Method for determining drillability based on digital core and mechanical properties of mineral components

Abstract

The present application relates to the field of rock mechanics, and discloses a method for determining the drillability of a digital core and mechanical properties based on mineral components, comprising the following steps: S1, obtaining a core sample to be determined, and analyzing the mineral crystal particle components and their contents of the core sample to be determined; S2, constructing a Voronoi polyhedron for representing mineral crystal particles; S3, assigning materials to the Voronoi polyhedron after grid division according to the mineral crystal particle contents of the core sample to be determined to obtain an initial digital core model; S4, optimizing the initial digital core model to obtain a final digital core model; S5, establishing a micro test drill bit model, and combining the micro test drill bit model with the final digital core model to obtain a drillability model; and S6, performing a drillability test on the drillability model to finally obtain a drillability evaluation of the drillability model. The present application uses a digital core to evaluate the drillability of a drill bit.

Description

TECHNICAL FIELD

[0001] The present application relates to the fields of petroleum and natural gas engineering, mine engineering and rock mechanics, and particularly relates to a method for determining the drillability of a digital core based on mineral components and mechanical properties. BACKGROUND

[0002] There are two main types of modeling methods for digital cores: one type is a physical experiment method, which directly images core samples by experimental instruments to construct digital cores, mainly including a sequential two-dimensional thin section stacking imaging method, a confocal laser scanning method and a non-destructive X-ray CT scanning imaging method; the other type is a mathematical method, which is based on high-precision two-dimensional thin section images and reconstructs three-dimensional digital cores through random simulation or geological process simulation. The physical experiment method for constructing digital cores has the advantages of directness and accuracy, but is relatively expensive. Since two-dimensional high-resolution thin section images of cores are often used in geological research, they are a relatively low-cost core data reflecting the microstructure of cores. Therefore, a mathematical method is proposed to reconstruct digital cores. At present, the main methods include random methods and process simulation methods. The random methods mainly include a Gaussian random field method, a simulated annealing method, a sequential indicator simulation method, a multiple-point geostatistics method and a Markov chain method.

[0003] In 1974, Joshi first proposed the Gaussian random field method for reconstructing three-dimensional digital cores. In 1997, Hazlet proposed the simulated annealing method for reconstructing three-dimensional digital cores. In 2003, Keehm reconstructed three-dimensional digital cores by using the sequential indicator simulation (SISIM) algorithm. The digital cores constructed by these three methods have poor connectivity when the porosity is low. In 2004, Okabe developed the multiple-point geostatistics method for reconstructing three-dimensional digital cores from two-dimensional thin section images of cores by referring to the commonly used geostatistics method in geological modeling. Wu et al. reconstructed three-dimensional digital cores based on a Markov random grid statistical model. The digital cores constructed by these two methods have good pore connectivity. Unlike the random method which introduces a random function to reconstruct digital cores, Dren and Bakke applied spheres with different particle radii to reconstruct digital cores by simulating the deposition process, compaction process and diagenetic process of rocks in 1997. The digital cores constructed by the process simulation method have good pore connectivity, but are generally only suitable for the reconstruction of digital cores of simple rocks with diagenetic processes.

[0004] From the current numerous digital core modeling methods, the research mainly focuses on the modeling method of porous reservoir digital cores, and the research on the whole digital core modeling method based on mechanical properties is less. Since the whole digital core modeling based on mechanical properties is relatively difficult, it is urgent to propose a complete digital core modeling method, so as to provide an effective method for the research on the mechanical properties of digital cores and the evaluation of drillability. SUMMARY

[0005] The present application provides a method for determining the drillability of a digital core based on mineral components and mechanical properties to solve the above problems.

[0006] The present application is realized by the following technical solutions:

[0007] A method for determining the drillability of a digital core based on mineral components and mechanical properties, comprising the following steps:

[0008] S1, obtaining a core sample to be determined, analyzing the mineral crystal particle components and their contents of the core sample to be determined;

[0009] S2, based on the mathematical theory of Thiessen polygons, constructing a Thiessen polyhedron for characterizing mineral crystal particles, and performing grid division on the Thiessen polyhedron;

[0010] S3, according to the mineral crystal particle content of the core sample to be determined, assigning materials to the Thiessen polyhedron after grid division to obtain an initial digital core model;

[0011] S4, importing the initial digital core model into finite element software for mechanical simulation analysis, comparing the simulation analysis results with the mineral crystal particle content of the core sample to be determined obtained by analysis, optimizing the initial digital core model, and obtaining a final digital core model;

[0012] S5, establishing a micro test drill bit model, combining the micro test drill bit model with the final digital core model to obtain a drillability model, calculating the strength criterion of the drillability model, and the damage factor of the final digital core model when drilling the final digital core model through the micro test drill bit model;

[0013] S6, performing a drillability test on the drillability model, based on the strength criterion and the damage factor, finally obtaining the appropriate drilling pressure grade and drilling time of the final digital core model, and finally obtaining the drillability evaluation of the drillability model.

[0014] As an optimization, in S1, the whole rock and clay mineral analysis experiment of the core sample to be determined is performed by SYT5163-2010 X-ray diffraction analysis method for clay minerals and common non-clay minerals in sedimentary rocks, and the mineral crystal particle content of the core sample to be determined is obtained.

[0015] As an optimization, the specific steps of S3 are:

[0016] S3.1, based on the material containing regularity characteristics of different regions of the core, regarding the Voronoi polyhedron after the grid division as a core sample to be determined, based on the regular region of the core sample to be determined, the corresponding mineral crystal particle material attribute assignment is performed at the corresponding position of the Voronoi polyhedron, and based on the irregular region of the core sample to be determined, the material attribute assignment of the mineral crystal particle is randomly performed at the corresponding position of the Voronoi polyhedron;

[0017] S3.2, if the internal part of the core sample to be determined contains micro-voids or micro-cracks, the micro-voids or micro-cracks are treated as materials without any attributes, and when the material attribute assignment is performed by using Fortran language, the surface layer grid is selected as the pore and crack on the surface of the mineral particle, and the void or crack is randomly or regularly generated between different mineral crystal particles, so as to obtain the initial digital core model.

[0018] As an optimization, when the material attribute assignment is performed, the content proportion of each mineral crystal particle is the same as the content proportion of each mineral crystal particle of the core sample to be determined.

[0019] As an optimization, the material parameters of the micro test drill bit model are set based on SYT5426-2016 "Petroleum and Natural Gas Drilling Engineering Rock Drilling Determination and Classification".

[0020] As an optimization, the micro test drill bit model comprises a drill bit body, two pieces of polycrystalline diamond compact are symmetrically fixed at the end of the drill bit body, the polycrystalline diamond compact is in a cylindrical shape, the outer diameter of the drill bit body is φ32mm, and the inclination angle between the polycrystalline diamond compact and the axis of the drill bit body is 20°, and the side inclination angle is 5°.

[0021] As an optimization, the hardness and size requirements of the micro test drill bit model are:

[0022] The polycrystalline diamond compact: the diameter is 13.44mm, the thickness is 4.5mm, the cutting edge chamfer is 0.3*45°, the density is 3250kg / m 3 , the yield strength is 1200MPa, the Young's modulus is 890GPa, and the Poisson's ratio is 0.07;

[0023] The drill bit body: the material is 35CrMo steel, the tensile strength is 985MPa, the yield strength is 835MPa, the Young's modulus is 207GPa, and the Poisson's ratio is 0.25.

[0024] As optimization, the strength criterion of the final digital core model is specifically set based on a Drucker-Prager plasticity model, and when drilling the final digital core model by the micro test drill bit model, the deviatoric stress is taken as a factor for the final digital core model to be damaged in the drilling process, and the Drucker-Prager plasticity model is specifically:

[0025] τ oct = τ0+ mσ oct ;

[0026] wherein

[0027]

[0028] , σ1, σ2, σ3 respectively represent the maximum principal stress, the intermediate principal stress and the minimum principal stress of the final digital core model, and the unit is MPa; α, k represent parameters related to the material cohesion C and the friction angle ζ of the final digital core model, and the cohesion and the friction angle of the core can be measured through core triaxial experiments, σ oct represents the normal stress on the octahedral plane; τ oct represents the shear stress; τ0 represents the initial shear stress.

[0029] As optimization, the solution of the damage factor caused by the damage of the final digital core model by the deviatoric stress is specifically to solve the damage factor of the final digital core model by calculating based on the change of the elastic modulus of the rock, and the specific formula is:

[0030]

[0031] wherein D is the damage factor, 0≤D≤1; E0 is the elastic modulus of the final digital core model in the complete state; E(t) is the elastic modulus of the material of the final digital core model after damage; ε d is the irreversible deformation of the final digital core model, and ε is the total deformation of the final digital core model. In the process of rock breaking by the PDC cutting tooth, when the plastic strain of the rock reaches the critical plastic strain, the rock starts to be damaged, at this time the damage factor D=0, and the resistance of the rock decreases with the increase of the plastic strain. When the plastic strain reaches the equivalent plastic strain at which the rock completely fails, the rock element is removed, and the damage factor D=1.

[0032] As optimization, the specific steps of S6 are:

[0033] S6.1, a first level of drilling pressure is applied to the micro test drill bit model of the drillability model;

[0034] S6.2, drilling the final digital core model by the micro test drill bit model, wherein the drilling pressure time is t0, the rotating speed of the micro test drill bit model is Ar / min, and A is a positive integer;

[0035] S6.3, obtaining the height from one end of the micro test drill bit model away from the final digital core model to a base surface by numerical experiment, wherein the base surface is in the same horizontal plane as the side of the final digital core model close to the micro test drill bit model;

[0036] S6.4, repeating S6.2-S6.3 twice for the same drilling position of the final digital core model, and obtaining three groups of the height from one end of the micro test drill bit model away from the final digital core model to the base surface, which are h0, h1 and h2 respectively, and the drilling pressure time of the last two times is t1 and t2 respectively;

[0037] S6.5, judging whether |h0-h2| is greater than a set height threshold value, if yes, jumping to S6.8, otherwise, jumping to S6.6;

[0038] S6.6, judging whether the drilling pressure level of the micro test drill bit model of the drillability model is three, if yes, jumping to S6.8, otherwise, jumping to S6.7;

[0039] S6.7, increasing one level on the basis of the original drilling pressure to be applied to the micro test drill bit model of the drillability model, and jumping to S6.2;

[0040] S6.8, ending.

[0041] As an optimization, the drillability evaluation of the drillability model is specifically evaluated by a drillability level value, and the specific formula of the drillability level value is:

[0042] K d = log2t+2 i-1 -1;

[0043] In the formula, K d is the drillability level value, t is the average drilling time, the unit is second (s), and i is the drilling pressure level.

[0044] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0045] Compared with the existing digital core modeling method, the mineral component-based digital core and the drillability determination method of mechanical properties in the application are a modeling method for the digital core of the whole rock mechanical properties, and the digital core is used for drill bit drillability evaluation. The modeling method firstly proposes a rock mechanics digital core model, firstly carries out core component experiment to obtain rock mineral component parameters, uses the mathematical theory and script of the tessellation polygon to establish the crystal particles representing the minerals, randomly assigns the material parameters to the crystal particle units by using the random constant function by controlling the mineral component ratio, and builds the digital core model after the assignment is completed; the drillability model of the PDC micro drill bit-digital core is established according to the standard SYT5426-2016, the drillability extreme value of the rock is calculated according to the same footage under different drilling pressures, and the drillability evaluation method based on the digital core is formed. Due to the randomness and regularity of the rock deposition process, two different assignment methods are set for the material parameters of the mineral particles, one is random assignment, the random function is used for random assignment of the mineral particles, and the randomness of the rock deposition is embodied; one method is the material assignment of the mineral particles by human intervention, the regular assignment is artificially performed according to the component distribution law of the solid core, and the real core model is better restored. BRIEF DESCRIPTION OF DRAWINGS

[0046] The drawings described herein are used to provide further understanding of the embodiments of the application, constitute a part of the application, and do not constitute a limitation on the embodiments of the application. In the drawings:

[0047] Figure 1 is a flowchart of an embodiment of the mineral component-based digital core and the drillability determination method of mechanical properties according to the application;

[0048] Figure 2 is a different mineral type and its distribution diagram of the digital core model according to the application;

[0049] Figure 3 is a structural schematic diagram of the micro test drill bit model according to the application;

[0050] Figure 4 is a structural schematic diagram of the drillability model according to the application;

[0051] Figure 5 is a numerical experiment parameter extraction position schematic diagram according to the application;

[0052] Figure 6 is a rock drillability classification reference diagram;

[0053] Figure 7 is a comparison diagram of the digital core and the actual core mechanical experiment results. DETAILED DESCRIPTION

[0054] In order to make the objects, technical solutions and advantages of the present application clearer, further detailed description will be given below in combination with embodiments and drawings. The schematic embodiments of the present application and the description thereof are only used to explain the present application and do not limit the present application.

[0055] The method of the present application is suitable for rock samples with small radius size and strong brittleness, and only needs to determine rock components and mechanical parameters to establish a digital core, and then establish a drillability model to evaluate the drillability of the rock, which greatly reduces the cost of field experiments and improves the efficiency. Next, the method of the present application will be described through embodiments. It should be noted that the method of the present application is performed on Naper and abaqus software.

[0056] This embodiment 1 provides a method for determining the drillability of a digital core based on mineral components and mechanical properties, as shown in Figure 1 The method comprises the following steps:

[0057] S1, obtaining a core sample to be determined, and analyzing the mineral crystal particle components and contents of the core sample to be determined;

[0058] According to the X-ray diffraction analysis method for clay minerals and common non-clay minerals in sedimentary rocks in SYT5163-2010, the core is subjected to whole rock and clay mineral analysis experiments, and the mineral crystal particle content of the core sample to be determined is detected.

[0059] In this embodiment, the core sample to be determined in the target area is provided, as well as the mechanical experimental parameters of the core sample to be determined and the experimental results of the core component to be determined. In this embodiment, the core in the target area is pretreated to obtain the core sample to be determined. The core sample to be determined can be a standard sample size or rock debris. At the same time, in this embodiment, the shape of the core sample to be determined can be a cylinder or a cube.

[0060] S2, based on the Voronoi mathematical theory, edit a program script to construct a Voronoi polyhedron for representing mineral crystal particles, and perform grid division on the Voronoi polyhedron. Through grid division, the micro-particles can be refined, and the particle size is further reduced to be closer to the real core.

[0061] In step S1 and step S2, the whole rock and clay mineral analysis experiment can obtain the content of each mineral crystal of the core sample to be determined more accurately, and the Voronoi polyhedron can better represent the characteristics of the microcrystal in the rock. The Voronoi polyhedron is based on the Voronoi polygon, which is a series of continuous polygons composed of vertical bisectors connecting two adjacent points. The distance from any point in a Voronoi polygon to the control point constituting the polygon is less than the distance to other polygon control points. The key to establishing the Voronoi polygon algorithm is to reasonably connect the discrete data points into a triangular network.

[0062] S3, according to the content of each mineral crystal particle of the core sample to be determined, the Voronoi polyhedron after the grid division is material valued for the digital core, and an initial digital core model is obtained;

[0063] The specific steps of S3 are as follows:

[0064] S3.1, based on the material content regularity characteristics of different regions of the core, the Voronoi polyhedron after the grid division is regarded as the core sample to be determined, the corresponding material properties of the mineral crystal particles are valued at the corresponding positions of the Voronoi polyhedron based on the regular region of the core sample to be determined, and the random material properties of the mineral crystal particles are valued at the corresponding positions of the Voronoi polyhedron based on the irregular region of the core sample to be determined;

[0065] S3.2, if the internal part of the core sample to be determined contains micro-voids or micro-cracks, the micro-voids or micro-cracks are treated as materials without any properties, the surface grid is selected as the pore and crack on the surface of the mineral particle when the material properties are valued by using Fortran language, and the voids or cracks are randomly or regularly generated between different mineral crystal particles, so that the initial digital core model is obtained. The micro-voids or micro-cracks refer to the voids or crack gap range of 0.001-0.1mm.

[0066] When the material properties are valued, the content proportion of each mineral crystal particle is the same as the content proportion of each mineral crystal particle of the core sample to be determined.

[0067] In the embodiment, the content of each mineral crystal particle in the core sample to be tested is measured according to experiments, material properties are assigned according to the total percentage of the content of each mineral crystal particle, and the random_seed and random_number random functions of Fortran language are used to simulate the irregular randomness of core formation. The material in different regions of the core has regularity characteristics, and when the material is assigned, the assignment region can be artificially selected according to the regularity of the core, a certain mineral is assigned, and the material properties of the remaining regions continue to be randomly assigned. For the micro-voids and micro-cracks in the core, the voids can be treated as materials without any properties. When the material parameters are randomly assigned, since the voids exist between the mineral particles, the voids or cracks are randomly or regularly generated between different minerals according to the selection judgment principle of the particle surface when the Fortran assignment is used, so as to simulate the microstructure inside the core.

[0068] As shown in Figure 2 , five kinds of mineral particles are arranged, the script is applied to the Voronoi polyhedron to assign the material of the mineral particles, and finally the initial digital core model representing the characteristics of the core sample to be tested is obtained.

[0069] S4, the initial digital core model is imported into the finite element software for mechanical simulation analysis, the results of the simulation analysis are compared with the content of each mineral crystal particle of the core sample to be tested obtained by analysis, the initial digital core model is optimized, and the final digital core model is obtained. The digital core model after assignment is used to perform rock mechanics simulation experiment by using the finite element software, the finite element simulation experiment and the actual mechanical experiment parameters are compared and analyzed, the digital core model is optimized, and the core modeling basis for drillability evaluation is prepared. According to the stress-strain curve and elastic modulus of the comparison between the actual mechanical experiment and the finite element simulation experiment, the initial digital core model is optimized.

[0070] S5, a micro test drill bit model is established, the micro test drill bit model is combined with the final digital core model to obtain a drillability model, and the strength criterion of the drillability model and the damage factor of the final digital core model when the micro test drill bit model drills the final digital core model are calculated.

[0071] Figure 3 and Figure 4In the embodiment, in step S5, a micro test drill bit model is established based on the drill bit size and material parameters of SYT5426-2016 "Rock Drilling Ability Determination and Classification of Petroleum and Natural Gas Drilling Engineering", and the specific operation is as follows: two pieces of polycrystalline diamond compact 301 (i.e. PDC piece 301) are symmetrically placed on the drill bit body 302, and the outer diameter of the assembled micro PDC test drill bit is φ32mm, the post-installation inclination angle of the compact is 20°, and the side inclination angle is 5°. The side inclination angle refers to the angle between the drill bit teeth (PDC piece) and the axial direction of the drill bit. The established micro test drill bit model is introduced into the final digital core model to establish a drillability model.

[0072] The hardness and size requirements of the micro test drill bit model in the drillability model are as follows:

[0073] The polycrystalline diamond compact has a diameter of 13.44mm, a thickness of 4.5mm, a cutting edge chamfer of 0.3x45°, a density of 3250kg / m 3 , a yield strength of 1200MPa, a Young's modulus of 890GPa, and a Poisson's ratio of 0.07.

[0074] The drill bit body is made of 35CrMo steel, has a tensile strength of 985MPa, a yield strength of 835MPa, a Young's modulus of 207GPa, and a Poisson's ratio of 0.25.

[0075] In the embodiment, the strength criterion of the final digital core model is specifically set based on a modified linear Drucker-Prager plasticity model, which reflects the influence of bulk stress on the strength of rock material. When the micro test drill bit model is used to drill the final digital core model, the deviatoric stress is taken as a factor for the destruction of the final digital core model during drilling, i.e. the deviatoric stress is taken as the cause of material failure. The Drucker-Prager plasticity model is specifically as follows:

[0076] τ oct =τ0+mσ oct ;

[0077] wherein

[0078]

[0079] wherein σ1, σ2, σ3 represent the maximum principal stress, the intermediate principal stress, and the minimum principal stress of the final digital core model, respectively, and the unit is MPa; the intermediate principal stress refers to half of the difference between the positive stress and the negative stress along a certain direction under a three-dimensional stress state, that is, σ2=(σ1+σ3) / 2; the maximum and minimum principal stresses can be measured through experiments, and the intermediate principal stress can be calculated according to the intermediate principal stress formula; α and k represent parameters related to the material cohesion C and the friction angle ζ of the final digital core model, and the cohesion and the friction angle of the core can be measured through a core triaxial experiment, σ oct represents the normal stress on the octahedral plane; τ oct represents the shear stress; τ0 represents the initial shear stress.

[0080] In order to characterize the damage degree of the rock during the drilling process, the rock damage can be calculated based on the change of the elastic modulus of the rock, that is, the damage factor caused by the damage of the deviatoric stress to the final digital core model during the drilling process can be converted into the calculation of the damage factor of the final digital core model based on the change of the elastic modulus of the rock, and the specific formula is as follows:

[0081]

[0082] wherein D is the damage factor, 0≤D≤1; E0 is the elastic modulus of the final digital core model in the complete state; E(t) is the elastic modulus of the material of the final digital core model after damage; ε d is the irreversible deformation of the final digital core model, and ε is the total deformation of the final digital core model. When the plastic strain of the rock reaches the critical plastic strain during the rock breaking process of the PDC cutting tooth, the rock begins to be damaged, at this time, the damage factor D=0, and the resistance of the rock becomes smaller with the increase of the plastic strain. When the plastic strain reaches the equivalent plastic strain at which the rock completely fails, the rock element is removed, and the damage factor D=1.

[0083] The deviatoric stress is the main factor causing damage, and the damage factor is a parameter for measuring damage, so the deviatoric stress affects the damage factor in a proportional relationship. According to the stress-strain curve in the rock damage process, the equivalent plastic strain of the rock is set as ε p , the critical plastic strain is , and the equivalent plastic strain at which the rock completely fails is When the plastic strain of the rock reaches the critical plastic strain during the rock breaking process of the PDC cutting tooth, the rock begins to be damaged, at this time, the damage factor D=0, and the resistance of the rock becomes smaller with the increase of the plastic strain. When the plastic strain reaches the equivalent plastic strain at which the rock completely fails, the rock element is removed, and the damage factor D=1. The equivalent plastic strain is used as the criterion for rock failure:

[0084]

[0085] With the rock damage D gradually increasing from 0 to 1, the equivalent plastic strain also gradually increases until the value reaches the equivalent plastic strain at complete failure of the rock, at which time the rock is broken, wherein, ε p represents the equivalent plastic strain of the rock; represents the equivalent plastic strain of the rock at complete failure.

[0086] The strength criterion and the damage factor are parameters that determine the mechanical properties of the rock; the greater the strength and the damage factor, the more difficult it is to break the rock, and therefore the greater the strength and the damage factor, the smaller the drilling time h and the longer the drilling depth t after the same time.

[0087] S6, drilling test is performed on the drillability model, and the drilling pressure level and the drilling time suitable for the final digital core model are finally obtained based on the strength criterion and the damage factor, and finally the drillability evaluation of the drillability model is obtained.

[0088] The specific steps of S6 are as follows:

[0089] S6.1, a first level of drilling pressure is applied to the micro test drill bit model of the drillability model;

[0090] S6.2, drilling is performed on the final digital core model by the micro test drill bit model, wherein the drilling pressure time is t0, and the rotating speed of the micro test drill bit model is Ar / min, and A is a positive integer;

[0091] S6.3, the height from one end of the micro test drill bit model away from the final digital core model to the base surface is obtained through numerical experiments, wherein the base surface and the side of the final digital core model close to the micro test drill bit model are in the same horizontal plane; drillability simulation is performed through simulation software ABAQUAS, and displacement and time can be extracted according to the results, so that the drilling depth of the drill bit, that is, the height from one end of the micro test drill bit model away from the final digital core model to the base surface, can be obtained.

[0092] S6.4, for the same drilling position of the final digital core model, S6.2-S6.3 are repeated twice to obtain three groups of heights from one end of the micro test drill bit model away from the final digital core model to the base surface, which are h0, h1 and h2 respectively, and the drilling pressure times of the last two times are t1 and t2 respectively;

[0093] S6.5, it is judged whether |h0-h2| is greater than a set height threshold value, if yes, the process jumps to S6.8, otherwise, the process jumps to S6.6;

[0094] S6.6, judge whether the drill pressure level applied on the micro test drill bit model of the drillability model is three levels, if yes, jump to S6.8, otherwise, jump to S6.7;

[0095] S6.7, increase one level on the basis of the original drill pressure applied on the micro test drill bit model of the drillability model, and jump to S6.2;

[0096] S6.8, end.

[0097] (1) 500N±10N (1st level) drill pressure is applied on the drill bit of the drillability model, the time is set to 128s, and the drill bit is rotated at a speed of 60r / min, the depth of the micro test drill bit is obtained through numerical experiment, and three groups of numerical simulation experiments with the same load are carried out on the same rock;

[0098] (2) After the numerical experiment, the time t1, t2 of the three groups of micro test drill bit positions at h1, h2 is extracted, wherein h1-h2=3mm, and the data is recorded; the model of the application is drilled by 3mm, simply speaking, the position with a distance of 3mm in the rock breaking process can be taken.

[0099] (3) Analyze the numerical experiment results under the 1st level drill pressure, if the drill bit has no obvious drilling (here, it means that the drilling distance is 0-1mm, which belongs to no obvious drilling), the drill pressure of the three groups of rock drillability models is applied to 1000N±10N (2nd level), and the numerical experiment is carried out again without changing other parameters, after the experiment, the data is extracted according to step (2) and recorded;

[0100] (4) Analyze the numerical experiment results under the 2nd level drill pressure, if the drill bit has no obvious drilling, the drill pressure of the three groups of rock drillability models is applied to 2000N±10N (3rd level), and the numerical experiment is carried out again without changing other parameters, after the experiment, the data is extracted according to step (2) and recorded;

[0101] (5) Analyze the numerical experiment results under the 3rd level drill pressure, if the drill bit has no obvious drilling, the numerical experiment is terminated.

[0102] The drillability of the drillability model is evaluated by the drillability level value, and the specific formula of the drillability level value is:

[0103] K d =log2t+2 i-1 -1;

[0104] In the formula, K d is the drillability level value, t is the average value of drilling time, unit: second (s), and i is the drill pressure level.

[0105] Figure 5 and Figure 6As shown, after the calculation is completed, the time t1, t2 at which the micro test drill bit position is at h1, h2 is extracted according to the results, where h1-h2=3mm, and the time is brought into K d =log2t+2 i-1 -1 formula to obtain the drillability extreme value K of the rock d According to the calculation results of the drillability calculation formula, the rock drillability can be divided into ten levels, and after obtaining the rock drillability level value of the PDC drill bit, the rock drillability classification table is checked to grade. Figure 6 The first three columns in the table represent time

[0106] Figure 7 As shown, in the present embodiment, the stress-strain curve obtained by comparing and analyzing the digital core finite element simulation experiment and the actual mechanical experiment can be seen that the fitting degree of the digital core simulation result and the actual test stress-strain curve is high, and the mechanical response law of the core in the loading process can be better reflected.

[0107] The core of the second member of Xujiahe group in a certain block is selected to carry out component content test experiment, and the component content is shown in Table 1.

[0108]

[0109] Three groups of final digital core models are established by using a script program, the drillability of the core is simulated, relevant data is extracted, and according to K d =log2t+2 i-1 -1, the results are shown in Table 2.

[0110]

[0111]

[0112] The above-described specific embodiments further detail the purposes, technical solutions and beneficial effects of the present application. It should be understood that the above-described embodiments are merely specific embodiments of the present application and are not intended to limit the protection scope of the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the protection scope of the present application.

Claims

1. A method for determining the drillability of a digital core based on the mineral composition and the mechanical properties of the digital core, characterized in that, It comprises the following steps: S1, obtaining a core sample to be determined, analyzing the mineral crystal particle composition and content of the core sample to be determined; S2, based on the mathematical theory of Thiessen polygon, a Thiessen polyhedron for characterizing mineral crystal particles is constructed, and the Thiessen polyhedron is meshed; S3, the material of the meshed Thiessen polyhedron is valued according to the content of each mineral crystal particle of the core sample to be determined, and an initial digital core model is obtained; S4, the initial digital core model is imported into a finite element software for mechanical simulation analysis, the results of simulation analysis are compared with the content of each mineral crystal particle of the core sample to be determined obtained by analysis, the initial digital core model is optimized, and a final digital core model is obtained; S5, a micro test drill bit model is established, the micro test drill bit model is combined with the final digital core model to obtain a drillability model, and the strength criterion of the drillability model and the damage factor of the final digital core model when the micro test drill bit model drills the final digital core model are calculated; S6, drillability test is performed on the drillability model, and finally the drillability evaluation of the drillability model is obtained based on the strength criterion and the damage factor.

2. The method for determining the drillability of a digital core based on mineral components and mechanical properties according to claim 1, characterized in that, In S1, the whole rock and clay mineral analysis experiment of the core sample to be determined is performed by SYT5163-2010 X-ray diffraction analysis method for clay minerals and common non-clay minerals in sedimentary rocks, and the content of each mineral crystal particle of the core sample to be determined is obtained.

3. The method for determining the drillability of a digital core based on mineral components and mechanical properties according to claim 1, characterized in that, The specific steps of S3 are as follows: S3.1, based on the material content regularity characteristics of different regions of the core, the meshed Thiessen polyhedron is regarded as the core sample to be determined, the material properties of the corresponding mineral crystal particles are valued at the corresponding positions of the Thiessen polyhedron based on the regularity regions of the core sample to be determined, and the material properties of the mineral crystal particles are randomly valued at the corresponding positions of the Thiessen polyhedron based on the irregularity regions of the core sample to be determined; S3.2, if the internal part of the core sample to be determined contains micro voids or micro cracks, the micro voids or micro cracks are treated as materials without any properties, the surface grid is selected as the pore and crack on the mineral particle surface when the material property valuation is performed by Fortran language, and the voids or cracks are randomly or regularly generated between different mineral crystal particles, and the initial digital core model is obtained.

4. The method for determining the drillability of a digital core based on mineral components and mechanical properties according to claim 1, characterized in that, When the material property valuation is performed, the content proportion of each mineral crystal particle is the same as the content proportion of each mineral crystal particle of the core sample to be determined.

5. The method for determining the drillability of a digital core based on mineral components and mechanical properties according to claim 4, characterized in that, The micro test drill bit model comprises a drill bit body, two pieces of polycrystalline diamond compact are symmetrically fixed at the end of the drill bit body, the polycrystalline diamond compact is in a cylindrical shape, the outer diameter of the drill bit body is φ32mm, and the inclination angle between the polycrystalline diamond compact and the axis of the drill bit body is 20°, and the side inclination angle is 5°.

6. The method for determining the drillability of a digital core based on mineral components and mechanical properties according to claim 5, characterized in that, The hardness and size requirements of the micro test drill bit model are as follows: The polycrystalline diamond compact has a diameter of 13.44 mm, a thickness of 4.5 mm, a cutting edge chamfer of 0.3*45°, and a density of 3250 kg / m 3 , a yield strength of 1200 MPa, a Young's modulus of 890 GPa, and a Poisson's ratio of 0.

07. The drill bit body is made of 35CrMo steel, has a tensile strength of 985 MPa, a yield strength of 835 MPa, a Young's modulus of 207 GPa, and a Poisson's ratio of 0.

25.

7. The method of determining the drillability of a digital core based on mineral composition and mechanical properties according to claim 6, characterized in that, The strength criterion of the final digital core model is specifically set based on a Drucker-Prager plasticity model, and when the final digital core model is drilled by the micro test drill bit model, the deviatoric stress is taken as a factor for the final digital core model to be destroyed in the drilling process, and the Drucker-Prager plasticity model is specifically: τ oct = τ0+ mσ oct ; wherein wherein σ1, σ2, σ3 represent the maximum principal stress, the intermediate principal stress, and the minimum principal stress of the final digital core model, respectively, in units of MPa; and α, k represent parameters related to the material cohesion C and the friction angle ζ of the final digital core model, and σ oct represents the normal stress on the octahedral plane; τ oct represents the shear stress; and τ0 represents the initial shear stress.

8. The method of determining the drillability of a digital core based on mineral composition and mechanical properties according to claim 7, characterized in that, The solution of the damage factor caused by the damage of the final digital core model by the deviatoric stress is specifically to solve the damage factor of the final digital core model by calculating based on the change of the elastic modulus of the rock, and the specific formula is: wherein D is a damage factor, 0≤D≤1; E0is the elastic modulus of the final digital core model in the intact state; E(t) is the elastic modulus of the material of the final digital core model after damage; ε d is the irreversible deformation of the final digital core model, and ε is the total deformation of the final digital core model. During the rock breaking process of the PDC cutting tooth, when the plastic strain of the rock reaches the critical plastic strain, the rock begins to be damaged, at which time the damage factor D=0, and the resistance of the rock decreases with the increase of the plastic strain. When the plastic strain reaches the equivalent plastic strain at which the rock completely fails, the rock element is removed, and the damage factor D=1.

9. The method of determining the drillability of a digital core based on mineral composition and mechanical properties according to claim 8, wherein, The specific steps of S6 are: S6.1, a first level of drilling pressure is applied to the micro test drill bit model of the drillability model; S6.2, drilling is performed on the final digital core model by the micro test drill bit model, wherein the drilling pressure time is t0, the rotational speed of the micro test drill bit model is Ar / min, and A is a positive integer; S6.3, the height from one end of the micro test drill bit model away from the final digital core model to the base surface is obtained by numerical experiment, wherein the base surface and the side of the final digital core model close to the micro test drill bit model are in the same horizontal plane; S6.4, for the same drilling position of the final digital core model, S6.2-S6.3 are repeated twice to obtain three groups of heights from one end of the micro test drill bit model away from the final digital core model to the base surface, which are h0, h1 and h2 respectively, and the drilling pressure times of the last two times are t1 and t2 respectively; S6.5, it is judged whether |h0-h2| is greater than a set height threshold, if yes, jump to S6.8, otherwise, jump to S6.6; S6.6, it is judged whether the drilling pressure level applied to the micro test drill bit model of the drillability model is three, if yes, jump to S6.8, otherwise, jump to S6.7; S6.7, one level of drilling pressure is added to the micro test drill bit model of the drillability model on the basis of the original drilling pressure, and jump to S6.2; S6.8, end.

10. The method of determining the drillability of a digital core based on mineral composition and mechanical properties according to claim 9, wherein, The drillability evaluation of the drillability model is specifically evaluated by a drillability level value, and the specific formula of the drillability level value is: K d = log2t + 2 i-1 - 1; wherein K d drillability level value; t - average value of drilling time, in seconds (s); i - number of weight on bit levels.

Images