A rock compressive strength evaluation method based on detrital mineral components, nano indentation and friction test

Abstract

The application discloses a rock compressive strength evaluation method based on a cutting mineral component, a nano indentation and a friction test, and relates to the field of oil and gas exploration and development, and is characterized in that: firstly, rock mineral phase relative content, nano indentation crack length average value, mineral nano indentation load-depth curve and a friction coefficient are obtained through a rock X-ray diffraction experiment, a nano indentation experiment and a friction experiment; subsequently, based on the energy conservation principle, each rock mineral elastic modulus and each rock mineral plane strain fracture toughness are calculated and obtained; further, according to the principle of the weighted superposition method, each mineral parameter is weighted and integrated according to its proportion to obtain rock fracture toughness characteristic parameters; finally, combined with the rock fracture toughness characteristic parameters and the rock friction coefficient, the rock uniaxial and triaxial compressive strength is calculated and obtained. The application can realize the accurate calculation of the rock compressive strength by using the downhole cutting test analysis, and provides support for the rock compressive strength evaluation of the formation lacking core samples.

Description

Technical Field

[0001] This invention belongs to the field of oil and gas exploration and development, and specifically relates to a method for evaluating the compressive strength of rocks based on rock fragment mineral composition, nanoindentation and friction testing. Background Technology

[0002] Accurate evaluation of rock mechanical properties is crucial for rock mass engineering construction, especially rock compressive strength, a key parameter in rock mass engineering design that directly affects the safety of rock mass structures. Traditional methods for obtaining rock compressive strength involve drilling rock cores to prepare standard cylindrical rock samples, followed by uniaxial or triaxial compression tests. In addition, some researchers have proposed various empirical models to evaluate rock compressive strength using Schmidt hammer tests and point load tests. Furthermore, artificial intelligence technology based on rock physical properties has been explored to address the problem of rock compressive strength evaluation. The principle is to first establish the relationship between rock mechanical properties and rock acoustic-electrophysical properties, and then use neural networks and machine learning models to predict rock compressive strength. While these methods can obtain relatively accurate compressive strength of rocks in the studied strata, they are typically time-consuming and costly. These limitations are particularly pronounced in jointed rock masses or shale formations, as the success rate of standard specimen preparation is low, increasing overall costs and leading to significant waste of valuable rock samples. Therefore, there is an urgent need for a new method to reduce dependence on rock sample size and shape, as well as the need for large amounts of data and analysis time, while still being able to accurately evaluate rock compressive strength.

[0003] With the continuous development of rock surface testing technology, nanoindentation technology has been widely used to study the mechanical behavior of rocks at the microscale. This technique has low requirements for sample size and does not impose strict limitations on sample shape. It involves applying a load of micro- to millinewtons to the surface of a rock sample using a diamond indenter, obtaining a nanoindentation load-depth curve. Based on the characteristics of this curve, micromechanical parameters of the rock, such as elastic modulus, hardness, and fracture toughness, can be determined. Numerous studies have shown a strong correlation between these micromechanical properties, such as the elastic modulus and hardness of the constituent minerals, and the macroscopic compressive strength of the rock. Therefore, the micromechanical properties measured by nanoindentation can be extended to predict the overall behavior of rocks, such as evaluating their compressive strength.

[0004] To overcome the limitations of sample size and shape requirements and reduce the economic and time costs of rock compressive strength evaluation, achieving accurate evaluation of rock compressive strength, this invention proposes a rock compressive strength evaluation method based on rock fragment mineral composition, nanoindentation, and friction testing. This method is based on microscopic fracture mechanics theory. By introducing the average indentation crack length and rock fracture toughness characteristic parameters obtained from rock fragment nanoindentation experiments, and combining them with the rock friction coefficient obtained from friction experiments, a novel rock compressive strength evaluation method is constructed, thereby achieving accurate calculation of rock compressive strength. The theoretical basis for establishing the rock compressive strength evaluation method based on rock fragment mineral composition, nanoindentation, and friction testing is as follows:

[0005] 1. Critical conditions for rock failure

[0006] The distribution of microcracks within a rock is random. If the tips of all microcracks originate from a common point and their directions are all within a defined angular range, as the external load on the rock increases, the shear force acting on the crack surface may gradually exceed the frictional resistance, leading to relative sliding along the crack surface. This sliding will create a concentration of tensile stress at the crack tip, potentially triggering crack initiation. Therefore, a critical condition for rock failure can be established, calculated using the following formula:

[0007]

[0008] In the formula: σ1 is the maximum principal stress, MPa; σ3 is the minimum principal stress, MPa; β is the angle between the principal stress direction and the crack surface, °; μ is the rock friction coefficient, dimensionless; π is pi, dimensionless; c is the average length of the rock nanoindentation crack, μm; K IC For the plane strain fracture toughness of rock, MPa·m 0.5 .

[0009] Simplifying formula (1) yields:

[0010]

[0011] Solving equation (2) yields:

[0012]

[0013] Based on the direction of the microcracks formed in the rock We can obtain:

[0014]

[0015] 2. Rock strength calculation model based on microscopic fracture mechanics

[0016] To evaluate rock fracture failure, a rock failure characteristic parameter needs to be determined as a criterion for complete rock failure. This failure characteristic parameter should be an invariant. This rock failure characteristic parameter is defined as follows:

[0017] First, we transform formula (4) to obtain:

[0018]

[0019] By taking the partial derivative of formula (5) with respect to σ1, the characteristic parameters of rock failure can be obtained.

[0020]

[0021] When σ1 = 0, we can obtain:

[0022]

[0023] Due to rock failure characteristic parameters Since it is an invariant, formulas (6) and (7) are equivalent. Therefore, by combining formulas (6) and (7), we can obtain the formula for calculating the triaxial compressive strength of rock:

[0024]

[0025] The average length of the indentation crack and the plane strain fracture toughness of the rock in Formula (8) can be determined by rock nanoindentation experiments. These are two control parameters of the mechanical response of the rock at the microscale. Summary of the Invention

[0026] This invention aims to address the problem of scarce formation core samples and the high cost and time-consuming nature of conventional rock compressive strength evaluation methods. To solve the difficulty of efficiently and cost-effectively evaluating the compressive strength of formation rocks, this invention proposes a rock compressive strength evaluation method based on rock fragment mineral composition, nanoindentation, and friction testing.

[0027] The technical solution adopted in this invention is as follows:

[0028] Step 1.1: Collect fresh rock fragments from the strata under study, prepare them into dry rock powder samples with a particle size of less than 75 μm, and use an X-ray diffractometer to test the rock mineral composition and the relative content of each rock mineral.

[0029] Step 1.2: Collect fresh rock fragments from the strata under study, select blocky rock fragments with the longest axis diameter of 0.8–1.2 cm, grind and polish them to prepare rock nanoindentation specimens; conduct experiments on the rock nanoindentation specimens using a nanoindentation instrument to obtain the nanoindentation load-depth curves of each rock mineral; observe the residual nanoindentation marks using a metallographic optical microscope, measure the crack length of the rock nanoindentation, and calculate the average value of the crack length of the rock nanoindentation.

[0030] Step 1.3: Calculate the elastic modulus and plane strain fracture toughness of each rock mineral based on the nanoindentation load-depth curve of each rock mineral.

[0031] Step 1.4: Based on the relative content of each rock mineral, the average length of the rock nano-indentation crack, and the plane strain fracture toughness of each rock mineral, the weighted superposition method is used to integrate and calculate the parameters of each mineral according to their proportion to obtain the characteristic parameters of rock fracture toughness.

[0032] Step 1.5: Collect fresh rock fragments from the strata under study, conduct rock fragment friction experiments, and obtain the rock friction coefficient;

[0033] Step 1.6: Based on the rock fracture toughness characteristic parameters and rock friction coefficient, calculate the rock compressive strength using the rock strength calculation model.

[0034] Furthermore, the specific steps of step 1.1 are as follows:

[0035] Step 1.1.1: Collect fresh rock fragments from the strata under study. Use a mortar and pestle to crush the rock fragments into particles with a diameter of <2mm. Take 5g of the crushed particle sample and grind it with an agate mortar and pestle to obtain a rock powder sample with a particle size of less than 75μm. Place the ground rock powder sample in an oven and dry it at 105±5℃ for 4 hours to remove moisture and obtain a dry rock powder sample.

[0036] Step 1.1.2: Weigh about 0.5g of dry rock powder sample, pour it into a glass sample holder, scrape it flat with a scraper, and press the powder sample with a glass plate until it fits tightly against the glass sample holder to obtain the X-ray diffraction test specimen.

[0037] Step 1.1.3: Place the X-ray diffraction test specimen into the X-ray diffractometer for the experiment, record the change of diffraction intensity with the diffraction angle, generate an X-ray diffraction pattern, and analyze to obtain the rock mineral composition and the relative content of each rock mineral.

[0038] Furthermore, the specific steps of step 1.2 are as follows:

[0039] Step 1.2.1: Collect fresh rock fragments from the strata under study. Visually inspect and select rock fragments without obvious cracks. Select blocky rock fragments with the longest axis diameter of 0.8 to 1.2 cm and grind their upper and lower ends flat to obtain rock fragment samples with flat upper and lower ends.

[0040] Step 1.2.2: The upper surface of the rock chip sample with flat upper and lower end faces is polished sequentially using polishing silk, 1μm diamond liquid, and 0.05μm oil-based oxide polishing liquid to obtain a rock nano-indentation specimen with an upper surface roughness ≤10nm.

[0041] Step 1.2.3: Place the rock nanoindentation specimen into the nanoindentation instrument and perform indentation experiments on each rock mineral on the upper surface of the rock nanoindentation specimen at a constant loading rate of 10 mN / s. When the maximum indentation load is 200 mN, maintain the 200 mN load for 15 s, and then unload at a constant unloading rate of 10 mN / s. Record the load and loading depth during the experiment and plot the nanoindentation load-depth curve for each rock mineral.

[0042] Step 1.2.4: Observe the indentation marks of the rock nanoindentation specimen using a metallographic optical microscope, screen out the residual marks with clear indentation cracks, measure the length of the rock nanoindentation crack for each residual mark, and calculate the average length of the rock nanoindentation crack. The calculation formula is as follows:

[0043]

[0044] In the formula: c is the average length of nanoindentation cracks in rock, μm; k is the total number of indentation cracks, an integer; l j Let be the length of the j-th crack, in μm.

[0045] Furthermore, the specific steps of step 1.3 are as follows:

[0046] Step 1.3.1: Based on the nanoindentation load-depth curves of each rock and mineral, calculate the elastic modulus of each rock and mineral. The calculation formula is as follows:

[0047]

[0048] In the formula: E (i) Let be the elastic modulus of the i-th rock mineral, in GPa; π be pi, dimensionless; S be the contact stiffness between the indenter and the rock nanoindentation specimen, in mN / nm, which is equal to the slope of the initial unloading stage of the nanoindentation load-depth curve; α be the geometric constant of the indenter, dimensionless; A c The contact area between the indenter and the rock nanoindentation specimen in the rock nanoindentation experiment is expressed in μm. 2 .

[0049] Step 1.3.2: Fit the nanoindentation load-depth curves of each rock and mineral to obtain the power function equations of the nanoindentation load-depth curves of each rock and mineral during the loading and unloading stages. The expressions are as follows:

[0050] Loading phase:

[0051] Uninstallation phase:

[0052] In the formula: P l(i) For the i-th type of rock mineral nanoindentation load during the loading phase, mN; P max(i) The maximum load of the nanoindentation for the i-th rock mineral is mN; h (i) h represents the depth of the nanoindentation of the i-th rock mineral, in nm. l(i) The depth of the nanoindentation of the i-th rock mineral at the moment when the maximum load is reached, in nm; n (i) P represents the power exponent of the nanoindentation load-depth curve for the i-th rock mineral during the loading phase; it is dimensionless. u(i) For the nanoindentation load of the i-th rock mineral during the unloading phase, mN; h f(i) h represents the depth of the residual nanoindentation of the i-th rock mineral, in nm. m(i) The maximum nanoindentation depth for the i-th rock mineral is given in nm; m. (i) denoted as the power exponent of the nanoindentation load-depth curve of the i-th rock mineral during the unloading phase, which is dimensionless.

[0053] Step 1.3.3: Based on the power function equations of the load-depth curves of nanoindentation for each rock and mineral during the loading and unloading stages, the total energy is obtained by integrating over the loading segment, and the elastic energy is obtained by integrating over the unloading segment. The total energy and elastic energy during the nanoindentation experiment of each rock and mineral are calculated using the following formulas:

[0054]

[0055] In the formula: U t(i) U represents the total energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm; e(i) Let be the elastic energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm.

[0056] Step 1.3.4: Based on the total energy during the nanoindentation experiment of each rock and mineral, calculate the pure plasticity of each rock and mineral during the nanoindentation experiment. The calculation formula is as follows:

[0057]

[0058] In the formula: U pp(i) denoted as , representing the pure plasticity of the i-th rock mineral during the nanoindentation experiment, in mN·μm.

[0059] Step 1.3.5: Based on the total energy, elastic energy, and pure plasticity during the nanoindentation experiment of each rock and mineral, calculate the fracture energy during the nanoindentation experiment of each rock and mineral. The calculation formula is as follows:

[0060]

[0061] In the formula: U c(i) Let be the fracture energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm.

[0062] Step 1.3.6: Based on the fracture energy during the nanoindentation experiment of each rock and mineral, the elastic modulus of each rock and mineral, and the contact area between the indenter and the specimen in the nanoindentation experiment, the plane strain fracture toughness of each rock and mineral is calculated. The calculation formula is as follows:

[0063]

[0064] Where: K IC(i) Let be the plane strain fracture toughness of the i-th rock mineral, in MPa·m 0.5 .

[0065] Furthermore, the formula for calculating the characteristic parameters of rock fracture toughness in step 1.4 is as follows:

[0066]

[0067] Where: K CS Here, is the characteristic parameter of rock fracture toughness, in MPa. This parameter is the equivalent parameter obtained by weighted integration of the plane strain fracture toughness of various rocks and minerals; r is the number of rock and mineral types, an integer; f i denoted as the relative content of the i-th mineral, in %; π is the ratio of pi, dimensionless.

[0068] Furthermore, the specific steps of step 1.5 are as follows:

[0069] Step 1.5.1: Collect fresh rock fragments from the strata under study, select blocky rock fragments with the longest axis diameter of 0.8 to 1.2 cm, grind the test surface of the blocky rock fragments to obtain a friction test rock fragment sample with a flat test surface.

[0070] Step 1.5.2: Fix the rock chip sample for friction test on the fixture of the friction and wear testing machine, and conduct the rock chip friction test under the experimental conditions of normal force of 10N and sliding speed of 0.1mm / s, and record the friction force and normal force data.

[0071] Step 1.5.3: Calculate the friction coefficient based on the frictional force and normal force data recorded in the rock cuttings friction experiment. The calculation formula is as follows:

[0072]

[0073] In the formula: μ is the rock friction coefficient, which is dimensionless; F f Friction force, N; F n The normal force is N.

[0074] Furthermore, the specific steps of step 1.6 are as follows:

[0075] Step 1.6.1: Based on the rock fracture toughness characteristic parameters and rock friction coefficient, calculate the uniaxial compressive strength of the rock. The calculation formula is as follows:

[0076]

[0077] In the formula: σ c σ is the uniaxial compressive strength of the rock, MPa; μ is the coefficient of friction of the rock, dimensionless.

[0078] Step 1.6.2: Based on the rock friction coefficient, rock fracture toughness characteristic parameters, and rock uniaxial compressive strength, calculate the rock triaxial compressive strength. The calculation formula is as follows:

[0079]

[0080] In the formula: σ p σ is the confining pressure, in MPa; σ is the confining pressure. p Triaxial compressive strength of rock under certain conditions, MPa.

[0081] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are:

[0082] 1. This invention, based on the theory of rock microfracture mechanics, describes the propagation behavior of microcracks in rock nanoindentations and quantitatively evaluates the length of these microcracks and the characteristics of the indentation load-depth curve. Furthermore, it uses the mechanical behavior of rock microfracture failure as a basis for judging its compressive strength. The resulting method for evaluating rock compressive strength originates from the microscopic nature of rock structural failure. Compared to traditional evaluation methods based on rock strength criteria, this method not only has a clearer physical meaning but also more accurately predicts the failure behavior and strength characteristics of rocks.

[0083] 2. In this invention, the parameters required for evaluating rock compressive strength can be obtained by utilizing rock fragment mineral composition, nanoindentation, and friction testing, thus enabling the evaluation of rock compressive strength. This invention effectively overcomes the problem of traditional methods failing to effectively evaluate rock compressive strength when formation core samples are scarce, and significantly reduces the economic and time costs of traditional core sample testing for rock compressive strength. Therefore, it provides an effective technical means for evaluating rock compressive strength in rock engineering construction—especially in oil engineering where downhole core samples are difficult to obtain. Attached Figure Description

[0084] Figure 1 This is a flowchart of a method for evaluating the compressive strength of rocks based on rock fragment mineral composition, nanoindentation, and friction testing.

[0085] Figure 2 A diagram showing the mineral composition and relative content of each mineral in the Qiongzhusi Formation shale and Dengying Formation dolomite from Well PS6.

[0086] Figure 3 Image of the nano-indentation specimen after grinding and polishing.

[0087] Figure 4 Nanoindentation load-depth curves for various rocks

[0088] Figure 5 Image showing the measurement of crack length in nanoindentation residues in rock.

[0089] Figure 6 Figure 1 shows the power function equation of the nanoindentation load-depth curve of the Qiongzhusi Formation shale and the Dengying Formation dolomite minerals in Well PS6, and the calculated parameters of the rock compressive strength.

[0090] Figure 7 Statistical results of plane strain fracture toughness and elastic modulus of various rocks and minerals.

[0091] Figure 8 Image of a friction test rock chip sample with a smooth test surface after grinding.

[0092] Figure 9 Error analysis diagram of rock compressive strength evaluation results Detailed Implementation

[0093] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further illustrated below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0094] A flowchart of a rock compressive strength evaluation method based on rock fragment mineral composition, nanoindentation, and friction testing, according to this invention, is shown below. Figure 1 As shown, the specific explanation is as follows:

[0095] 1. Fresh rock fragments from the strata under study were collected and prepared into dry rock powder samples with a particle size of less than 75 μm. X-ray diffraction was used to determine the rock mineral composition and the relative content of each mineral. The specific process is as follows:

[0096] (1) Collect fresh rock fragments from the strata under study. Use a mortar to crush the rock fragments into particles with a diameter of <2mm. Take 5g of the crushed particle sample and grind it with an agate mortar to obtain a rock powder sample with a particle size of less than 75μm to ensure the uniformity of the sample and the accuracy of the test. Place the ground rock powder sample in an oven and dry it at 105±5℃ for 4 hours to remove moisture and obtain a dry rock powder sample.

[0097] (2) Weigh about 0.5g of dry rock powder sample, pour it into the glass sample holder, scrape it flat with a scraper, press the powder sample with a glass plate until it fits tightly against the glass sample holder, so that the sample surface is flush with the sample holder surface, and avoid protrusions or depressions, and prepare X-ray diffraction test specimen.

[0098] (3) Turn on the X-ray diffractometer and preheat it to ensure stable instrument performance. Place the X-ray diffraction test specimen into the X-ray diffractometer, start the test program to scan the specimen, record the change of diffraction intensity with diffraction angle, generate X-ray diffraction pattern, and analyze to obtain rock mineral composition and relative content of each rock mineral.

[0099] 2. Fresh rock fragments from the strata were collected for study. Blocky rock fragments with a longest axis diameter of 0.8–1.2 cm were selected, ground, and polished to prepare rock nanoindentation specimens. The specimens were then tested using a nanoindentation instrument to obtain the load-depth curves of the nanoindentation for each rock mineral. The residual nanoindentation marks were observed using a metallographic optical microscope, and the crack lengths of the rock nanoindentation were measured. The average crack length of the rock nanoindentation was calculated. The specific process is as follows:

[0100] (1) Collect fresh rock fragments from the strata under study. Visually inspect and select rock fragments with no obvious cracks and uniform texture. Select blocky rock fragments with the longest axis diameter of 0.8 to 1.2 cm. Use 400 to 800 mesh metallographic sandpaper to polish the upper and lower end faces until they are flat, and obtain rock fragment samples with flat upper and lower end faces.

[0101] (2) Polishing the upper surface of the rock chip sample with flat upper and lower end faces is done by polishing silk, and then further polishing the upper surface with 1μm diamond liquid, and then finely polishing the upper surface with 0.05μm oil-based oxide polishing liquid. By gradually reducing the abrasive particle size, the surface of the upper surface of the rock chip sample with flat upper and lower end faces is polished to obtain a rock nano-indentation specimen with an upper surface roughness ≤10nm.

[0102] (3) Place the rock nanoindentation specimen into the nanoindentation apparatus and perform indentation experiments on each rock mineral on the upper surface of the rock nanoindentation specimen at a constant loading rate of 10 mN / s. When the maximum indentation load is 200 mN, maintain the 200 mN load for 15 s. Then unload at a constant unloading rate of 10 mN / s and record the load and loading depth during the experiment. Based on this, plot the nanoindentation load-depth curves of each rock mineral.

[0103] (4) The indentation marks of the rock nanoindentation specimens were observed using a metallographic optical microscope. Residual marks with clear indentation cracks were screened out. The length of the rock nanoindentation crack of each residual mark was measured using the measuring tool on the microscope. The average length of the rock nanoindentation crack was calculated using the following formula:

[0104]

[0105] In the formula: c is the average length of nanoindentation cracks in rock, μm; k is the total number of indentation cracks, an integer; l j Let be the length of the j-th crack, in μm.

[0106] 3. Based on the nanoindentation load-depth curves of each rock and mineral, the elastic modulus and plane strain fracture toughness of each rock and mineral are calculated. The specific process is as follows:

[0107] (1) Based on the nanoindentation load-depth curves of each rock and mineral, and combined with the inherent parameters of the nanoindenter, the elastic modulus of each rock and mineral is calculated. The calculation formula is as follows:

[0108]

[0109] In the formula: E (i) Let be the elastic modulus of the i-th rock mineral, in GPa; π be pi, dimensionless; S be the contact stiffness between the indenter and the rock nanoindentation specimen, in mN / nm, which is equal to the slope of the initial unloading stage of the nanoindentation load-depth curve; α be the geometric constant of the indenter, dimensionless; A c The contact area between the indenter and the rock nanoindentation specimen in the rock nanoindentation experiment is expressed in μm. 2 ;

[0110] (2) Fit the nanoindentation load-depth curves of each rock and mineral. For the loading and unloading stages, the power function model is used to process the curve data of each rock and mineral respectively. The parameters are optimized by the least squares method to obtain the power function equations of the nanoindentation load-depth curves of each rock and mineral in the loading and unloading stages. The expressions are as follows:

[0111] Loading phase:

[0112] Uninstallation phase: In the formula: P l(i) For the i-th type of rock mineral nanoindentation load during the loading phase, mN; P max(i) The maximum load of the nanoindentation for the i-th rock mineral is mN; h (i) h represents the depth of the nanoindentation of the i-th rock mineral, in nm. l(i) The depth of the nanoindentation of the i-th rock mineral at the moment when the maximum load is reached, in nm; n (i) P represents the power exponent of the nanoindentation load-depth curve for the i-th rock mineral during the loading phase; it is dimensionless. u(i) For the nanoindentation load of the i-th rock mineral during the unloading phase, mN; h f(i) h represents the depth of the residual nanoindentation of the i-th rock mineral, in nm. m(i) The maximum nanoindentation depth for the i-th rock mineral is given in nm; m. (i) , where is the power exponent of the nanoindentation load-depth curve of the i-th rock mineral during the unloading phase, and is dimensionless;

[0113] (3) Based on the power function equations of the load-depth curves of nanoindentation for each rock and mineral during the loading and unloading stages, the total energy is obtained by integrating over the loading segment, and the elastic energy is obtained by integrating over the unloading segment. The total energy and elastic energy of each rock and mineral during the nanoindentation experiment are calculated using the following formulas:

[0114]

[0115] In the formula: U t(i) U represents the total energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm; e(i) Let be the elastic energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm;

[0116] (4) Based on the total energy during the nanoindentation experiment of each rock and mineral, calculate the pure plasticity of each rock and mineral during the nanoindentation experiment. The calculation formula is as follows:

[0117]

[0118] In the formula: U pp(i) The pure plasticity during the nanoindentation experiment of the i-th rock mineral is expressed in mN·μm.

[0119] (5) Based on the total energy, elastic energy, and pure plasticity of each rock and mineral during the nanoindentation experiment, and according to the energy balance relationship, the fracture energy of each rock and mineral during the nanoindentation experiment is calculated by subtracting the sum of elastic energy and pure plasticity from the total energy. The calculation formula is as follows:

[0120]

[0121] In the formula: U c(i) Let be the fracture energy during the nanoindentation experiment of the i-th rock mineral, in mN·μm;

[0122] (6) Based on the fracture energy, elastic modulus, and contact area between the indenter and the specimen in the nanoindentation test of each rock mineral, the plane strain fracture toughness of each rock mineral is calculated. The calculation formula is as follows:

[0123]

[0124] Where: K IC(i) Let be the plane strain fracture toughness of the i-th rock mineral, in MPa·m 0.5 .

[0125] 4. Based on the relative content of each rock mineral, the average length of the rock nanoindentation crack, and the plane strain fracture toughness of each rock mineral, a weighted superposition method is used to integrate the parameters of each mineral according to their proportion to obtain the characteristic parameters of rock fracture toughness. The calculation formula is as follows:

[0126]

[0127] Where: K CS Here, is the characteristic parameter of rock fracture toughness, in MPa. This parameter is the equivalent parameter obtained by weighted integration of the plane strain fracture toughness of various rocks and minerals; r is the number of rock and mineral types, an integer; f i The relative content of the i-th mineral is %.

[0128] 5. Collect fresh rock fragments from the strata under study, conduct rock fragment friction experiments, and obtain the rock friction coefficient. The specific process is as follows:

[0129] (1) Collect fresh rock fragments from the strata under study, select blocky rock fragments with the longest axis diameter of 0.8 to 1.2 cm, and polish their test surfaces with sandpaper step by step to remove surface protrusions and impurities until they are flat and smooth, thus obtaining a friction test rock fragment sample with a flat test surface.

[0130] (2) Fix the rock chip sample for friction test on the fixture of the friction and wear test machine, set the experimental parameters of normal force 10N and sliding speed 0.1mm / s, start the friction and wear test machine to carry out rock chip friction test, and record the friction force and normal force data in real time during the experiment.

[0131] (3) Calculate the friction coefficient based on the friction force and normal force data recorded in the rock cuttings friction experiment. The calculation formula is as follows:

[0132]

[0133] In the formula: μ is the rock friction coefficient, which is dimensionless; Ff Friction force, N; F n The normal force is N.

[0134] 6. Based on the rock fracture toughness characteristic parameters and rock friction coefficient, and using a rock strength calculation model, the rock compressive strength is calculated. The specific process is as follows:

[0135] (1) Calculate the uniaxial compressive strength of the rock based on the rock fracture toughness characteristic parameters and the rock friction coefficient. The calculation formula is as follows:

[0136]

[0137] In the formula: σ c denoted as uniaxial compressive strength of rock, in MPa.

[0138] (2) Based on the rock friction coefficient, rock fracture toughness characteristic parameters and rock uniaxial compressive strength, and combined with formulas (8) and (18), the rock compressive strength calculation model is obtained as shown in formula (21). The rock compressive strength is calculated using this model:

[0139]

[0140] In the formula: σ p σ is the confining pressure, in MPa; σ is the confining pressure. p Triaxial compressive strength of rock under certain conditions, MPa.

[0141] Implementation Cases

[0142] This case study evaluates the compressive strength of rocks in the Qiongzhusi and Dengying Formations of a well area in the Sichuan Basin. Rock cuttings samples were taken from well PS6 in this area. The specific steps are as follows:

[0143] Step 1: Fresh shale fragments from the 7655–7700m section of the Qiongzhusi Formation and fresh dolomite fragments from the 7990–8010m section of the Dengying Formation in Well PS6 were collected. The fragments were crushed to particles <2mm using a mortar and pestle. 5g of the crushed fragments were taken and ground in an agate mortar to obtain a rock powder sample with a particle size less than 75μm. The ground rock powder sample was placed in an oven and dried at 105±5℃ for 4 hours to remove moisture, obtaining a dry rock powder sample. Approximately 0.5g of the dry rock powder sample was weighed, poured into a glass sample holder, leveled with a scraper, and pressed with a glass plate until it fits tightly against the glass sample holder, thus preparing the X-ray diffraction test specimen. Turn on the X-ray diffractometer and preheat it. Place the X-ray diffraction specimen into the X-ray diffractometer, start the test program to scan the specimen, record the change in diffraction intensity with the diffraction angle, generate an X-ray diffraction pattern, and analyze to obtain the rock mineral composition and the relative content of each rock mineral. The mineral composition and relative content of each mineral in the Qiongzhusi Formation shale and Dengying Formation dolomite of Well PS6 are shown in the figure. Figure 2 .

[0144] Step 2: Fresh shale cuttings from the 7655–7700m section of the Qiongzhusi Formation and fresh dolomite cuttings from the 7990–8010m section of the Dengying Formation in Well PS6 were collected. Cuttings without obvious cracks and with uniform texture were visually inspected and selected. Blocky cuttings with a longest axis diameter of 0.8–1.2 cm were selected and their upper and lower surfaces were progressively polished with 400–800 grit metallographic sandpaper until smooth. The upper surface was then polished sequentially using polishing silk, 1μm diamond liquid, and 0.05μm oil-based oxide polishing liquid to obtain a rock nanoindentation specimen with an upper surface roughness ≤10nm. The polished rock nanoindentation specimen is shown below. Figure 3 Polished rock nanoindentation specimens from the Qiongzhusi and Dengying Formations of Well PS6 were placed in a nanoindenter. Indentation experiments were performed on the upper surface of each rock mineral using a constant loading rate of 10 mN / s. The maximum indentation load of 200 mN was maintained for 15 seconds. Then, unloading was performed at a constant unloading rate of 10 mN / s. Real-time load and loading depth data were accurately recorded throughout the process, and nanoindentation load-depth curves for each rock mineral were plotted. The nanoindentation load-depth curves for each rock mineral are shown below. Figure 4 The indentation marks of the nanoindentation specimens were observed using a metallographic optical microscope. Residual indentations with clear indentation cracks were selected. The length of the rock nanoindentation cracks in each residual indentation was measured using the microscope's built-in measuring tool. The measurement of the rock nanoindentation crack lengths in the nanoindentation residual indentations is shown in the figure. Figure 5 Then, the average length of the nano-indentation crack in the rock is calculated using formula (9).

[0145] Step 3: Based on the nanoindentation load-depth curves of various rocks and minerals in the Qiongzhusi Formation shale and Dengying Formation dolomite of Well PS6, Figure 6 The basic parameters are as follows: the geometric constant of the indenter in the rock nanoindentation experiment is taken as 1.034. The elastic modulus of each rock mineral is calculated using formula (10). The statistical results of the elastic modulus of each rock mineral are shown in the figure. Figure 7 The load-depth curves of nanoindentation for each rock and mineral were fitted, and the power function equations for the load-depth curves of nanoindentation for each rock and mineral during the loading and unloading stages are shown in the figure. Figure 6 Because the contents of clay minerals, quartz, calcite, and feldspar in the dolomite of the Dengying Formation in well PS6 are all less than 1%, and the dolomite content is 98.3% (see...). Figure 2In this implementation case, only dolomite mineral is considered in the strength evaluation of dolomite. According to the power function equation of the nanoindentation load-depth curve of each rock mineral, the total energy and elastic energy of each rock mineral in the nanoindentation experiment are calculated using formulas (13) and (14). According to the total energy of each rock mineral in the nanoindentation experiment, the pure plastic properties of each rock mineral in the nanoindentation experiment are calculated using formula (15). According to the total energy, elastic energy and pure plastic properties of each rock mineral in the nanoindentation experiment, the fracture energy of each rock mineral in the nanoindentation experiment is calculated using formula (16). According to the fracture energy of each rock mineral in the nanoindentation experiment, the elastic modulus of each rock mineral, and the contact area between the rock nanoindentation experiment indenter and the rock nanoindentation specimen, the plane strain fracture toughness of each rock mineral is calculated using formula (17). The statistical results of the plane strain fracture toughness of each rock mineral are shown in […]. Figure 7 .

[0146] Step 4: Based on the relative content of each rock mineral, the average length of the rock nano-indentation crack, and the plane strain fracture toughness of each rock mineral, the characteristic parameters of rock fracture toughness are calculated using formula (18).

[0147] Step 5: Collect fresh shale cuttings from the 7655–7700m section of the Qiongzhusi Formation and fresh dolomite cuttings from the 7990–8010m section of the Dengying Formation in Well PS6. Select blocky cuttings with a longest axis diameter of 0.8–1.2 cm, and polish their test surfaces with sandpaper step by step to remove surface protrusions and impurities until they are smooth and flat, obtaining friction test cuttings samples with smooth test surfaces. See [link to relevant documentation]. Figure 8 The rock chip sample for the friction test was fixed on the fixture of the friction and wear testing machine. The experimental parameters were set to 10 N normal force and 0.1 mm / s sliding speed. The friction and wear testing machine was started to carry out the rock chip friction test. The friction force and normal force data during the experiment were recorded in real time. The friction coefficient was calculated according to formula (19).

[0148] Step 6: Calculate the uniaxial compressive strength of the rock using formula (20) based on the rock fracture toughness characteristic parameters and the rock friction coefficient. Calculate the triaxial compressive strength of the rock using formula (21) based on the rock friction coefficient, rock fracture toughness characteristic parameters, and rock uniaxial compressive strength. A comparison between the rock compressive strength calculated by the method described in this invention and the results of the rock triaxial compression test is shown below. Figure 9 .from Figure 9 As can be seen from the above, the determination coefficient R of the shale compressive strength calculated using the method of the present invention is... 2 The mean absolute percentage error (AAREP) and root mean square error (RMSE) were 0.8967, 8.28%, and 16.89, respectively. The Rcompressive strength of the dolomite was calculated. 2The AAREP and RMSE values ​​were 0.9659, 4.77%, and 15.57, respectively, indicating that the rock compressive strength evaluated by this invention has high accuracy.

[0149] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for evaluating the compressive strength of rocks based on rock fragment mineral composition, nanoindentation, and friction testing, characterized in that, The method includes the following steps: Step 1.1: Collect fresh rock fragments from the strata under study, prepare them into dry rock powder samples with a particle size of less than 75µm, and use an X-ray diffractometer to test the rock mineral composition and the relative content of each rock mineral; Step 1.2: Collect fresh rock fragments from the strata under study, select blocky rock fragments with the longest axis diameter of 0.8~1.2cm, grind and polish them to prepare rock nanoindentation specimens; conduct experiments on the rock nanoindentation specimens using a nanoindentation instrument to obtain the nanoindentation load-depth curves of each rock mineral; observe the residual nanoindentation marks using a metallographic optical microscope, measure the crack length of the rock nanoindentation, and calculate the average crack length of the rock nanoindentation. Step 1.3: Based on the nanoindentation load-depth curves of each rock and mineral, calculate the elastic modulus and plane strain fracture toughness of each rock and mineral. Step 1.4: Based on the relative content of each rock mineral, the average length of the rock nano-indentation crack, and the plane strain fracture toughness of each rock mineral, the weighted superposition method is used to integrate and calculate the parameters of each mineral according to their proportion to obtain the characteristic parameters of rock fracture toughness. Step 1.5: Collect fresh rock fragments from the strata under study, conduct rock fragment friction experiments, and obtain the rock friction coefficient; Step 1.6: Based on the rock fracture toughness characteristic parameters and rock friction coefficient, calculate the rock compressive strength using the rock strength calculation model; The formula for calculating the characteristic parameters of rock fracture toughness in step 1.4 is as follows: In the formula: K CS The characteristic parameter of rock fracture toughness is MPa. This parameter is the equivalent parameter after weighted integration of the plane strain fracture toughness of various rocks and minerals. r The number of rock mineral types, an integer; f i For the first i The relative content of the minerals, % K IC(i) For the first i Planar strain fracture toughness of a type of rock mineral, MPa·m 0.5 π is the ratio of a circle's circumference to its diameter, and it is dimensionless. c The average length of nanoindentation cracks in the rock; The specific steps of step 1.6 are as follows: Step 1.6.1: Based on the rock fracture toughness characteristic parameters and rock friction coefficient, calculate the uniaxial compressive strength of the rock. The calculation formula is as follows: In the formula: σ c The uniaxial compressive strength of the rock is given in MPa. μ is the rock friction coefficient, dimensionless; Step 1.6.2: Based on the rock friction coefficient, rock fracture toughness characteristic parameters, and rock uniaxial compressive strength, calculate the rock triaxial compressive strength. The calculation formula is as follows: In the formula: σ p The confining pressure is in MPa. σ The confining pressure is σ p Triaxial compressive strength of rock under certain conditions, MPa.

2. The method for evaluating the compressive strength of rock based on rock fragment mineral composition, nanoindentation, and friction testing according to claim 1, characterized in that, The specific steps of step 1.1 are as follows: Step 1.1.1: Collect fresh rock fragments from the strata under study. Use a mortar and pestle to crush the rock fragments into particles with a diameter of <2mm. Take 5g of the crushed particle sample and grind it with an agate mortar and pestle to obtain a rock powder sample with a particle size of less than 75μm. Place the ground rock powder sample in an oven and dry it at 105±5℃ for 4 hours to remove moisture and obtain a dry rock powder sample. Step 1.1.2: Weigh about 0.5g of dry rock powder sample, pour it into the glass sample holder, scrape it flat with a scraper, and press the powder sample with a glass plate until it fits tightly against the glass sample holder to obtain the X-ray diffraction test specimen. Step 1.1.3: Place the X-ray diffraction test specimen into the X-ray diffractometer for the experiment, record the change of diffraction intensity with the diffraction angle, generate an X-ray diffraction pattern, and analyze to obtain the rock mineral composition and the relative content of each rock mineral.

3. The method for evaluating the compressive strength of rock based on rock fragment mineral composition, nanoindentation, and friction testing according to claim 1, characterized in that, The specific steps of step 1.2 are as follows: Step 1.2.1: Collect fresh rock fragments from the strata under study, visually inspect and screen out rock fragments without obvious cracks, select blocky rock fragments with the longest axis diameter of 0.8~1.2cm, grind the upper and lower end faces of the fragments to obtain rock fragment samples with flat upper and lower end faces. Step 1.2.2: The upper surface of the rock chip sample with flat upper and lower end faces is polished sequentially using polishing silk, 1μm diamond liquid, and 0.05μm oil-based oxide polishing liquid to obtain a rock nano-indentation specimen with an upper surface roughness ≤10nm. Step 1.2.3: Place the rock nanoindentation specimen into the nanoindenter and perform indentation experiments on each rock mineral on the upper surface of the rock nanoindentation specimen at a constant loading rate of 10 mN / s. When the maximum indentation load is 200 mN, maintain the 200 mN load for 15 s, and then unload at a constant unloading rate of 10 mN / s. Record the load and loading depth during the experiment and plot the nanoindentation load-depth curve for each rock mineral. Step 1.2.4: Observe the indentation marks of the rock nanoindentation specimen using a metallographic optical microscope, screen out the residual marks with clear indentation cracks, measure the length of the rock nanoindentation crack for each residual mark, and calculate the average length of the rock nanoindentation crack. The calculation formula is as follows: In the formula: c The average length of nanoindentation cracks in the rock; k The total number of indentation cracks, an integer; l j For the first j The length of the crack.

4. The method for evaluating the compressive strength of rock based on rock fragment mineral composition, nanoindentation, and friction testing according to claim 1, characterized in that, The specific steps of step 1.3 are as follows: Step 1.3.1: Based on the nanoindentation load-depth curves of each rock and mineral, calculate the elastic modulus of each rock and mineral. The calculation formula is as follows: In the formula: E (i) For the first i The elastic modulus of a rock mineral, in GPa; π is pi, dimensionless; S The contact stiffness between the indenter and the rock nanoindentation specimen in the rock nanoindentation experiment is expressed in mN / nm, and its value is equal to the slope of the initial stage of the unloading of the nanoindentation load-depth curve. α is the geometric constant of the indenter in rock nanoindentation experiments, which is dimensionless; A c The contact area between the indenter and the rock nanoindentation specimen in the rock nanoindentation experiment is expressed in μm. 2 ; Step 1.3.2: Fit the nanoindentation load-depth curves of each rock and mineral to obtain the power function equations of the nanoindentation load-depth curves of each rock and mineral during the loading and unloading stages. The expressions are as follows: In the formula: P l(i) For the loading phase i Nanoindentation load on various rocks and minerals, mN; P max(i) For the first i Maximum load of nanoindentation in a certain type of rock mineral, mN; h (i) For the first i Nanoindentation depth of rock minerals, in nm; h l(i) The moment when the maximum load is just reached i Nanoindentation depth of rock minerals, in nm; n (i) For the loading phase i The power exponent of the nanoindentation load-depth curve of a certain rock mineral, dimensionless; P u(i) For the uninstallation phase i Nanoindentation load on various rocks and minerals, mN; h f(i) For the first i The depth of residual nanoindentations in rock minerals, in nm; h m(i) For the first i Maximum nanoindentation depth of a certain rock mineral, in nm; m (i) For the uninstallation phase i The power exponent of the nanoindentation load-depth curve of a certain rock mineral, dimensionless; Step 1.3.3: Based on the power function equations of the load-depth curves of nanoindentation for each rock and mineral during the loading and unloading stages, the total energy is obtained by integrating over the loading segment, and the elastic energy is obtained by integrating over the unloading segment. The total energy and elastic energy during the nanoindentation experiment of each rock and mineral are calculated using the following formulas: In the formula: U t(i) For the first i The total energy during the nanoindentation experiment on a rock and mineral, mN·μm; U e(i) For the first i Elastic energy during nanoindentation experiments on various rocks and minerals, mN·μm; Step 1.3.4: Based on the total energy during the nanoindentation experiment of each rock and mineral, calculate the pure plasticity of each rock and mineral during the nanoindentation experiment. The calculation formula is as follows: In the formula: U pp(i) For the first i Pure plasticity of various rocks and minerals during nanoindentation experiments, mN·μm; Step 1.3.5: Based on the total energy, elastic energy, and pure plasticity during the nanoindentation experiment of each rock and mineral, calculate the fracture energy during the nanoindentation experiment of each rock and mineral. The calculation formula is as follows: In the formula: U c(i) For the first i Fracture energy during nanoindentation experiments on various rocks and minerals, mN·μm; Step 1.3.6: Based on the fracture energy during the nanoindentation experiment of each rock and mineral, the elastic modulus of each rock and mineral, and the contact area between the indenter and the specimen in the nanoindentation experiment, the plane strain fracture toughness of each rock and mineral is calculated. The calculation formula is as follows: In the formula: K IC(i) For the first i Planar strain fracture toughness of a type of rock mineral, MPa·m 0.5 .

5. The method for evaluating the compressive strength of rock based on rock fragment mineral composition, nanoindentation, and friction testing according to claim 1, characterized in that, The specific steps of step 1.5 are as follows: Step 1.5.1: Collect fresh rock fragments from the strata under study, select blocky rock fragments with the longest axis diameter of 0.8~1.2cm, grind the test surface of the blocky rock fragments to obtain a friction test rock fragment sample with a flat test surface; Step 1.5.2: Fix the rock cutting sample for friction test on the fixture of the friction and wear testing machine, and conduct the rock cutting friction test under the experimental conditions of normal force of 10N and sliding speed of 0.1mm / s, and record the friction force and normal force data; Step 1.5.3: Calculate the friction coefficient based on the frictional force and normal force data recorded in the rock cuttings friction experiment. The calculation formula is as follows: In the formula: μ is the rock friction coefficient, dimensionless; F f Friction force, N; F n The normal force is N.

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