain its stress distribution characteristics. In actual integrated well construction and fracturing processes, the formation fractures formed by the extension of the cement sheath fractures induce stress in the formation during propagation. That is, the stress on the annulus is actually the superposition of in-situ stress and induced stress generated by formation fractures. Based on this, relationships are established between the normal stress of the annulus and the in-situ and induced stresses, as well as between the shear stress of the annulus and the in-situ and induced stresses. Based on these relationships, the first stress parameters (normal stress parameter and shear stress parameter) on the annulus wall are obtained. The first stress parameters on the annulus wall are calculated using the following expression:

[0084]

[0085]

[0086]

[0087] Where, σ x σ represents the component of the normal stress along the x-axis. y τ represents the component of the normal stress along the y-axis. xy σ represents the shear stress component along the xy axis. H σ represents the maximum horizontal principal stress of the formation. h K represents the minimum principal stress of the formation. I denoted by , r represents the distance from any point on the wall of the annulus to the tip of the cement sheath crack, and θ' represents the angle between the line connecting any point on the wall of the annulus to the tip of the cement sheath crack and the direction of the maximum horizontal principal stress of the formation.

[0088] Furthermore, assuming that when the cement sheath crack extends into the annular gap, the approximation angle between the crack tip and the inner wall of the annular gap is β, a coordinate system about the approximation angle is established with the current crack tip as the origin. The first stress parameter on the wall of the annular gap, obtained through calculation, is then mapped to this coordinate system, thereby transforming the first stress parameter into the stress distribution characteristics of the annular gap. The stress distribution characteristics of the annular gap are obtained using the following expression:

[0089]

[0090]

[0091]

[0092] Where β represents the approach angle between the cement annulus crack and the annulus gap, σ βx σ βy Let τ represent the normal stress components along the x-axis and y-axis of the annulus wall in a coordinate system related to the approximation angle, respectively. β This represents the shear stress component of the annular gap wall in a coordinate system related to the approximation angle.

[0093] Furthermore, in step S140, based on the annular stress distribution characteristics, the likelihood of the cement sheath fracture forming a formation fracture is diagnosed, and if formation fracture formation is possible, the propagation characteristics of the formed formation fracture are predicted. When the cement sheath fracture extends to interact with the annular gap, the extension of the cement sheath fracture may cause the annular gap to open or shear. After the cement sheath fracture extends to form a formation fracture, the annular gap will experience through-fracture. Regardless of the failure mode, the extension characteristics of the cement sheath fracture (extension path, fracture propagation width, etc.) will change. Therefore, in this embodiment, by analyzing the failure modes of the annular gap, it is determined whether the cement sheath fracture can form a formation fracture, and if formation fracture formation is determined to be possible, the propagation characteristics of the formation fracture are predicted.

[0094] Next, based on the distribution characteristics of shear stress in the annular stress distribution characteristics, it is determined whether the annular gap has shear slip characteristics. If it is determined that it does not, the possibility of the cement annular crack extending through the annular gap is diagnosed by combining the tensile strength of the rock in the corresponding formation. Specifically, as the shear stress acting on the wall of the annular gap increases, the annular gap will experience shear slip. In this embodiment, the shear displacement of the annular gap is calculated according to the Westergaard function in fracture mechanics and the shear displacement formula for a type II fracture surface (single-sided) in an infinite medium. Therefore, when the calculated shear displacement result is zero, it is determined that the annular gap does not have shear slip characteristics. Furthermore, under the premise that the annular gap does not have shear slip characteristics, when the maximum principal stress acting on the wall of the annular gap is greater than the tensile strength of the rock in the formation, it is determined that the cement annular crack can extend through the annular gap. The shear displacement of the annular gap is calculated using the following expression:

[0095]

[0096] k = 3 - 4ν (15)

[0097] G = E / 2(1+ν) (16)

[0098] Among them, u s Let represent shear displacement, k represent Kolosov constant, G represent shear modulus, l1 represent annular gap half-length, x represent coordinates of any point on the crack surface, and E represent Young's modulus.

[0099] When determining whether a cement shear fracture can extend through the annulus, the critical characteristics of the annulus at the point of shear failure are used to determine whether shear failure has occurred. If shear failure has not occurred, the cement shear fracture is considered capable of forming a formation fracture. Based on the critical characteristics of the annulus at the point of shear failure, the non-critical state of the current annulus is determined to be caused by excessive shear stress. If the shear stress is excessive, shear failure is considered to have occurred; otherwise, it is considered not to have occurred. When the annulus has not experienced shear failure, the cement shear fracture can form a formation fracture.

[0100] Furthermore, based on the critical characteristics of the annular gap when it is about to undergo shear failure, the shear failure state of the annular gap is determined to ascertain whether shear failure has occurred. The condition for reaching the critical characteristics is expressed by the following expression:

[0101] |τ β |=s0-μσ βy (17)

[0102] Where s0 represents the cohesive force of the annular gap wall, and μ represents the friction coefficient of the annular gap wall.

[0103] In this embodiment of the application, according to the condition satisfied by the annular gap to reach the critical characteristic, when |τ β |>s0-μσ βy When |τ| < τ, shear slip will occur in the annular gap; when |τ| < τ, |τ| β |<s0-μσ βy At this time, the annular gap will not experience shear failure.

[0104] Under conditions where the annulus has not experienced shear failure, when the maximum principal stress acting on the annulus wall exceeds the tensile strength of the formation rock, and the annulus is in a critical state of shear failure, a cement annulus fracture can form a formation fracture. The maximum principal stress on the annulus wall when the annulus is in a critical state of shear failure is calculated using the following expression:

[0105]

[0106] Where σ1” represents the maximum principal stress on the annular gap wall.

[0107] Next, after determining that the cement sheath fracture can form a formation fracture, the critical distance between the cement sheath fracture and the formation when the formation fracture forms is calculated using the following expression:

[0108]

[0109] Where, r c The value represents the critical distance, and K represents the consistency coefficient of the fracturing fluid as determined in the laboratory.

[0110] In this embodiment, during the process of a cement sheath crack extending to form an annular crack but not yet penetrating the annular gap, the annular crack gradually turns parallel to the maximum horizontal principal stress of the annular gap wall. However, when the cement sheath crack extends to penetrate the annular gap, the turning angle of the annular crack has not yet reached parallel to the maximum horizontal principal stress of the annular gap wall. Therefore, this invention also determines the penetration direction of the cement sheath crack when it extends to penetrate the annular gap by obtaining the turning angle characteristics of the annular crack and combining them with the maximum horizontal principal stress of the annular gap wall. Furthermore, the penetration direction is integrated with the formation fracture initiation characteristics to obtain the crack propagation characteristics of the cement sheath crack from penetrating the annular gap to penetrating the formation. The turning angle characteristic parameter of the annular crack is calculated using the following expression:

[0111]

[0112] Where γ' represents the turning angle of the annular crack, and Atn represents the function symbol.

[0113] In addition, during the process of the cement sheath crack extending to form an annular crack but not penetrating the annular gap, this application marks the angle of the annular crack and the angle of the direction of the maximum horizontal principal stress on the wall of the annular gap in a counterclockwise direction, and calculates the difference between the angle of the annular crack and the angle of the maximum horizontal principal stress. If the result is positive, the annular crack formed by the cement sheath crack in the annular gap will penetrate upwards into the annular gap.

[0114] Furthermore, the method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures described in this invention also utilizes the distribution characteristics of normal stress in the annular stress distribution characteristics, combined with the pressure parameters of the fluid within the annular fracture, to calculate the opening characteristics of the annular fracture, thereby obtaining the propagation characteristics of the annular fracture formed by the cement sheath fracture. Specifically, when the pressure of the fluid within the annular fracture formed by the cement sheath fracture is greater than the distribution characteristics of normal stress in the annular stress distribution characteristics, the annular fracture will undergo opening failure. At this time, the pressure of the fluid within the annular fracture is obtained to calculate the opening characteristics of the annular fracture, thus using the opening characteristics of the annular fracture as the propagation characteristics of the annular fracture formed by the cement sheath fracture. The pressure of the fluid within the annular fracture is calculated using the following expression:

[0115] p = σ βy (twenty one)

[0116] p = σ h +p net (twenty two)

[0117] Where p represents the pressure of the fluid inside the slit, p net This represents the original formation pressure.

[0118] Furthermore, based on the theory of elasticity, when the annular gap undergoes opening failure, the width of the annular crack formed by the cement annulus crack is calculated using the following expression to obtain the opening characteristics of the annular crack:

[0119]

[0120] Where w represents the opening width of the annular crack, and H f This indicates the height of the annular gap.

[0121] Furthermore, after determining that the cement sheath fracture can form a formation fracture, the fracture width is obtained and used as the first propagation feature of the formation fracture. Specifically, the first propagation feature characterizing the width of the formation fracture is obtained using the following expression:

[0122]

[0123] in, E' represents the average width of the fracture in segment k when the perforation fracture at point i in segment j extends to segment n1. K represents the elastic modulus of the rock. IC This indicates the type I fracture toughness of the rock. This represents the length of a single wing when the perforation fracture at the i-th segment of the j-th section of the inclined shaft extends to the n1-th segment. This represents the length of the fracture extension in the preceding m-th segment when the perforation fracture at the i-th segment of the j-th well section extends to the n1-th segment.

[0124] Furthermore, after determining that the cement sheath fracture can form a formation fracture, the intra-fracture frictional pressure drop of the formation fracture is also obtained, and this intra-fracture frictional pressure drop is used as the second extension characteristic of the formation fracture. Since the pressure drop of power-law fluid flowing through the formation fracture can be approximated as laminar flow between infinitely large parallel plates, in this embodiment, the following expression is used to calculate the frictional resistance (Δp) of the k-th segment before the n1-th segment when the perforation fracture at the i-th location in the j-th segment of the deviated well extends to the n1-th segment. fi,j To obtain a second extension feature characterizing the intra-fracture frictional pressure drop in formation fractures:

[0125]

[0126]

[0127] in, This indicates the frictional resistance of the k-th segment preceding the i-th perforation fracture in the j-th segment of the deviated well, when the fracture extends to the n1-th segment. Here, n represents the fracturing fluid flow index. This represents the length of segment k when the perforation fracture at point i in segment j of the deviated well extends to segment n1. This represents the average width of the fracture in the k-th segment when the perforation fracture at the i-th location in the j-th segment of the deviated well extends to the n1-th segment. K represents the flow rate of the perforation fracture at the i-th perforation point in the j-th section of the deviated well as it extends to the n1-th section. f This represents the consistency coefficient of the power-law type fracturing fluid within the fracture.

[0128] Next, based on the calculated frictional resistance of the k-th segment before the n1-th segment when the perforated fracture at the i-th location in the j-th segment of the deviated well extends to the n1-th segment, the second extension characteristic characterizing the intra-fracture frictional pressure drop is calculated using the following expression:

[0129]

[0130] in, This represents the total frictional pressure drop of the fracture when the perforation fracture at point i in section j of the deviated well extends to section n1.

[0131] Furthermore, after determining that the cement sheath fracture can form a formation fracture, the total propagation time of the formation fracture is also obtained, and this total propagation time is used as the third propagation characteristic of the formation fracture. In the embodiments of this application, when the first fracture extends to the n1th segment, the length of the first fracture is always ΔL during the time period t(n1)-t(n1-1). Based on this premise, the total time required for propagation is calculated. The third propagation characteristic, representing the total propagation time of the formation fracture, is obtained using the following expression:

[0132]

[0133] Where c represents the filtration coefficient, Q f1 ΔL represents the displacement, ΔL represents the length of the extended fracture, t represents the extension time, n2 represents the well section number, and t(n2) represents the total extension time from the initial fracture in the formation to the n2th section.

[0134] In this embodiment of the invention, the well segments traversed by the initial fracture as it expands to form a formation fracture and continues to expand within the formation, as well as the well segments to be traversed, are marked as segment n1, segment n2, ..., segment n according to the order in which the fractures initiate. n The third extension feature described in this embodiment of the invention is the total extension time taken from the initial crack to the n2th segment. It should be noted that this invention does not specifically limit the extension node of the initial crack; those skilled in the art can set it according to actual needs.

[0135] Next, based on the crack initiation and propagation characteristics of the hydraulic fracturing cracks predicted in the embodiments of this application, the initiation mechanism and propagation law of single-point hydraulic fracturing with annular gap are analyzed for two cases: well cemented sheath (no annular gap) and poorly cemented cemented sheath (with annular gap).

[0136] In this embodiment of the application, the basic parameters involved in predicting the initiation and propagation characteristics of hydraulic fracturing fractures are shown in Table 1:

[0137] Table 1 Basic Parameters Table

[0138]

[0139]

[0140] Next, based on the predicted fracture initiation and propagation characteristics of the fracturing fractures in the embodiments of this application, the initiation mechanism and propagation law of sliding sleeve single-point fracturing with annular gaps are analyzed for two cases: good cementing quality and poor cementing quality. Furthermore, by comparing the fracture initiation and propagation characteristics of sliding sleeve fracturing and perforation fracturing, the propagation law of sliding sleeve single-point fracturing is further revealed. Finally, by analyzing the propagation trajectory, displacement, pressure, and fracture width variation laws of the fracturing fractures in sliding sleeve single-point fracturing, the obtained variation laws are used to guide the field construction of integrated well construction sliding sleeve single-point fracturing.

[0141] In actual construction, sliding sleeve single-point fracturing involves a multi-process fracturing process where the propagating fracture initiates and propagates through the first interface between the cement sheath and the casing assembly, then through the second interface between the cement sheath and the formation, before initiating and propagating further within the formation. Perforation fracturing, on the other hand, is a single-process fracturing process where a perforation penetrates the cement sheath and directly initiates and propagates the fracture within the formation. The processes by which these two fracturing methods generate propagating fractures differ, resulting in different pressure distributions, propagation trajectories, and fracture width distributions.

[0142] Specifically, under the same construction conditions, the fracturing pressure generated by sliding sleeve single-point fracturing is higher than that of perforation fracturing, and the rate of increase in fracture initiation and propagation pressure over time is lower for sliding sleeve single-point fracturing than for perforation fracturing. According to the principle of perforation fracturing, the perforation creates a small-aperture dominant fluid inlet channel during fracturing, causing the fracturing fluid to concentrate its action in the perforation direction to find the optimal fracturing initiation location. Therefore, the fracturing pressure generated by sliding sleeve single-point fracturing is higher than that of perforation fracturing. At the same time, perforation fracturing also generates additional small-aperture friction, making the rate of increase in fracture initiation and propagation pressure over time faster than that of sliding sleeve single-point fracturing.

[0143] Figure 3 This is an example diagram illustrating the pressure variation state during the initiation and propagation of sliding sleeve single-point fracturing and perforation fracturing, as described in the method for predicting the initiation and propagation characteristics of fracturing fractures according to embodiments of this application. (Refer to...) Figure 3 Under the same construction conditions, the fracture pressure of single-point fracturing with a sliding sleeve is 76 MPa and the fracture initiation and propagation pressure is 70 MPa, while the fracture pressure of perforation fracturing is 74 MPa and the fracture initiation and propagation pressure is 73 MPa. Furthermore, at 2100 s, the fracture initiation and propagation pressure of perforation fracturing is greater than that of single-point fracturing with a sliding sleeve.

[0144] Next, under the same construction environment, the length and turning radius of the extended fracture in the sliding sleeve single-point fracturing are longer than those in the perforation fracturing, and the fracture width is also larger than that in the perforation fracturing. Figure 4 This is an example diagram illustrating the changes in the propagation trajectory of sliding sleeve single-point fracturing and perforation fracturing in a method for predicting the initiation and propagation characteristics of fracturing fractures according to embodiments of this application. (Refer to...) Figure 4During fracturing, both the sliding sleeve in single-point fracturing and the perforation in perforation fracturing change direction until the direction of change is parallel to the direction of the maximum horizontal principal stress. However, due to the energy loss caused by the additional orifice friction in perforation fracturing, the length and turning radius of the propagation fracture in single-point sliding sleeve fracturing are longer than those in perforation fracturing, and the fracture width is also larger.

[0145] Next, under the same construction conditions, the length of the extended fracture in single-point fracturing of the sliding sleeve is longer than that in perforation fracturing, and the fracture width is also larger in single-point fracturing. Figure 5 This is an example diagram illustrating the variation in fracture width during sliding sleeve single-point fracturing and perforation fracturing, as described in the method for predicting the initiation and propagation characteristics of fracturing fractures according to embodiments of this application. (Refer to...) Figure 5 In both sliding sleeve single-point fracturing and perforation fracturing, the width of the propagating fracture gradually decreases as the fracture extends. That is, the fracture width is largest at the fracture opening and smallest at the fracture tip. However, due to the energy loss caused by the additional orifice friction in perforation fracturing, the propagating fracture in sliding sleeve single-point fracturing is longer and wider than that in perforation fracturing.

[0146] In practical applications, the properties of the annulus can affect the propagation of fracturing fractures and the fracturing effect of fracturing operations. Figure 6 This is an example diagram illustrating the influence of cementing quality on the propagation pressure of a single-point fracturing fracture in a sliding sleeve, within the method for predicting the initiation and propagation characteristics of fracturing fractures according to embodiments of this application. Through analysis of... Figure 6 Analysis reveals that cementing quality significantly impacts the initiation and propagation of fractures in single-point fracturing using a sliding sleeve. Next, we will illustrate the influence of annular gap properties on fracture initiation and propagation pressures in two scenarios: well-bonded cement sheath (no annular gap) and poorly bonded cement sheath (with annular gap).

[0147] Next, under the same construction conditions, the fracturing pressure of the well section with good cementing quality is the same as that of the well section with poor cementing quality, and no local pressure buildup occurs in the well section with good cementing quality. However, the fracture initiation and propagation pressure of the well section with good cementing quality is greater than that of the well section with poor cementing quality. This is because the fracture initiation and propagation pressure of the propagating fracture in the well section with poor cementing quality decreases after reaching the fracturing pressure of the cement sheath. The presence of the annulus causes local pressure buildup and stress concentration between the cement sheath and the wellbore. As a result, with the continuous injection of fracturing fluid, the propagating fracture breaks through the annulus and initiates fracturing in the formation, and the fracture initiation and propagation pressure decreases again. Figure 7 This is an example diagram illustrating the effect of displacement on the propagation trajectory of a single-point hydraulic fracturing fracture in a sliding sleeve, within the method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures according to embodiments of this application. (Refer to...) Figure 7 For well sections with good cementing quality and those with poor cementing quality, the greater the fracturing fluid displacement within the extended fracture, the greater the fracture initiation and propagation pressure. In the extended fracture, the fracturing fluid displacement is 9m³ / s.3 Under the condition of / min, the fractures all start to initiate on the cement sheath. At this time, the fracture pressure of both the well section with good cementing quality and the well section with poor cementing quality is 76 MPa. The fracture initiation and propagation pressure of the well section with good cementing quality is 70 MPa, while the fracture initiation and propagation pressure of the well section with poor cementing quality is 67 MPa.

[0148] Furthermore, under the same construction conditions, in well sections with good cementing quality and those with poor cementing quality, a larger fracturing fluid discharge results in a higher net pressure and a larger turning radius for the propagating fracture. This is further supported by... Figure 7 Analysis shows that the turning radius of the expanded fractures in well sections with good cementing quality is smaller than that in well sections with poor cementing quality.

[0149] Furthermore, during single-point fracturing with a sliding sleeve, the turning radius of the extended fracture in the well section with good cementing quality is smaller than that in the well section with poor cementing quality, and the extended fracture in the well section with good cementing quality will not deviate from the sliding sleeve position to initiate and extend again. During single-point fracturing with a sliding sleeve, the higher the viscosity of the annulus wall, the greater the net pressure, the slower the filtration loss, the longer the extended fracture length, and the larger the turning radius. Figure 8 This is an example diagram illustrating the effect of viscosity on the propagation trajectory of a single-point hydraulic fracturing fracture in a sliding sleeve, as described in an embodiment of this application for predicting the initiation and propagation characteristics of hydraulic fracturing fractures. Through analysis of... Figure 8 Analysis shows that the turning radius of the extended fractures in well sections with good cementing quality is smaller than that in well sections with poor cementing quality. Furthermore, in well sections with good cementing quality, the extended fractures will not deviate from the position of the sliding sleeve and will not re-initiate or extend.

[0150] Furthermore, during single-point fracturing with a sliding sleeve, the width of the extended fracture in the well section with good cementing quality is smaller than that in the well section with poor cementing quality. Moreover, the larger the fracturing fluid discharge and the greater the net pressure, the wider the extended fracture becomes. Figure 9 This is an example diagram illustrating the effect of displacement on the width of a single-point hydraulic fracturing fracture in a sliding sleeve, within the method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures according to embodiments of this application. Through... Figure 9 Analysis shows that the width of the extended fractures in well sections with good cementing quality is smaller than that in well sections with poor cementing quality.

[0151] This invention provides a method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures. The method designs a fracturing tool assembly that meets the requirements of integrated well construction and considers the annular gap formed between the outer wall of the cement sheath and the wellbore. Then, based on the structural parameters of the fracturing tool assembly, a single-point fracturing model for the current well section, considering the annular gap factor, is established. Next, using the single-point fracturing model, combined with the initiation criteria of the cement sheath and formation, the initiation characteristics of cement sheath fractures and formation fractures formed by the extension of cement sheath fractures are obtained, along with the stress distribution characteristics of the annular gap. Finally, based on the annular gap stress distribution characteristics, it is determined whether the cement sheath fracture can penetrate the annular gap and form a formation fracture, and if it can, the propagation characteristics of the formed formation fracture are predicted. This invention achieves accurate prediction of the initiation and propagation characteristics of hydraulic fracturing fractures under integrated well construction methods, and simultaneously reveals the initiation and propagation laws of hydraulic fracturing fractures formed by sliding sleeve single-point fracturing.

[0152] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

[0153] Of course, the present invention may have other various embodiments. Without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and modifications according to the present invention, but these corresponding changes and modifications should all fall within the protection scope of the claims of the present invention.

[0154] Those skilled in the art will understand that the modules or steps of the present invention described above can be implemented using general-purpose computing devices. They can be centralized on a single computing device or distributed across a network of multiple computing devices. Optionally, they can be implemented using computer-executable program code, thereby storing them in a storage device for execution by a computing device, or fabricating them separately as individual integrated circuit modules, or fabricating multiple modules or steps as a single integrated circuit module. Thus, the present invention is not limited to any particular hardware and software combination.

[0155] While the embodiments disclosed in this invention are as described above, the content is merely for the purpose of facilitating understanding of the invention and is not intended to limit the invention. Any person skilled in the art to which this invention pertains may make any modifications and variations in form and detail of the implementation without departing from the spirit and scope disclosed herein; however, the scope of patent protection for this invention shall still be determined by the scope defined in the appended claims.

Claims

1. A method for predicting the initiation and propagation characteristics of hydraulic fracturing fractures, comprising: The design meets the requirements of integrated well construction fracturing tool assembly, wherein the fracturing tool assembly includes casing assembly and cement sheath, and an annular gap is formed between the outer wall of the cement sheath and the well wall; Based on the structural parameters of the fracturing tool assembly, a single-point fracturing model for the current well section is established; Using the single-point fracturing model, the stress of the sliding sleeve at the wellbore wall in the casing assembly is analyzed, and the wellbore wall stress is converted into cement sheath stress and formation stress of the corresponding well section. This is to obtain the first fracturing characteristic of the cement sheath fracture and the second fracturing characteristic of the formation fracture formed by the extension of the cement sheath fracture, and to obtain the stress distribution characteristics of the annulus. In the step of obtaining the stress distribution characteristics of the annulus, when the cement sheath fracture extends to the annulus, the stress analysis of the annulus is performed, and the stress state corresponding to the annulus is determined according to the analysis results. Based on this, the first stress parameter on the wall of the annulus is calculated, and the first stress parameter is converted into the stress distribution characteristics of the annulus using the following expression: in, β This represents the approximation angle between the cement annulus crack and the annulus gap. , These represent the coordinates along the approximation angle in the coordinate system. x axis and along y The normal stress components of the annular gap wall. Indicates the stress intensity factor. r This represents the distance from any point on the wall of the annular gap to the tip of the cement annular crack. θ' This represents the angle between the line connecting any point on the wall of the annulus to the tip of the cement annulus crack and the direction of the maximum horizontal principal stress in the formation. This represents the shear stress components of the annular gap wall in a coordinate system related to the approximation angle. Indicates the maximum horizontal principal stress of the formation. Indicates the minimum horizontal principal stress of the formation; Based on the annular stress distribution characteristics, the possibility of the cement annular fracture forming a formation fracture is diagnosed, and the propagation characteristics of the formed formation fracture are predicted when it is possible to form. In the step of diagnosing the possibility of the cement annular fracture forming a formation fracture based on the annular stress distribution characteristics, the distribution characteristics of shear stress in the annular stress distribution characteristics are used to determine whether the annular fracture has shear slip characteristics. If it is determined that it does not have shear slip characteristics, the possibility of the cement annular fracture extending to penetrate the annular fracture is diagnosed in combination with the tensile strength of the rock in the corresponding formation. Wherein, when it is possible to penetrate, the critical characteristics of the annular fracture when shear failure occurs are used to determine whether the annular fracture has undergone shear failure. Wherein, when shear failure does not occur, it is determined that the cement annular fracture can form a formation fracture. Further, in the step of determining whether the annular fracture has undergone shear failure, the shear failure state of the annular fracture is determined based on the critical characteristics of the annular fracture when it is about to undergo shear failure, so as to determine whether the annular fracture has undergone shear failure. Wherein, the condition satisfied to reach the critical characteristics is represented by the following expression: in, This represents the shear stress components of the annular gap wall in a coordinate system related to the approximation angle. This represents the cohesion of the annular gap walls. μ This represents the friction coefficient of the annular gap wall. In the coordinate system related to the approximation angle, along y The normal stress component of the wall of the annular gap.

2. The method according to claim 1, characterized in that, The step of obtaining the first crack initiation feature includes: By analyzing the stress state of the cement sheath, the threshold value of the first characteristic parameter characterizing cement sheath failure is calculated, thereby obtaining the first crack initiation pressure, wherein... If the triaxial stress of the cement ring is all under tensile stress, then the range of the maximum principal stress that satisfies the first relationship is taken as the first threshold in the first characteristic parameter threshold. The first relationship characterizes the correlation between the maximum principal stress in the cement ring stress and the tensile strength of the cement ring, wherein the first relationship is expressed by the following expression: in, This indicates the maximum principal stress of the cement ring. Indicates the tensile strength of the cement ring. If the triaxial stress of the cement ring is in a tensile-compressive-compressive stress state or a tensile-tensile-compressive stress state, then the range of values ​​of the minimum principal stress that satisfies the second relationship is taken as the second threshold in the first characteristic parameter threshold. The second relationship characterizes the correlation between the minimum principal stress in the cement ring stress and the compressive strength of the cement ring, wherein the second relationship is expressed by the following expression: in, This represents the minimum principal stress of the cement ring. This indicates the compressive strength of the cement ring.

3. The method according to claim 1 or 2, characterized in that, The step of obtaining the second crack initiation feature includes: Based on the maximum tensile stress criterion, the threshold value of the second characteristic parameter characterizing formation fracturing is calculated, thereby obtaining the second fracturing initiation pressure. The range of minimum principal stress values ​​satisfying the third relationship is used as the second characteristic parameter threshold. The third relationship characterizes the correlation between the minimum principal stress in the formation stress and the tensile strength of the formation rock. The third relationship is expressed by the following expression: in, Indicates the minimum principal stress of the formation. According to the representation of the tensile strength of rock, Indicates formation pressure. This represents the Biot elastic coefficient.

4. The method according to claim 3, characterized in that, The method also utilizes Mohr's circle stress analysis to obtain the crack initiation angle characteristic parameter in the second crack initiation characteristic.

5. The method according to claim 1, characterized in that, The method further includes: By utilizing the distribution characteristics of normal stress in the annular stress distribution characteristics, combined with the pressure parameters of the fluid inside the annular crack, the opening characteristics of the annular crack are calculated to obtain the propagation characteristics of the annular crack formed by the cement annulus crack: in, This indicates the opening width of the annular crack. Represents Poisson's ratio. Indicates the minimum horizontal principal stress of the formation. E Indicates Young's modulus. Indicates the height of the annular gap. p net Indicates the original formation pressure. In the coordinate system related to the approximation angle, along y The normal stress component of the wall of the annular gap.

6. The method according to claim 1, characterized in that, In the step of predicting the propagation characteristics of the formed formation fractures, a first propagation characteristic representing the width of the formation fractures is obtained using the following expression: in, Indicates the first inclined shaft j well section number i The perforation crack extended to the first n 1st segment k The average width of the crack in the segment, This represents the elastic modulus of rock. This indicates the type I fracture toughness of the rock. Indicates the first inclined shaft j well section number i The perforation crack extended to the first n wing length at segment 1 Indicates the first inclined shaft j well section number i The perforation crack extended to the first n The first paragraph m The length of the crack propagation in the segment.

7. The method according to claim 1, characterized in that, The method also utilizes the following expression to obtain a second extended characteristic characterizing the intra-fracture frictional pressure drop of the formation fracture: in, This indicates that the perforation fracture at point i in section j of the deviated well extends to point i. n The total frictional pressure drop of the crack in section 1. Indicates the fracturing fluid flow index. Indicates the first inclined shaft j well section number i The perforation crack extended to the first n 1st segment k The length of the segment Indicates the first inclined shaft j well section number i The perforation crack extended to the first n 1st segment k The average width of the crack in the segment, Indicates the first inclined shaft j well section number i The perforation crack extended to the first n Crack flow rate at stage 1 Indicates the height of the crack. This represents the consistency coefficient of the power-law type fracturing fluid within the fracture.

8. The method according to claim 1, characterized in that, The method also utilizes the following expression to obtain a third propagation feature characterizing the total propagation time of the formation fractures: in, Indicates the filtration loss coefficient. Indicates the height of the annular gap. Indicates engine displacement. Indicates the extended seam length, t Indicates extended time, n 2 indicates the well section location number. t ( n 2) Indicates that the fracture in the first fracturing stratum extends to the th... n Total extension time for 2 segments.

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