Rock main fracture direction prediction method based on wave velocity and nuclear magnetic imaging

Abstract

The application discloses a rock main fracture direction prediction method based on wave velocity and nuclear magnetic imaging, relates to the rock fracture prediction technical field, and comprises the following steps: step one, sampling and numbering the sample, and dividing the sample into five horizons according to the height; step two, measuring the 360° ring wave velocity of each horizon, and obtaining rock initial P wave and S wave data; step three, nuclear magnetic test before damage; step four, mechanical damage test, loading and damaging the sample into five groups, synchronously collecting acoustic emission AE signals and testing the real-time wave velocity in different stress stages during the loading process; step five, nuclear magnetic test after damage; and step six, data arrangement.The rock main fracture direction prediction method based on wave velocity and nuclear magnetic imaging is adopted, the fracture direction is pre-judged, the instability early warning is supported, different confining pressure environments are adapted, multiple means are mutually verified, and the prediction result reliability is improved.

Description

Technical Field

[0001] This invention relates to the field of rock fracture prediction technology, and in particular to a method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging. Background Technology

[0002] Predicting rock failure using wave velocity variation characteristics has gradually become an important direction in rock mechanics research. Further exploration is needed regarding the quantitative coupling relationship between confining pressure, wave velocity, and damage in randomly fractured limestone, as well as failure prediction models considering in-situ stress release. The development characteristics of natural fractures and the complexity of their impact on surrounding rock stability have received widespread attention; however, most studies focus on the mechanical characteristics and fracture mechanisms of regularly developed fractures. Systematic research on the wave velocity evolution and failure prediction methods for heterogeneous fractured limestone under different confining pressure conditions is still lacking. Especially for heterogeneous rocks containing primary fractures, the combined effect of internal fracture structure and stress conditions causes wave velocity to exhibit directional and complex variation patterns. Particularly under confining pressure, the fracture closure and propagation processes further affect wave velocity propagation characteristics, thus increasing the complexity of failure prediction. Summary of the Invention

[0003] The purpose of this invention is to provide a method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging, which solves the problem that existing methods for predicting the main fracture direction of rocks lack research on heterogeneous fractured limestone.

[0004] To achieve the above objectives, this invention provides a method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging, comprising the following steps: Step 1: Sampling and numbering the samples. Rock samples are taken on-site and processed to prepare standard bodies with a diameter-to-height ratio of 2:1. The prepared samples are randomly named and numbered and divided into 5 layers according to height. Step 2: Wave velocity measurement. 360° circumferential wave velocity measurements are performed on each layer at 20° intervals. Each angle is tested 3 times and the average value is taken to obtain the initial P-wave and S-wave data of the sample. Step 3: Pre-destruction NMR test. The sample is placed in a triaxial synchronous loading multi-field coupled NMR imaging system and saturated with water for 24 hours under vacuum pressure of 40 MPa. The NMR test is performed on the water-saturated sample before destruction to obtain the initial porosity distribution and T2 spectrum characteristics, and NMR imaging is performed. Step 4: Mechanical destructive test. The specimens are divided into five groups and confining pressure conditions are set for each group. The confining pressure conditions are calculated by the earth pressure formula. The multi-field coupled mechanical test system developed by deep earth resources is used to carry out loading and destructive testing. Acoustic emission (AE) signal acquisition and real-time wave velocity testing are carried out simultaneously at different stress stages during the loading process. Step 5: Post-destruction NMR test. The sample after mechanical destruction is subjected to NMR test again to obtain the initial porosity distribution and T2 spectrum characteristics after destruction, and NMR imaging is performed. Step 6: Data processing. Integrate initial wave velocity anisotropy data, AE and dynamic wave velocity data throughout the loading process, and NMR data before and after damage to study the damage evolution law of rocks under multi-field coupling conditions.

[0005] Preferably, in step two, the reflection and transmission behavior of the P-wave at the fracture interface is described by the Zoeppritz equation or its linearized form, and the reflection coefficient... With transmission coefficient for: In the formula, It is the wave impedance.

[0006] Preferably, the longitudinal wave velocity of the sample tested in step two is... and transverse wave velocity Length is: In the formula, This refers to the distance between the centers of the two pressure heads when they are joined together. This refers to the P-wave propagation time. The propagation time of the S-wave; This is the built-in time.

[0007] Preferably, in step three The relationship between the pore geometry and the pore characteristics is expressed as follows: In the formula, This refers to the surface relaxation time caused by the interaction between the fluid and the pore wall. The transverse surface relaxation strength. It is the ratio of pore surface area to volume.

[0008] Preferably, in step three, the nuclear magnetic resonance imaging originates from the spatial coding mechanism of the nuclear magnetic signal under the action of a magnetic field gradient. Under low-field conditions, the NMR imaging signal intensity at spatial location r is expressed as: In the formula, For nuclear magnetic resonance imaging signal intensity, For hydrogen nucleus density, For the repetition time, The interval time, and These represent the local longitudinal and lateral relaxation times, respectively.

[0009] Preferably, in step four , The value is defined as follows: In the formula, The average frequency of the acoustic emission signal. The steepness of the rise in acoustic emission signal. The number of rings exceeding the threshold. For duration, For the rising time, The amplitude is denoted as .

[0010] Preferably, the mechanical properties in step four are determined using the elastic modulus and Poisson's ratio.

[0011] Preferably, the rock pore sizes in steps three and five are categorized as follows: micropore diameter range is... Corresponding to capillary-bound fluid; mesopore diameter range is The range of large pore diameters is .

[0012] Therefore, the present invention employs the aforementioned method for predicting the main fracture direction of rocks based on wave velocity and NMR imaging. Its advantages include the ability to effectively predict the main failure direction before rock instability occurs by combining indicators such as wave velocity reduction and cumulative acoustic emission count. It adapts to different confining pressure environments, uses multiple methods to corroborate each other, and improves the reliability of the prediction results. Before failure, acoustic emission energy will directionally accumulate along the direction of maximum wave velocity, providing evidence for the prediction conclusion from the perspective of dynamic fracture evolution. Post-failure NMR imaging can directly verify the spatial location of the main fracture.

[0013] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0014] Figure 1 This is a schematic diagram of a structural embodiment of the rock main fracture direction prediction method based on wave velocity and nuclear magnetic resonance imaging according to the present invention. Figure 2 This is a front view of an embodiment of the rock main fracture direction prediction method based on wave velocity and nuclear magnetic resonance imaging of the present invention; Figure 3 This is a side view of an embodiment of the rock main fracture direction prediction method based on wave velocity and nuclear magnetic resonance imaging of the present invention; Figure 4 This is an initial wave velocity characteristic diagram of the limestone of this invention. Figure 4 (a) is a relatively intact limestone rock sample L4. Figure 4(b) Sample L1 is a limestone rock specimen filled with white calcite veins. Figure 4 (c) is sample L11, a gray-black fractured matrix limestone rock. Figure 4 (d) Rock sample L17, which contains both white calcite vein filling and gray-black fracture matrix. Figure 4 (e) is parallel type limestone rock sample L7, Figure 4 (f) is sample L16 of oblique-type limestone rock; Figure 5 This is a schematic diagram showing the direction of maximum wave velocity in the limestone sample of this invention. Figure 5 (a) is a relatively intact limestone rock sample L4. Figure 5 (b) Sample L1 is a limestone rock specimen filled with white calcite veins. Figure 5 (c) is sample L11, a gray-black fractured matrix limestone rock. Figure 5 (d) Rock sample L17, which contains both white calcite vein filling and gray-black fracture matrix. Figure 5 (e) is parallel type limestone rock sample L7, Figure 5 (f) is sample L16 of oblique-type limestone rock; Figure 6 The stress-strain curves of limestone under uniaxial UCS are shown. Figure 7 The stress-strain curves of limestone under a confining pressure of 5 MPa are shown. Figure 8 The stress-strain curve of limestone under a confining pressure of 15 MPa; Figure 9 The stress-strain curves of limestone under a confining pressure of 25 MPa are shown. Figure 10 The stress-strain curve of limestone under a confining pressure of 35 MPa; Figure 11 This is a diagram showing the acoustic emission location and energy distribution during the loading process of the rock sample in this invention. Figure 11 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 11 (b) Rock sample L9 is confined under a pressure of 15 MPa. Figure 11 (c) Rock sample L17 is confined under a pressure of 35 MPa; Figure 12 This is a diagram showing the rock failure types of the rock sample from this invention. Figure 12 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 12 (b) Rock sample L9 is confined under a pressure of 15 MPa. Figure 12 (c) Rock sample L17 is confined under a pressure of 35 MPa; Figure 13 This is a diagram showing the distribution of T2 in limestone under different confining pressures according to the present invention. Figure 13 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 13(b) The confining pressure of rock sample L6 is 5 MPa. Figure 13 (c) Rock sample L9 is under a confining pressure of 15 MPa. Figure 13 (d) represents the confining pressure of rock sample L16 at 25 MPa. Figure 13 (e) is the confining pressure of rock sample L17 at 35 MPa; Figure 14 This is a diagram showing the distribution of limestone pore size under different confining pressures according to the present invention. Figure 14 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 14 (b) The confining pressure of rock sample L6 is 5 MPa. Figure 14 (c) Rock sample L9 is under a confining pressure of 15 MPa. Figure 14 (d) represents the confining pressure of rock sample L16 at 25 MPa. Figure 14 (e) is the confining pressure of rock sample L17 at 35 MPa; Figure 15 This is a diagram showing the cumulative porosity distribution characteristics of limestone before and after failure under different confining pressures, according to the present invention. Figure 15 (a) before the destruction, Figure 15 (b) after the destruction; Figure 16 These are the T1-T2 spectra of limestone before failure under different confining pressures according to the present invention. Figure 16 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 16 (b) The confining pressure of rock sample L6 is 5 MPa. Figure 16 (c) Rock sample L9 is under a confining pressure of 15 MPa. Figure 16 (d) represents the confining pressure of rock sample L16 at 25 MPa. Figure 16 (e) is the confining pressure of rock sample L17 at 35 MPa; Figure 17 These are the T1-T2 spectra of limestone after failure under different confining pressures according to the present invention. Figure 17 (a) is the confining pressure of rock sample L2, which is 0 MPa. Figure 17 (b) The confining pressure of rock sample L6 is 5 MPa. Figure 17 (c) Rock sample L9 is under a confining pressure of 15 MPa. Figure 17 (d) represents the confining pressure of rock sample L16 at 25 MPa. Figure 17 (e) is the confining pressure of rock sample L17 at 35 MPa; Figure 18 These are nuclear magnetic resonance imaging (NMR) images of the rock sample before and after failure under the confining pressure L2 of this invention. Figure 18 (a) To disrupt the pre-confrontational MRI, Figure 18 (b) To disrupt forward-looking MRI, Figure 18 (c) is the main-view MRI image after destruction. Figure 18 (d) is a top-down MRI image after the damage. Detailed Implementation

[0015] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0016] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning understood by one of ordinary skill in the art to which this invention pertains. The terms "first," "second," and similar terms used in this invention do not indicate any order, quantity, or importance, but are merely used to distinguish different components. Terms such as "comprising" or "including" mean that the element or object preceding the word encompasses the elements or objects listed following the word and their equivalents, without excluding other elements or objects. Terms such as "connected" or "linked" are not limited to physical or mechanical connections, but can include electrical connections, whether direct or indirect. Terms such as "upper," "lower," "left," and "right" are used only to indicate relative positional relationships; when the absolute position of the described object changes, the relative positional relationship may also change accordingly.

[0017] Example Please see Figures 1-18 This invention provides a method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging, comprising the following steps: Step 1: Sampling and numbering of samples. Rock samples are taken on-site and processed to prepare standard cylinders with a diameter of 50 mm and a height of 100 mm. The prepared limestone samples are randomly named and numbered and divided into 5 layers according to height: 10, 25, 40, 55, and 70 mm layers.

[0018] Step 2: Wave velocity measurement. 360° circumferential wave velocity measurements are performed on each layer at 20° intervals. Each angle is tested 3 times and the average value is taken to obtain the initial P-wave and S-wave data of the rock.

[0019] For an isotropic elastic medium, the propagation velocities of the P-wave and S-wave are respectively: In the formula, Bulk modulus Shear modulus For density.

[0020] The presence of fissures primarily weakens the volumetric compressibility of the rock mass, thus affecting its bulk modulus. For shear modulus The changes are relatively limited. The reflection and transmission behavior of P-waves at the fracture interface can be described by the Zoeppritz equation or its linearized form, and its approximate reflection coefficient... With transmission coefficient is: where, is the wave impedance.

[0021] Discontinuity of wave impedance at fracture interfaces will cause amplitude variation. In laboratory rock wave velocity tests, as can be seen from the velocity formula, the longitudinal wave velocity of the rock under test and the shear wave velocity are expressed as: where, is the distance between centers when two indenters are butted; is the P-wave propagation time; is the S-wave propagation time; is the built-in time.

[0022] Step 3: Nuclear magnetic test before failure. Place the specimen in a triaxial synchronous loading multi-field coupled nuclear magnetic resonance imaging system, perform vacuum pressurization at 40 MPa for 24 hours to saturate the specimen with water, conduct a nuclear magnetic test on the water-saturated specimen before failure to obtain the initial pore distribution and T2 spectrum characteristics, and perform NMRI, where NMRI refers to nuclear magnetic resonance imaging.

[0023] Step 4: Mechanical failure test. Divide the specimens into five groups and set confining pressure conditions respectively, use the deep resource development multi-field coupled mechanical test system to perform loading failure, and synchronously perform AE signal acquisition and real-time wave velocity test at different stress stages during the loading process, where AE refers to acoustic emission.

[0024] The RA-AF distribution method realizes crack classification by virtue of the characteristic difference of AE signals under tensile failure and shear failure modes. During tensile failure, energy is released instantaneously, and the corresponding AE signal is characterized by fast rising speed, short duration, large amplitude, and many ringing counts; the energy release process of shear failure is more gentle, and the AE signal characteristics are completely opposite to those of tensile failure. Based on this difference, shear failure exhibits high RA value and low AF value in RA-AF analysis, while tensile failure presents the characteristics of high AF value and low RA value.

[0025] As shown in Figure 2 , AF / RA<k represents the shear crack propagation mode, while AF / RA>k indicates the occurrence of the tensile crack propagation mode, and many scholars take the diagonal slope k of 1 as the threshold for distinguishing shear and tensile types. The AF and RA values are as follows: where, The average frequency of the acoustic emission signal. The steepness of the rise in acoustic emission signal. The number of rings exceeding the threshold. For duration, For the rising time, The amplitude is denoted as .

[0026] Step 5: Post-destruction NMR test. The limestone sample after mechanical destruction is subjected to NMR test again to obtain the initial porosity distribution and T2 spectrum characteristics after destruction, and NMR imaging is performed.

[0027] In steps three and five, the NMR signals originate from the magnetic moment and angular momentum of hydrogen nuclei in the porous fluid. When a saturated sample is placed in a stable magnetic field and excited by a radio frequency pulse, the hydrogen nuclei generate measurable relaxation signals, the characteristics of which are closely related to the internal pore structure and geometric properties of the material. From a microscopic perspective, atomic nuclei in a high-energy state gradually recover to a low-energy state after the radio frequency pulse is removed; this recovery process is called… Relaxation. After the radio frequency pulse is removed, a phase difference gradually appears in the in-phase atomic nuclei, and the transverse magnetization vector gradually disappears. This decay process is called relaxation. Relaxation.

[0028] Compared to , It has advantages such as short testing time and strong sensitivity to pore size distribution, and is therefore more widely used in the study of pore structure in rocks and coal. For fluid-saturated porous media, the transverse relaxation time... It can be represented as: In the formula, Let be the volume relaxation time of the fluid. This refers to the surface relaxation time caused by the interaction between the fluid and the pore wall. This represents the diffusion relaxation time caused by molecular diffusion.

[0029] Under laboratory conditions, when using the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence for testing, the magnetic field distribution is relatively uniform, and low-viscosity fluids (such as water) are typically used as the pore fluid. Under these conditions, the effects of bulk relaxation and diffusion relaxation are negligible, and T2 is mainly controlled by surface relaxation mechanisms. Therefore, the relationship between T2 and pore geometry can be approximated as: In the formula, This refers to the surface relaxation time caused by the interaction between the fluid and the pore wall. The transverse surface relaxation strength. It is the ratio of pore surface area to volume.

[0030] Since the S / V ratio is inversely proportional to the pore size, smaller pores correspond to shorter T2 values, while larger pores or fissures exhibit longer T2 relaxation times. Using a two-dimensional inversion formula, the two-dimensional spectrum of the T1-T2 fluid signal can be obtained, where T1 / T2 reflects the fluid's flowability. A higher T1 / T2 value indicates poorer fluid flowability; conversely, a lower T1 / T2 value indicates stronger fluid flowability. Figure 3 As shown, Region I is generally defined as hydrogen in hard organic materials; Region II represents hydrogen in hydroxyl groups; Region III represents inorganic pores or bound water in kerogen; Organic micropores / nanopores (IV) (the water in these organic pores is bound by surface tension and has low fluidity); Inorganic mesopores (V) (the water is relatively free but still partially bound and has moderate fluidity); Macropores / cracks (VI) (representing water in inorganic macropores (>50 nm) or cracks, which is almost free-flowing and has high fluidity). The region associated with methane in porous media (VII) represents methane (CH4) in porous media. Methane, as a gas, has a long relaxation time and the highest fluidity.

[0031] In addition to T2 spectral analysis, nuclear magnetic resonance imaging (NMR imaging) can provide visualization information on the spatial distribution of pore fluids and structural heterogeneity within rocks and coal. By applying a controllable magnetic field gradient, NMR imaging can locate nuclear magnetic signals at different spatial locations, enabling imaging characterization of pore fluid migration and fracture development characteristics within the sample.

[0032] The theoretical basis of nuclear magnetic resonance imaging (NMR) stems from the spatial encoding mechanism of NMR signals under the influence of a magnetic field gradient. Under low-field conditions, the NMR imaging signal intensity at spatial location r can be expressed as: In the formula, For nuclear magnetic resonance imaging signal intensity, For hydrogen nucleus density, and These are the local longitudinal and lateral relaxation times, respectively. The repetition time is the time interval between two consecutive radio frequency excitation pulses (typically 90° excitation pulses). The interval time is the time elapsed from the moment the radio frequency excitation pulse (90° pulse) is emitted until the peak signal is acquired by the receiver and the echo signal is generated. Under low field, water-saturated conditions and short echo time, the imaging signal is mainly controlled by the hydrogen nucleus density and T2 relaxation characteristics.

[0033] Due to the complex characteristics of rock pores, a unified standard for classifying pore size using T2 has not yet been established. In this experiment, the rock pore sizes were categorized as follows: micropores with a diameter range of r < 0.1 μm, corresponding to capillary-bound fluids; mesopores with a diameter range of 0.1 μm < 1 μm; and macropores with a diameter range of r > 1 μm, both corresponding to movable fluid pores.

[0034] Step 6: Data processing. Integrate initial wave velocity anisotropy data, AE and dynamic wave velocity data throughout the loading process, and NMR data before and after damage to study the damage evolution law of rocks under multi-field coupling conditions.

[0035] Specific example: The limestone sample was taken from a mine in the province. The limestone core was relatively intact, with random white calcite vein structures and a small number of karst pores. After sampling, the on-site samples underwent secondary processing to prepare standard cylinders with a diameter of 50 mm and a height of 100 mm, conforming to international rock mechanics testing standards. The prepared limestone samples were randomly named L1 to L20. Table 1 shows the basic information of the tested limestone. The height and diameter errors after processing were small, meeting the basic requirements of the test.

[0036] Initial circumferential wave velocity characteristic analysis.

[0037] Prior to the experiment, the surface characteristics of the limestone were observed and analyzed in detail. Based on the apparent morphology of the upper and lower end faces of the limestone, the limestone in this experiment can be divided into four types of characteristics, as follows: Figure 4 As shown, the red arrow indicates the direction of minimum wave velocity, and the blue arrow indicates the direction of maximum wave velocity. Specifically, the types are: ① relatively intact limestone (without obvious primary fissures such as infill), ② limestone filled only with obvious white calcite veins, ③ limestone with obvious gray-black argillaceous fissure matrix, and ④ limestone with both white calcite vein filling and gray-black argillaceous fissure matrix.

[0038] The wave velocities of five layers measured from top to bottom on the limestone sample were superimposed and displayed on the rose garden, as shown below. Figures 4-5 As shown, the surface morphology and initial wave velocity of the four types of samples exhibit the following relationship: ① The initial circumferential 360° P-waves of relatively intact limestone are relatively uniform, and the wave velocity directionality of each layer is not obvious, with a relatively large wave velocity, and the cumulative wave velocity value reaches 24500 m / s. ② Limestone dominated by white calcite veins is characterized by white calcite veins, which can be divided into a main infill (a distinct white calcite vein infill band) and a primary-secondary merging feature (a white calcite vein infill band supplemented by secondary small white calcite vein infill). The wave velocity of this type of limestone has a certain directionality, and the cumulative wave velocity value is slightly smaller, around 24000 m / s.

[0039] ③ Gray-black limestone with a predominantly argillaceous fractured matrix: This type of limestone exhibits secondary fracture filling and shear zones. The gray-black limestone matrix contains microfractures, argillaceous material, or organic matter, resulting in a tortuous sound wave propagation path. Consequently, it displays a more pronounced initial wave velocity directionality, with wave velocities generally lower than the previous two types, accumulating to approximately 20,000 m / s.

[0040] ④ Limestone with both white calcite vein filling and gray-black limestone matrix (this type accounts for the vast majority): This type of limestone is mainly composed of gray-black limestone matrix, and secondary fracture filling / shear zones exist. It also exhibits directionality. When passing through the white calcite vein filling and limestone matrix, the wave velocity increases, with the initial cumulative wave velocity value being around 18,500 m / s.

[0041] Table 1 Basic Mechanical Parameters of Limestone

[0042] When the ultrasonic sensor is parallel and coincident with the white calcite vein, the initial wave velocity increases; when the ultrasonic sensor is obliquely aligned with the white calcite vein and the black-filled fissures are not obvious, the initial wave velocity increases with the area of ​​white filling that penetrates the calcite vein. Figures 4-5 As shown. The main reason is that the white calcite vein itself has a dense structure, low porosity, completely cemented fractures, high contact stiffness, and an elastic modulus higher than that of argillaceous limestone matrix with well-developed microfractures. When the wave velocity is relatively uniform, there are two main directions with high initial wave velocity: the wave velocity increases in the oblique direction and the wave velocity is even greater in the region parallel to the white filling of the calcite vein.

[0043] The initial ultrasonic wave velocity decreases when passing through the gray-black argillaceous primary or secondary fracture-filled / shear zones. Calcite veins / calcite-filled fractures, with higher brightness than the surrounding rock, are mostly distributed in vein-like patterns along continuous fractures, with clear boundaries; the wave velocity increases when passing through calcite veins. Due to geomechanical influences, the limestone matrix is ​​more compact along the direction parallel to the main calcite veins / calcite-filled fractures, resulting in a higher initial ultrasonic wave velocity compared to other areas.

[0044] The mechanical properties of limestone under confining pressure were investigated. The limestone tested exhibited good compaction and linear elastic characteristics before peak values ​​under different confining pressures. Increasing the confining pressure relatively reduced the rock's anisotropy, meaning the difference in elastic modulus among limestone samples within the same group decreased. Increasing the confining pressure also improved the rock's peak strength. Starting from a uniaxial compressive strength of 50 MPa, increasing the confining pressure by 5 MPa and 15 MPa resulted in strengths of approximately 130 MPa and 170 MPa, respectively. When the confining pressure increased to 25 MPa and 35 MPa, the strength increased to approximately 220 MPa. Under confining pressures of 25 MPa and 35 MPa, the strength did not increase significantly but rather tended to increase gradually. Figure 10As shown, the strength of this type of limestone does not increase significantly after a certain confining pressure, and the differences in homogeneity (i.e., elastic modulus) tend to decrease.

[0045] The small Poisson ratio of the limestone in this experiment indicates that the radial displacement or radial volume change of this type of limestone is small in the linear elastic stage, with significant changes concentrated after the peak point. Furthermore, high confining pressure significantly alters the post-peak properties of the rock; after 15 MPa, the post-peak residual stress increases, and after 25 MPa, the rock exhibits a change from brittle to ductile characteristics.

[0046] Under confining pressure, the proportion of shear cracks generally increases. This is evident in the proportion of shear cracks: 10.2% at 5 MPa, 15% at 15 MPa, and increasing to 37.9% at 25 MPa. However, under high confining pressure (35 MPa), the proportion of shear cracks decreases, reaching 11.8%. Therefore, besides confining pressure altering rock tensile and shear crack patterns, the original infilled fissures in the limestone itself may also change the rock failure mode. The peak value of the AE ring count also indicates that from 15,000 rings on a single axis to <1,000 rings at 35 MPa confining pressure, the spatial location of large-energy events shifts from a dispersed to a concentrated distribution, with some small events also dispersed, reflecting that confining pressure inhibits large-scale cracks and promotes distributed damage accumulation. Furthermore, under low confining pressure, the stress-strain curve shows a phased fluctuation before the peak value, which then becomes a gradual rise as the confining pressure increases, indicating that confining pressure regulates the microscopic mechanism from a mixed-type to a shear failure evolution.

[0047] Confining pressure altered the failure mode of limestone from brittle to ductile by regulating ultrasonic rise, AE activation, and crack type distribution. These results can serve as composite indicators for rock engineering stability assessment; for example, the peak strength criticality can be predicted using the cumulative AE count threshold (>10^5 times) and wave velocity reduction (>10%).

[0048] Under uniaxial compression conditions (0 MPa, L2 sample), AE events were distributed throughout the time interval (0–1400 s), exhibiting a high-density clustered distribution with peak frequencies ranging from 0–600 kHz, and fewer high-frequency events (>1000 kHz). The probability density distribution showed a single high-density peak region (probability density 9.55e-4), indicating that the failure process was mainly characterized by sudden brittle fracture, with rapid microcrack propagation leading to concentrated energy release, and a short duration (total duration approximately 1200 s).

[0049] As the confining pressure increased to 5 MPa (L6 sample), the time axis expanded to approximately 2000 s, and the scatter plot distribution became relatively concentrated, with the high-density region of AE events shifting from the front end to the middle and rear. The peak probability density decreased to 1.10e-4, and the distribution became more uniform, reflecting that the confining pressure began to suppress brittle behavior and promote progressive damage accumulation. Similarly, at a confining pressure of 15 MPa (L9 sample), the number of scatter plot clusters increased, but the density rose, with a peak value of approximately 2.84e-4. The scatter plot distribution showed significant temporal diffusion, and the proportion of high-frequency events decreased (peak frequencies were mostly concentrated in the 0–600 kHz range). The probability density tended to level off, and the distribution area widened, indicating enhanced plastic deformation.

[0050] Under high confining pressure (25 MPa and 35 MPa, samples L16 and L17), the scatter distribution was dispersed, and the proportion of low-frequency events (<400 kHz) increased. The probability density distribution further broadened, with peak values ​​decreasing to approximately 4.96e-5 (25 MPa) and 5.44e-5 (35 MPa), respectively, reflecting that pressure-induced ductile failure was dominant. AE events were mainly distributed releases with low amplitude and high frequency, and the total event duration increased, indicating improved rock bearing capacity and a more gradual failure process. Furthermore, the average peak frequency decreased with confining pressure (from ~800 kHz at 0 MPa to ~400 kHz at 35 MPa), indicating that microcrack propagation under high confining pressure was dominated by low-frequency shear, while under low confining pressure it was dominated by high-frequency tension. The gradual accumulation of shear and compaction damage under high confining pressure promoted ductile flow.

[0051] The evolution characteristics of pore distribution in steps three and five, Figures 13-14 The distribution patterns of T2 and pore size in limestone under different confining pressures are shown. Under uniaxial conditions (0 MPa, L2), micropores to mesopores predominate before failure. After failure, the T2 spectrum shifts towards longer relaxation times and the main peak intensifies, with a significant increase in the proportion of macropores, indicating enhanced fracture opening and connectivity, dominated by fracture propagation. Under low confining pressure (5 MPa, L6), the T2 spectrum amplitude increases significantly after failure, but the increase in macropores is limited, with pore evolution mainly characterized by micropore compression and reconstruction. Under moderate confining pressure (15 MPa, L9), the T2 spectrum shows a continuous distribution and extends towards longer T2 values, with a slight increase in mesopores and macropores, reflecting a certain degree of interconnection in the pore network.

[0052] As the confining pressure further increased (25 MPa, L16; 35 MPa, L17), although the T2 spectrum amplitude increased, the peak shape converged towards shorter T2 values, exhibiting obvious fluid redistribution characteristics. The pore size distribution results showed that the proportion of micropores continued to increase after the damage, while the development of macropores was inhibited, indicating that pore compaction and structural densification were dominant under high confining pressure conditions.

[0053] In summary, as the confining pressure increases from 0 MPa to 35 MPa, the pore size distribution shows that macropores increase under low confining pressure (promoting cracking), but the increase weakens or even reverses under medium to high confining pressure, mesopores decrease relatively, and the proportion of micropores increases. This is consistent with the mechanism by which confining pressure inhibits the propagation of fracture: under low pressure, brittle fracture propagates macropores, while under high pressure, plastic deformation compresses mesopores, leading to a decrease in overall porosity and fluid confinement.

[0054] Figure 15 The cumulative porosity distribution characteristics of limestone before and after failure under different confining pressures are shown. Both before and after failure, the cumulative porosity distribution tends to saturate with increasing pore size, but the evolution characteristics differ significantly. Before failure, the pore volume fraction in the small-diameter segment (approximately 0–2 μm) increases rapidly, indicating that the original structure was dominated by micropores. With increasing confining pressure (0→35 MPa), the pore distribution gradually becomes more homogeneous, and the cumulative curve shifts upward overall. After failure, the proportion of pores in the small-diameter segment decreases significantly, while the contribution of medium and large-diameter pores (>2 μm) increases. The curve growth slows down, but the distribution range expands, indicating that pore expansion and connectivity occurred during the loading and failure process, and micro-fractures transformed into macro-fractures. Simultaneously, under high confining pressure conditions (25–35 MPa), the curve lengths are more similar, indicating that the confining pressure has a certain restraining effect on inhibiting pore expansion, thus relatively weakening the degree of structural damage. Overall, the failure process manifests as a transformation of pores from a fine, dispersed type to an expanded, interconnected type.

[0055] Figure 16 , Figure 17 The T1-T2 spectra of limestone before and after failure under different confining pressures are shown. As the confining pressure increases, the spectra show that the signal migrates from the confined region (III, IV) to the free region (V, VI, VII). The initial stage promotes the development of fractures and macropores (enhancing the signal in VI and VII), but higher confining pressures lead to weakening in other regions. Fluids concentrate in specific areas, indicating enhanced heterogeneity and local closure of the pore network.

[0056] The results of nuclear magnetic resonance imaging (NMR) and the verification of damage predictions, combined with experimental results under different confining pressures (5–35 MPa), reveal a spatial correspondence between the anisotropy of the cumulative initial wave velocity before rock failure and the original infilled fracture structure revealed by NMR. Under low confining pressure, the direction of the main fracture after limestone failure is significantly influenced by the direction along the calcite vein (white infilled fracture), and most secondary fractures can be predicted by the direction of the maximum wave velocity. Under medium to high confining pressure, the inhibitory effect of confining pressure on the random propagation of fractures gradually controls the evolution of new fractures to the direction of the dominant wave velocity. Notably, when the failure direction of the main fracture is not parallel to the direction of the maximum wave velocity, both the main fracture and the secondary fracture tend to be parallel to the direction of the maximum wave velocity. The directional accumulation of acoustic emission energy in the direction of the maximum wave velocity at the failure front, and the spatial verification of the main fracture zone by NMR after failure, demonstrate that the combined method based on wave velocity and NMR imaging can effectively predict the main failure direction before rock instability.

[0057] Therefore, this invention employs the aforementioned method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging. This method can effectively predict the main failure direction before rocks become unstable and fail. Combined with indicators such as wave velocity reduction and cumulative acoustic emission count, it can also assist in determining the critical state of rock peak strength, providing a basis for rock engineering stability assessment. Initial wave velocity can characterize the evolution law of fractures. The dominant direction of initial wave velocity in the rock mass can be used to predict the development trend of fractures, distinguish the wave velocity response characteristics of limestone with different primary fracture types, and clarify the influence of calcite veins and argillaceous fractures on wave velocity anisotropy. Adaptable to different confining pressure environments, the prediction law has universality. Under low confining pressure, most secondary fractures in the rock mass can be predicted based on the direction of the maximum initial wave velocity. Under medium to high confining pressure, the confining pressure constrains the random expansion of fractures, and the evolution of new fractures is gradually dominated by the dominant direction of wave velocity. Regardless of the confining pressure, both primary and secondary fractures eventually tend to be parallel to the direction of the maximum wave velocity, and the prediction law is not limited by the confining pressure range. Multiple methods are used to corroborate each other and improve the reliability of the prediction results. On the one hand, the acoustic emission energy before the destruction will be directionally concentrated along the direction of the maximum wave velocity, which corroborates the prediction conclusion from the perspective of dynamic fracture evolution. On the other hand, NMRI imaging after the destruction can intuitively verify the spatial location of the main fracture. At the same time, the wave velocity prediction model constructed in this paper, which takes into account the confining pressure and the randomness of the fracture, has good reliability and realizes the complementarity of multi-source information from static fracture imaging, ultrasonic detection, and dynamic acoustic emission monitoring.

[0058] By distinguishing the differences in primary fracture structures, refined rock mass fracture prediction can differentiate the fracture characteristics and wave velocity response characteristics of four types of limestone: intact limestone, single calcite vein filling, single argillaceous fracture, and composite fracture. It can clarify the influence of the vein orientation and sensor placement angle on the initial wave velocity, accurately identify the dominant wave velocity direction for rock masses with different primary filling fractures, and improve the accuracy of fracture direction prediction for heterogeneous fractured rock masses.

[0059] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging, characterized in that, Includes the following steps: Step 1: Sampling and numbering the samples. Rock samples are taken on-site and processed to prepare standard bodies with a diameter-to-height ratio of 2:

1. The prepared samples are randomly named and numbered and divided into 5 layers according to height. Step 2: Wave velocity measurement. 360° circumferential wave velocity measurements are performed on each layer at 20° intervals. Each angle is tested 3 times and the average value is taken to obtain the initial P-wave and S-wave data of the sample. Step 3: Pre-destruction NMR test. The sample is placed in a triaxial synchronous loading multi-field coupled NMR imaging system and saturated with water for 24 hours under vacuum pressure of 40 MPa. The NMR test is performed on the water-saturated sample before destruction to obtain the initial porosity distribution, T2 spectrum characteristics, and NMRI. Step 4: Mechanical failure test. The specimens are divided into five groups and confining pressure conditions are set for each group. The confining pressure conditions are calculated by the earth pressure formula. The multi-field coupled mechanical test system for deep earth resources is used to carry out loading failure. During different stress stages in the loading process, AE signal acquisition and real-time wave velocity testing are carried out simultaneously. Step 5: Post-destruction NMR test. The sample after mechanical destruction is subjected to NMR test again to obtain the initial porosity distribution and T2 spectrum characteristics after destruction, and NMRI is performed. Step 6: Data processing. Integrate initial wave velocity anisotropy data, AE and dynamic wave velocity data throughout the loading process, and NMR data before and after damage to study the damage evolution law of rocks under multi-field coupling conditions.

2. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 1, characterized in that, In step two, the reflection and transmission behavior of the P-wave at the fracture interface is described by the Zoeppritz equation or its linearized form, and the reflection coefficient... With transmission coefficient for: In the formula, It is the wave impedance.

3. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 2, characterized in that, The longitudinal wave velocity of the sample tested in step two and transverse wave velocity Length is: In the formula, This refers to the distance between the centers of the two pressure heads when they are joined together. This refers to the P-wave propagation time. The propagation time of the S-wave; This is the built-in time.

4. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 3, characterized in that, In step three The relationship between the pore geometry and the pore characteristics is expressed as follows: In the formula, This refers to the surface relaxation time caused by the interaction between the fluid and the pore wall. The transverse surface relaxation strength. It is the ratio of pore surface area to volume.

5. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 4, characterized in that, In step three, nuclear magnetic resonance imaging originates from the spatial coding mechanism of NMR signals under the influence of a magnetic field gradient. Under low-field conditions, the NMR imaging signal intensity at spatial location r is expressed as: In the formula, For nuclear magnetic resonance imaging signal intensity, For hydrogen nucleus density, For the repetition time, The interval time, and These represent the local longitudinal and lateral relaxation times, respectively.

6. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 5, characterized in that, In step four , The value is defined as follows: In the formula, The average frequency of the acoustic emission signal. The steepness of the rise in acoustic emission signal. The number of rings exceeding the threshold. For duration, For the rising time, The amplitude is denoted as .

7. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 6, characterized in that, In step four, the mechanical properties are determined using the elastic modulus and Poisson's ratio.

8. The method for predicting the main fracture direction of rocks based on wave velocity and nuclear magnetic resonance imaging according to claim 7, characterized in that, In steps three and five, the rock pore size is categorized as follows: micropore diameter range is... This corresponds to capillary-bound fluid; The range of the diameter of the medium hole is The range of large pore diameters is .

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