Ultra-deep well bottom rock brittle-plastic transition index acquisition method and evaluation method

Abstract

The invention provides an ultra-deep well bottom rock brittle-plastic transition index obtaining method and an evaluation method, and relates to the technical field of rock mechanical parameter calculation, the method comprises the following steps: obtaining mudstone characteristic parameters of ultra-deep well bottom rock according to an ultra-deep well bottom rock theoretical stress-strain curve; determining an extra-deep well bottom rock energy block on the theoretical stress-strain curve; obtaining the relative magnitude of rock dissipated energy before the characterization peak, the relative magnitude of rock self-fracture capability after the characterization peak and the intensity of energy change in the characterization rock fracture process of the ultra-deep well bottom rock; and determining the rock brittleness-plasticity transformation index of the ultra-deep well bottom rock. The ultra-deep well bottom rock brittleness-plasticity transformation index provided by the invention can provide a theoretical basis for the development of key technologies such as drill bit model selection, well wall stability evaluation, fracturing construction transformation and efficient rock breaking acceleration during actual drilling.

Description

Technical Field

[0001] The present invention relates to the technical field of rock mechanics parameter calculation, and particularly relates to a method for obtaining the brittle-plastic transformation index of bottom-hole rocks in ultra-deep layers and an evaluation method therefor. Background Art

[0002] With the continuous growth of energy demand and the in-depth development of resource distribution, the drilling of ultra-deep wells at the ten-thousand-meter level has become an important field for energy resource acquisition and geological science research. Ultra-deep well drilling is not only a key way to extract deeply buried oil and gas resources, but also an important means to explore the crustal structure, study geological activities, and utilize geothermal energy. However, in the process of achieving these goals, a core challenge is faced: the mechanical properties of deep rocks have not been fully understood and accurately described. In China, although the drilling of ultra-deep wells above 8000 meters has gradually become normal, in the actual drilling process, only the rock fragmentation mechanism of the complex ultra-deep wellbore environment at about 7000 meters has been revealed, and there is still a lack of evaluation means for the drillability of deeper rocks, and the prediction accuracy is low. This knowledge gap directly restricts the improvement of drilling efficiency and the guarantee of drilling safety. Summary of the Invention

[0003] The purpose of the present invention is to provide a method for obtaining the brittle-plastic transformation index of bottom-hole rocks in ultra-deep layers and an evaluation method therefor, which can provide a theoretical basis for the development of key technologies such as bit selection, wellbore stability evaluation, fracturing construction transformation, and efficient rock breaking and speed increase during actual drilling.

[0004] In the first aspect, the present invention provides a method for obtaining the brittle-plastic transformation index of bottom-hole rocks in ultra-deep layers, which is characterized in that shale characteristic parameters of bottom-hole rocks in ultra-deep layers are obtained according to the theoretical stress-strain curve of bottom-hole rocks in ultra-deep layers;

[0005] Based on the shale characteristic parameters of the bottom-hole rocks in ultra-deep layers, an energy block of the bottom-hole rocks in ultra-deep layers is determined on the theoretical stress-strain curve;

[0006] According to the energy block of the bottom-hole rocks in ultra-deep layers, the relative magnitude of the dissipated energy of pre-peak rocks representing the bottom-hole rocks in ultra-deep layers, the relative magnitude of the self-fracture ability of post-peak rocks representing the bottom-hole rocks in ultra-deep layers, and the severity of the energy change during the rock fracture process are obtained;

[0007] According to the relative magnitude of the dissipated energy of pre-peak rocks representing the bottom-hole rocks in ultra-deep layers, the relative magnitude of the self-fracture ability of post-peak rocks representing the bottom-hole rocks in ultra-deep layers, and the severity of the energy change during the rock fracture process, the brittle-plastic transformation index of the bottom-hole rocks in ultra-deep layers is determined.

[0008] Furthermore, in some embodiments of the present application, the shale characteristic parameters include Poisson's ratio, Young's modulus, yield modulus, post-peak modulus, theoretical post-peak modulus, peak stress, and residual stress;

[0009] The ultra-deep well bottom rock energy block at least includes total dissipated energy, elastic energy, supplementary energy, and dissipated energy caused by damage.

[0010] Further, in some embodiments of the present application, the brittle-plastic transition index B of the ultra-deep well bottom rock E is determined by the average value of the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self-fracture ability of the rock after the peak, and the degree of severity of the energy change during the rock fracture process. The specific calculation method is shown in Equation (1):

[0011]

[0012] where B i1 is the relative magnitude of the dissipated energy of the rock before the peak, B i2 is the relative magnitude of the self-fracture ability of the rock after the peak, and B i3 is the degree of severity of the energy change during the rock fracture process.

[0013] Further, in some embodiments of the present application, the relative magnitude B of the dissipated energy of the rock before the peak i1 is determined by the dissipated energy before the peak of the ultra-deep well bottom rock, and the specific calculation formula is shown in Equation (2):

[0014]

[0015] where dW e and dW td are the elastic energy before the peak and the total dissipated energy before the peak, respectively;

[0016] The relative magnitude B of the self-fracture ability of the rock after the peak i2 is determined by the dissipated energy and supplementary energy of the rock caused by damage in the ultra-deep well bottom rock, and the specific calculation formula is shown in Equation (3):

[0017]

[0018] where dW k and dW a are the dissipated energy and supplementary energy caused by cracks after the peak, respectively;

[0019] The degree of severity B of the energy change during the rock fracture process i3 is determined by the elastic energy of the ultra-deep well bottom rock, and the specific calculation formula is shown in Equation (4):

[0020]

[0021] Among them, E, D, M, and H are the elastic modulus, yield modulus, post-peak modulus characterization, and theoretical post-peak modulus characterization, respectively.

[0022] Furthermore, in some embodiments of the present application, the elastic energy dW before peak characterization e is calculated as shown in Equation (5):

[0023]

[0024] where E is the elastic modulus; σ b is the peak stress characterization;

[0025] The total dissipated energy dW before peak characterization td is calculated as shown in Equation (6):

[0026]

[0027] where σ b′ is the theoretical peak stress characterization;

[0028] The dissipated energy dW caused by post-peak cracks k is calculated as shown in Equation (7):

[0029]

[0030] where σ c is the magnitude of the residual stress; M is the post-peak modulus characterization;

[0031] The supplementary energy dW a includes post-peak plastic energy and residual energy, and the supplementary energy dW a is calculated as shown in Equation (8):

[0032]

[0033] Furthermore, in some embodiments of the present application, the acquisition of the stress-strain curve of the ultra-deep well bottom rock includes the following steps:

[0034] Obtain the ultra-deep well bottom rock and prepare a rock mechanics sample;

[0035] Use the rock mechanics sample to conduct a rock triaxial compression experiment, construct a rock damage model, and obtain the stress-strain curve of the ultra-deep well bottom rock.

[0036] Furthermore, in some embodiments of the present application, the rock damage model is:

[0037]

[0038] Wherein, E is the elastic modulus, μ is the Poisson's ratio, ε1 is the strain value at a certain point, and ε cc is the strain after coordinate transformation based on the original coordinates, σ3 is the confining pressure applied in the triaxial compression experiment, is the residual stress, a is the initial damage degree, r is the inherent growth rate of damage, c is the internal crack shape factor, and σ1 is the stress value obtained from ε1 under the damage model.

[0039] Furthermore, in some embodiments of the present application, the rock mechanics sample is a cylindrical rock sample with a diameter of 25 mm to 50 mm, a height of 50 mm to 125 mm, and a width-diameter ratio of 2.0 to 2.5.

[0040] In a second aspect, the present application also provides a method for evaluating the brittle-plastic transition of deep-bottom rocks in a well. The brittle-plastic transition index value of the deep-bottom rocks in the well is obtained according to the method for obtaining the brittle-plastic transition index of deep-bottom rocks in the well described in the first aspect. According to the magnitude of the brittle-plastic transition index value, it is determined whether the deep-bottom rocks in the well are in a brittle state, a plastic state, or in a brittle-plastic transition;

[0041] When the brittle-plastic transition index value is greater than 0.5, the deep-bottom rocks in the well are in a brittle state; when the brittle-plastic transition index value is less than 0.5, the deep-bottom rocks in the well are in a brittle state; when the brittle-plastic transition index value is within the range of (0.425, 0.585), the deep-bottom rocks in the well may undergo a brittle-plastic transition at any time.

[0042] The method for obtaining the brittle-plastic transition index of deep-bottom rocks in the well provided by the embodiments of the present application introduces a rock damage model to construct the theoretical stress-strain curve of deep-bottom rocks in the well, and obtains shale characteristic parameters such as Poisson's ratio, Young's modulus, yield modulus, post-peak modulus, theoretical post-peak modulus, peak stress, and residual stress according to the stress-strain curve. Based on the shale characteristic parameters and the theoretical stress-strain curve, energy partitioning is performed to obtain and calculate the plastic energy, elastic energy, supplementary energy, and rock dissipation energy caused by damage of deep-bottom rocks in the well, so as to obtain the brittle-plastic transition index. This index acquisition method realizes the calculation of the brittle-plastic transition index of deep-bottom rocks in the well. The calculation result of the index has a good correlation with the brittle-plasticity of the rock itself. At the same time, it can also well distinguish the brittle-plasticity degree of the rock. The predicted rock mechanical properties are consistent with the known laws. Based on the calculation results, the rock fragmentation process can be effectively predicted. Therefore, it can well assist in the selection of drill bits during actual drilling, and provide a theoretical basis for the development of key technologies such as wellbore stability evaluation, fracturing construction transformation, and efficient rock breaking and speed increase. Description of the Drawings

[0043] Figure 1It is a schematic flowchart in the method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock provided by the present invention;

[0044] Figure 2 It is the stress-strain curve obtained from the rock mechanics triaxial compression experiment adopted in the method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock provided by the present invention;

[0045] Figure 3 It is the change diagram of the rock damage model with the increase of damage in the method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock provided by the present invention;

[0046] Figure 4 It is the photo of the experimental rock sample after the rock mechanics triaxial compression experiment in the method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock provided by the present invention;

[0047] Figure 5 It is the energy division schematic diagram of the theoretical stress-strain curve of the rock in the method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock provided by the present invention. Specific embodiments

[0048] To make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0049] A new index that can accurately reflect the brittle-plastic transition characteristics of deep rocks helps to reveal the fragmentation mechanism of rocks at a depth of 10,000 meters, which is directly related to the mechanical safety, efficiency, and cost during the drilling process, and has a profound impact on the safety of tunnel construction and the effect of fracturing and reconstruction of oil and gas reservoirs. It can provide a scientific basis for key technologies such as bit design and selection, wellbore stability evaluation, fracturing construction transformation, and efficient rock fragmentation and speed increase. There are no less than 80 indexes obtained from the research on rock brittleness and plasticity at home and abroad, including hardness testing, mineral composition analysis, strength parameter evaluation, stress-strain curve analysis, elastic parameter evaluation, and strain energy. However, the parameters of the above-mentioned indexes for studying rock brittleness and plasticity do not fully consider the integrity of the rock fracture process and are difficult to describe the entire behavior range of rocks from brittleness to plasticity. Especially for the research on rocks in ultra-deep drilling, it is even rarer. Therefore, for ultra-deep drilling (≥9000m), a reasonable rock brittle-plastic index is needed, which fully considers the integrity of the fracture process and requires the ability to monotonically describe the development process of brittle fracture and also describe the entire behavior range of rocks from absolute plasticity to absolute brittleness. Based on this technical problem, the brittle-plastic transition index of the ultra-deep bottom-hole rock proposed by the present invention is based on energy evolution and rock damage. The energy of the rock fragmentation process is divided into three stages: energy accumulation - energy consumption - energy release. During the energy accumulation stage of the rock, rock damage accumulates slowly, and most of the energy is stored in the rock in the form of elastic energy. During the energy dissipation stage of the rock, the rock transforms from the elastic stage to the yield stage. During this process, due to the accumulation of microcracks inside the rock before, the rock damage intensifies, mainly dominated by dissipated energy. During the energy release stage of the rock, due to the different types of rocks, the brittle-plasticity of the rock itself will be significantly manifested, that is, different brittle-plastic transitions occur. The rock fractures due to the elastic energy before the peak, and the fracture is promoted by the plastic energy after the peak. The more brittle the rock is, the less energy is required after the peak. Therefore, in this application, a rock triaxial compression experiment is carried out on a rock mechanics sample to obtain the stress-strain curve of the ultra-deep bottom-hole rock, and based on this, the characteristic parameters of mudstone are obtained, and the energy blocks of the three stages of energy accumulation - energy consumption - energy release are drawn based on the characteristic parameters of mudstone, and then the energy in the energy blocks is calculated, and based on this, the brittle-plastic transition index of the ultra-deep bottom-hole rock is determined, so that it can fully consider the integrity of the rock fracture process and describe the entire behavior range of rocks from absolute plasticity to absolute brittleness. Refer to Figure 1 , the specific steps of the method for obtaining the brittle-plastic transition index of the ultra-deep bottom-hole rock are as follows:

[0050] Obtain the ultra-deep bottom-hole rock and prepare a cylindrical rock mechanics sample with a diameter of 25 mm to 50 mm, a height of 50 mm to 125 mm, and a width-diameter ratio of 2.0 to 2.5;

[0051] Use the rock mechanics sample to carry out a rock triaxial compression experiment and construct a rock damage model:

[0052]

[0053] Among them, E is the elastic modulus, μ is the Poisson's ratio, ε1 is the strain value at a certain point, and ε cc is the strain after coordinate transformation based on the original coordinates, σ3 is the confining pressure applied in the triaxial compression experiment, is the residual stress, a is the initial damage degree, r is the inherent growth rate of damage, c is the internal crack shape factor, and σ1 is the stress value obtained from ε1 under the damage model.

[0054] According to the rock damage model, construct the theoretical stress-strain curves of the rock at the bottom of the ultra-deep well under different damages, and obtain the characteristic parameters of the theoretical stress-strain curves. The parameters at least include: Poisson's ratio, Young's modulus, yield modulus, post-peak modulus, theoretical post-peak modulus, peak stress, theoretical peak stress, and residual stress;

[0055] Based on the above characteristic parameters, draw the energy blocks of the rock at the bottom of the ultra-deep well on the theoretical stress-strain curve. The energy blocks at least include total dissipated energy, elastic energy, supplementary energy, and dissipated energy caused by damage;

[0056] Calculate the energy magnitudes of the above-mentioned dissipated energy, elastic energy, supplementary energy, and dissipated energy caused by damage respectively, and determine the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self-breaking ability of the rock after the peak, and the severity of the energy change during the rock fracture process based on the calculation results.

[0057] Among them, the dissipated energy includes the pre-peak elastic energy dW e and the pre-peak total dissipated energy dW e ; the calculation formula of the pre-peak dissipated energy dW e is shown in Equation (5):

[0058]

[0059] Among them, E is the elastic modulus; σ b is the peak stress;

[0060] The specific calculation formula of the pre-peak total dissipated energy dW td is shown in Equation (6):

[0061]

[0062] Among them, σ b′ is the theoretical peak stress;

[0063] The dissipated energy caused by damage includes the dissipated energy caused by pre-peak cracks and the dissipated energy caused by post-peak cracks. Among them, the dissipated energy caused by pre-peak cracks is directly involved in the calculation of the total pre-peak dissipated energy, while the dissipated energy dW k caused by post-peak cracks is calculated as shown in Equation (7):

[0064]

[0065] where σ c is the magnitude of the residual stress; M is the modulus representing post-peak;

[0066] The supplementary energy dW a includes the plastic energy and residual energy representing post-peak. The calculation formula of the supplementary energy dW a is shown in Equation (8):

[0067]

[0068] The relative magnitude B i1 of the dissipated energy of pre-peak rock is determined by the dissipated energy of pre-peak of the rock at the bottom of the ultra-deep well. Its specific calculation formula is shown in Equation (2):

[0069]

[0070] where dW e , dW td are the elastic energy representing pre-peak and the total dissipated energy representing pre-peak respectively;

[0071] The relative magnitude B i2 of the self-fracture ability of post-peak rock is determined by the dissipated energy and supplementary energy of the rock caused by damage at the bottom of the ultra-deep well. Its specific calculation formula is shown in Equation (3):

[0072]

[0073] where dW k , dW a are the dissipated energy and supplementary energy caused by post-peak cracks respectively;

[0074] The severity B i3 of the energy change during the rock fracture process is determined by the elastic energy of the rock at the bottom of the ultra-deep well. Its specific calculation formula is shown in Equation (4):

[0075]

[0076] where E, D, M, and H are the elastic modulus, yield modulus, modulus representing post-peak, and theoretical modulus representing post-peak respectively.

[0077] After determining the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self - fracture ability of the rock after the peak, and the severity of the energy change during the rock fracture process, the brittle - plastic transition index B of the rock at the bottom of the ultra - deep well required by this application can be determined through the following formula E , and the specific determination method is as follows: It is calculated from the calculation formula shown in Equation (1):

[0078]

[0079] where B i1 is the relative magnitude of the dissipated energy of the rock before the peak, B i2 is the relative magnitude of the self - fracture ability of the rock after the peak, and B i3 is the severity of the energy change during the rock fracture process.

[0080] The brittle - plastic transition index B of the rock at the bottom of the ultra - deep well obtained in this application E is within the range of (1, 0). Among them, the brittle - plastic transition index corresponding to the ideal brittleness of the rock at the bottom of the ultra - deep well is 1, the brittle - plastic transition index corresponding to the ideal plasticity of the rock at the bottom of the ultra - deep well is 0, and the range of the brittle - plastic transition index corresponding to the brittle - plastic transition of the rock at the bottom of the ultra - deep well is (0.425, 0.585). Therefore, according to the magnitude of the brittle - plastic transition index value of the rock, it can be determined whether the rock at the bottom of the ultra - deep well is in a brittle state, a plastic state, or in a brittle - plastic transition state. The specific evaluation method is as follows:

[0081] When the brittle - plastic transition index value of the rock is greater than 0.5, the rock at the bottom of the ultra - deep well is in a brittle state; when the brittle - plastic transition index value of the rock is less than 0.5, the rock at the bottom of the ultra - deep well is in a plastic state; when the brittle - plastic transition index value of the rock is within the range of (0.425, 0.585), the rock at the bottom of the ultra - deep well may undergo a brittle - plastic transition at any time.

[0082] The above - mentioned technical solutions of this application will be described below in conjunction with specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations of this application.

[0083] Embodiment

[0084] This embodiment provides a method for constructing and verifying the accuracy and rationality of the brittle - plastic transition index of the rock at the bottom of the ultra - deep well with mudstone as the rock sample. It includes the following steps:

[0085] S1 Obtain and manufacture rock mechanics experimental samples;

[0086] Specifically, taking a well depth of 9,000 meters as an example, the mud density used on-site is 1.8 ppg. Then the bottom-hole pressure is 162 MPa, the bottom-hole temperature is 160 °C, the mudstone is sampled from the well section at 8,078 m below the wellbore, and the rock sample is cored perpendicular to the cushion layer and processed into a cylinder with a diameter of about 25 mm to 50 mm, a height of 50 mm to 125 mm, and a width-to-diameter ratio of 2.0 to 2.5.

[0087] S2 Conduct a triaxial compression test on the rock mechanics test sample to obtain the stress-strain curve of the rock at the bottom of the ultra-deep well.

[0088] Specifically, conduct a mechanical test on the rock mechanics test sample prepared in step S1. This rock mechanics test applies stress to the above-mentioned rock mechanics test sample according to the procedures of the triaxial rock mechanics test until it fails. After the test is completed, the axial stress-strain curve of the complete rock mechanics test sample is obtained. For example, the stress-strain curve obtained during a certain test is the reference Figure 2 shown in the image.

[0089] S3 Introduce a rock damage model to construct the theoretical stress-strain curve of the rock at the bottom of the ultra-deep well, and obtain the characteristic parameters of the theoretical stress-strain curve. The parameters at least include: Poisson's ratio, elastic modulus, yield modulus, post-peak modulus, theoretical post-peak modulus, peak stress, theoretical peak stress, and residual stress.

[0090] Specifically, this rock damage model is based on the Usher function and a unified and complete damage evolution model under triaxial rock compression tests. Based on this damage evolution model, the constitutive equation expression for the entire deformation process of rock under conventional triaxial compression:

[0091]

[0092] where E is the elastic modulus, μ is Poisson's ratio, ε1 is the strain value at a certain point, ε cc is the strain after coordinate transformation based on the original coordinates, σ3 is the confining pressure applied in the triaxial compression test, is the residual stress, a is the initial damage degree, r is the inherent growth rate of damage, and c is the internal crack shape factor. σ1 is the stress value obtained from ε1 under the damage model. After determining the confining pressure and the test rock sample, the initial damage degree and the inherent growth rate of damage remain unchanged. The variables in the formula will only be the internal crack shape factor and the strain magnitude. When the rock damage decreases, the internal crack shape factor increases, and the peak stress in the stress-strain curve rises. Specifically, as shown in the reference Figure 3 shown: the change diagram of the rock damage model after damage increases.

[0093] According to the obtained theoretical stress-strain curve, the characteristic parameters of mudstone are shown in Table 1. The crushed samples obtained through the triaxial compression test of the rock are as shown in the reference Figure 4 (the blue line in the figure is the trend line of the crack).

[0094] Table 1

[0095]

[0096] Table 1: Results Table of Mudstone Characteristic Parameters (Unit: MPa)

[0097] S4 Based on the characteristic parameters, draw the energy block of the rock at the bottom of the ultra-deep well on the theoretical stress-strain curve. The energy block at least includes the total dissipated energy, elastic energy, supplementary energy, and dissipated energy caused by damage;

[0098] Specifically, according to the reference Figure 5 shown is the divided energy block, where S1 is the dissipated energy, S2 is the elastic energy, S3 is the post-peak plastic energy, S4 is the residual energy, the post-peak plastic energy and the residual energy together constitute the supplementary energy, S5 and S6 are the dissipated energy caused by damage. Their difference lies in the dissipated energy caused by damage before the peak and the dissipated energy caused by damage after the peak. Among them, the dissipated energy caused by damage before the peak is the dissipation caused by the yield stage of the rock, so it directly participates in the calculation of the total dissipated energy before the peak.

[0099] S5 Calculate the corresponding energy magnitude based on the energy block, and obtain the brittle-plastic transition index of the rock at the bottom of the ultra-deep well from the energy magnitude and characteristic parameters.

[0100] Specifically, according to the obtained characteristic parameters, calculate the brittle-plastic transition index of the rock at the bottom of the ultra-deep well proposed in the present invention. The calculation formula is:

[0101]

[0102] Where B i1 is the relative magnitude representing the dissipated energy of the rock before the peak, B i2 is the relative magnitude representing the self-fracture ability of the rock after the peak, B i3 is the degree of severity of the energy change during the rock fracture process.

[0103] Furthermore, the calculation formula for the relative magnitude representing the dissipated energy of the rock before the peak is:

[0104]

[0105] Where dW e and dW td are the elastic energy before the peak and the total dissipated energy before the peak respectively;

[0106] The calculation formula for the relative magnitude of the post-peak rock self-breaking ability is as follows:

[0107]

[0108] where dW k and dW a represent the dissipated energy and supplementary energy caused by post-peak cracks respectively;

[0109] The calculation formula for the relative magnitude of the severity of the energy change during the rock failure process is as follows:

[0110]

[0111] where E, D, M, and H are the elastic modulus, yield modulus, post-peak modulus, and theoretical post-peak modulus respectively.

[0112] Furthermore, the calculation formula for the pre-peak elastic energy is as follows:

[0113]

[0114] The calculation formula for the total pre-peak dissipated energy is as follows:

[0115]

[0116] The calculation formula for the dissipated energy caused by post-peak cracks is as follows:

[0117]

[0118] The calculation formula for the supplementary energy is as follows:

[0119]

[0120] where E and M are the elastic modulus and post-peak modulus, σ b and σ b′ are the peak stress and theoretical peak stress, and σ c is the magnitude of the residual stress.

[0121] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, rather than limiting them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the various embodiments of the present application, and they should all be covered within the scope of the claims and the specification of the present application. In particular, as long as there is no structural conflict, the technical features mentioned in each embodiment can be combined in any way. The present application is not limited to the specific embodiments disclosed in the text, but includes all technical solutions falling within the scope of the claims.

Claims

1. Method for obtaining brittle-plastic transition index of rock at ultra-deep well bottom, characterized in that: Obtain the mudstone characteristic parameters of the rock at the ultra-deep well bottom according to the theoretical stress-strain curve of the rock at the ultra-deep well bottom; determine the energy block of the rock at the ultra-deep well bottom on the theoretical stress-strain curve based on the mudstone characteristic parameters of the rock at the ultra-deep well bottom; Obtain the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self-fracture ability of the rock after the peak, and the severity of the energy change during the rock fracture process according to the energy block of the rock at the ultra-deep well bottom; Determine the brittle-plastic transition index of the rock at the ultra-deep well bottom according to the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self-fracture ability of the rock after the peak, and the severity of the energy change during the rock fracture process.

2. The method for obtaining the brittle-plastic transition index of the rock at the bottom of the ultra-deep well according to claim 1, characterized in that, The mudstone characteristic parameters include Poisson's ratio, Young's modulus, yield modulus, post-peak modulus, theoretical post-peak modulus, peak stress, and residual stress.

3. The method for obtaining the brittle-plastic transition index of the rock at the bottom of ultra-deep wells according to claim 1, characterized in that The energy block of the rock at the ultra-deep well bottom includes at least total dissipated energy, elastic energy, supplementary energy, and dissipated energy caused by damage.

4. The method for obtaining the brittle-plastic transition index of the bottom-hole rock in ultra-deep formations according to claim 2, characterized in that, The brittle-plastic transformation index B of the rock at the bottom of the ultra-deep well E is determined by the average value of the relative magnitude of the dissipated energy of the rock before the peak, the relative magnitude of the self-fracture ability of the rock after the peak, and the value representing the severity of the energy change during the rock fracture process. The specific calculation method is shown in Equation (1): Among which B i1 represents the relative magnitude of the dissipated energy of the rock before the peak, and B i2 represents the relative magnitude of the self - fracture ability of the rock after the peak, and B i3 represents the intensity of the energy change during the rock fracture process.

5. The method for obtaining the brittle-plastic transition index of ultra-deep well bottom rock according to claim 4, characterized in that The relative magnitude B of the dissipated energy of the rock before the peak is characterized i1 The value is determined by the dissipated energy of the rock at the bottom of the ultra-deep well before the peak, and its specific calculation formula is shown in Equation (2): where dW e and dW td represent the elastic energy before the peak and the total dissipated energy before the peak, respectively; The relative magnitude B of the self-fracture ability of the rock after the characterization peak i2 The value is determined by the rock dissipation energy and replenishment energy caused by damage to the rock at the bottom of the ultra-deep well, and its specific calculation formula is shown in Equation (3): where dW k and dW a are the dissipated energy and the supplementary energy caused by the crack after the peak respectively; The severity B of the energy change during the rock fracture process i3 is determined by the elastic energy of the rock at the bottom of the ultra-deep well, and its specific calculation formula is shown in Equation (4): Wherein, E, D, M, and H are elastic modulus, yield modulus, post-peak modulus, and theoretical post-peak modulus respectively.

6. The method for obtaining the brittle-plastic transition index of deep well bottom rock according to claim 5, wherein, The calculated formula for the elastic energy dW before the characteristic peak e is shown in Equation (5) as follows: where E is the elastic modulus; σ b represents the peak stress; The total dissipated energy dW before the characteristic peak td is calculated as shown in Equation (6): where σ b′ represents the theoretical peak stress; The dissipated energy dW caused by the post-characterization peak crack k is calculated as shown in Equation (7): where σ c is the magnitude of the residual stress; M is the post-peak modulus; The supplementary energy dW a includes the post-peak plastic energy and the residual energy, and the supplementary energy dW a is calculated as shown in Equation (8):

7. The method for obtaining the brittle-plastic transition index of deep bottom-hole rock according to claim 1, wherein The acquisition of the stress-strain curve of the rock at the ultra-deep well bottom includes the following steps: Obtain the rock at the ultra-deep well bottom and prepare a rock mechanics sample; Conduct a rock triaxial compression experiment using the rock mechanics sample, construct a rock damage model, and obtain the stress-strain curve of the rock at the ultra-deep well bottom.

8. The method for obtaining the brittle-plastic transition index of the rock at the bottom of the ultra-deep well according to claim 7, characterized in that, The rock damage model is: Among them, E is the elastic modulus, μ is the Poisson's ratio, ε1 is the strain value at a certain point, and ε cc is the strain after coordinate transformation based on the original coordinates, σ3 is the confining pressure applied in the triaxial compression experiment, is the residual stress, a is the initial damage degree, r is the inherent growth rate of damage, c is the internal crack shape factor, and σ1 is the stress value obtained from ε1 under the damage model.

9. The method for obtaining the brittle-plastic transition index of the rock at the bottom of the ultra-deep well according to claim 7, characterized in that, The rock mechanics sample is a cylindrical rock sample with a diameter of 25-50 mm, a height of 50-125 mm, and an aspect ratio of 2-2.

5.

10. An evaluation method for brittle-plastic transition of deep-bottom well rocks, characterized in that Obtain the brittle-plastic transition index value according to the method for obtaining the brittle-plastic transition index of the rock at the ultra-deep well bottom according to any one of claims 1-9, and determine whether the rock at the ultra-deep well bottom is in a brittle state, a plastic state, or in a brittle-plastic transition according to the magnitude of the brittle-plastic transition index value; When the brittle-plastic transition index value is greater than 0.5, the rock at the ultra-deep well bottom is in a brittle state; when the brittle-plastic transition index value is less than 0.5, the rock at the ultra-deep well bottom is in a brittle state; when the brittle-plastic transition index value is within the range of (0.425, 0.585), the rock at the ultra-deep well bottom may undergo a brittle-plastic transition at any time.

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