A method for monitoring the wetting range of coal and rock water injection based on ultrasonic point and surface array

Abstract

A method for monitoring the wetting range of coal and rock by water injection based on ultrasonic point-surface array includes: selecting a cubic coal and rock sample; drilling a straight hole from the center of any face to the center point of the cube and drying it; arranging a piezoelectric ceramic assembly at the center point of the cube through the drill hole, which is coupled to the hole wall; uniformly arranging an equidistant array of acoustic emission transducers on the six faces of the cube, which are coupled to the coal and rock sample and form an independent monitoring path with the piezoelectric ceramic assembly in the hole; first exciting the piezoelectric ceramic assembly, then sequentially exciting the transducers on each face, retaining two monitoring data points with similar monitoring paths, and performing analysis and calculation using a local isotropic model; injecting water into the drill hole, analyzing the same monitoring path, and finally until the water completely leaches out of the cubic coal and rock, measuring the final P-wave propagation velocity and time; determining the spatial coordinates of the water transmission range on the monitoring path, and determining the water transmission range through data envelopment analysis.

Description

Technical Field

[0001] This invention relates to a method for monitoring the wetting range of coal and rock water injection, specifically a method for monitoring the wetting range of coal and rock water injection based on an ultrasonic point-surface array. Background Technology

[0002] Currently, true triaxial coal and rock water injection seepage experiments are an important experimental method for studying technical measures such as dust suppression, gas control, and fire prevention and extinguishing in coal and rock water injection. However, current real-time visual monitoring methods for coal and rock (CT, MRI) cannot shield against interference from the metal indenter of true triaxial equipment. Therefore, consistent spatiotemporal evolution of coal and rock water injection remains a research bottleneck in this field. Given that the technology of embedding acoustic emission transducers within the true triaxial metal indenter has been successfully developed—for example, invention patent CN201610155869.2 discloses an acoustic emission test device that integrates acoustic emission sensors within a true triaxial chamber—significantly reducing the numerous interference signals generated by external acoustic emission sensors and achieving the authenticity and reliability of acoustic emission acquisition signals. Furthermore, because ultrasound can sensitively respond to changes in the internal density of coal and rock, it is essential to research a method for monitoring the wetting range of coal and rock water injection based on an acoustic emission transducer array. Summary of the Invention

[0003] This invention application, by overcoming the technical background problems, designs a method for monitoring the wetting range of water-injected coal and rock under a point-to-surface transducer array that can be embedded in a true triaxial equipment pressure head. Specifically, the process includes: Step 1: Select a cubic coal and rock sample, drill a straight hole from the center of any face to the center point of the cube, seal the hole with sealing material, and dry at 105 ℃ for 24 hours; arrange a piezoelectric ceramic assembly at the center point of the cube inside the drill hole, coupled to the hole wall, and evenly arrange equidistant arrays of acoustic emission transducers on the six faces of the cube, coupled to the coal and rock sample, and forming an independent monitoring path with the piezoelectric ceramic assembly inside the hole; Step 2: First, excite the piezoelectric ceramic assembly, the transducer receives the data, and determine the speed of ultrasonic wave propagation and the propagation time of P-wave between them; then, excite the transducers on each face in sequence, the piezoelectric ceramic assembly receives the data, and retain the monitoring data of similar forward and reverse sides of the same monitoring path; Step 3: Inject water into the drill hole, repeat the operation of Step 2 until the water completely leaches out of the cubic coal and rock sample, record the data of the same monitoring path in real time, retain the data set with the same P-wave velocity, and record the P-wave propagation speed and time; calculate the water wetting distance based on the obtained data.

[0004] Preferably, the piezoelectric ceramic assembly consists of hemispherical and cylindrical piezoelectric ceramics.

[0005] The beneficial effects of this invention are as follows: This invention employs a point-area array of acoustic emission transducers to monitor the water wetting range, and leverages the wave's sensitive response to changes in the density of the coal medium to locate the water wetting front. It uses a transducer self-excitation method for non-destructive monitoring of wave propagation in water-bearing porous media. The similarity of monitoring results in both forward and reverse propagation directions is used as a data screening criterion to determine the uniformity of the coal and rock structure along the ultrasonic wave propagation monitoring path. This achieves the monitoring and location of the water wetting range in injected coal and rock, breaking through the research bottleneck of the spatiotemporal evolution law of coal and rock water injection. Attached Figure Description

[0006] Figure 1 This is a structural diagram of the present invention application;

[0007] Figure 2 This is a schematic diagram of the data filtering principle of the present invention;

[0008] Among them: cylindrical piezoelectric ceramic 1; hemispherical piezoelectric ceramic 2; transducer 3; sealing material 4; oscilloscope 5; power amplifier 6; display 7; acoustic emission instrument 8; water injection pipe 9. Detailed Implementation

[0009] The present invention will now be described in detail with reference to the accompanying drawings and embodiments:

[0010] A method for monitoring the wetting range of coal and rock water injection based on ultrasonic point-area array, see attached examples. Figure 1 The main equipment used includes cylindrical piezoelectric ceramic 1, hemispherical piezoelectric ceramic 2, transducer 3, sealing material 4, oscilloscope 5, power amplifier 6, display 7, acoustic emission instrument 8, and water injection pipe 9. Piezoelectric ceramics 1 and 2, transducer 3, oscilloscope 5, power amplifier 6, and display 7 are connected to the acoustic emission instrument 8. The power amplifier 6 is connected to cylindrical piezoelectric ceramic 1 and hemispherical piezoelectric ceramic 2.

[0011] A 10 cm × 10 cm × 10 cm cubic coal and rock sample was subjected to a straight-line drilling from the center of any face to the center of the cube, with a drilling depth of 5 cm. The sample was then dried at 105 °C for 24 h to remove moisture.

[0012] A piezoelectric ceramic assembly consisting of a cylindrical piezoelectric ceramic 1 and a hemispherical piezoelectric ceramic 2 is arranged at the top of the borehole, and the piezoelectric ceramic is coupled to the borehole wall to ensure that ultrasonic waves can be excited and received in all directions. The borehole is then sealed with sealing material 4, and the tail end of the water-filled borehole has a 20 mm sealing area.

[0013] The piezoelectric ceramic array, consisting of a hemispherical and a cylindrical component, is used to couple the borehole surface, ensuring that the excited waveform does not deflect when passing through the ceramic-bearing interface.

[0014] An array of acoustic emission transducers is evenly arranged on the six faces of a cube, forming a point-to-surface array of piezoelectric ceramic groups in the central borehole and transducers on each face. Each piezoelectric ceramic group in the borehole and each transducer on the face can form an independent monitoring path. The transducers are coupled to the coal surface, with equidistant distances between each pair. 7031 coupling grease is used to couple the transducers to the coal and rock.

[0015] The transducer 3 and the piezoelectric ceramic assembly are connected to the acoustic transmitter 8, which can excite and receive waveforms.

[0016] Before water injection, the cylindrical piezoelectric ceramic 1 and the hemispherical piezoelectric ceramic 2 at the center are excited, and the transducers 3 on each surface obtain waveform data. Since the distance between the two transducers is constant, the speed v of ultrasonic wave propagation between the two transducers can be determined. 0i and the propagation time t of the P wave 0i .

[0017] Internal excitation involves placing a probe into an internal cavity for excitation. After excitation, the acoustic emission probes placed on each face of the cubic sample will receive acoustic signals.

[0018] Subsequently, the transducers 3 on each surface are excited in sequence, and the piezoelectric ceramic assembly inside the borehole receives the data.

[0019] like Figure 2 As shown, data analysis is used to determine whether the monitoring data from two separate monitoring paths are similar. If the two monitoring data are similar, it indicates that the monitoring path was relatively uniform before water injection, and a local isotropic model can be used for analysis and calculation. If the P-wave velocity values ​​measured in the two measurements differ significantly, it proves that the monitoring path exhibits significant anisotropy, leading to a significant deflection of wave propagation, and thus this set of data can be discarded.

[0020] Water is injected into the borehole through water injection pipe 9, with the pressure set to the experimental pressure n MPa. Real-time data monitoring is possible after water injection. Similar to step one, the P-wave propagation velocities measured when the piezoelectric ceramic assembly inside the borehole and the transducer 3 on the surface are excited are analyzed along the same monitoring path to determine if the P-wave propagation velocities are the same. Data sets with identical P-wave velocities are selected, and the P-wave propagation velocity is recorded as v. ni The propagation time t of the P-wave ni Finally, until the water completely leaches out the cubic coal rock, the final P-wave propagation velocity is measured to be v. hi P-wave propagation time t hi .

[0021] Based on the local isotropic model, the coal and rock within the monitoring path are considered relatively homogeneous, and the propagation velocity of ultrasonic P-waves in the un-water-injected area is assumed to be v. 0i The propagation speed of ultrasonic P-waves in the water injection area is v ni The monitoring path length is LThen, based on the t measured during the water injection process... ni Calculate the water wetting distance x.

[0022] Based on the water wetting distance x and the spatial information of the monitoring path, the spatial coordinates of the water transmission range on the monitoring path can be determined, and finally the water transmission range can be determined through data envelopment analysis.

[0023] Before the measurement of the cuboid sample, a database of the relationship between the wetting range and acoustic waves of the same material is established. Multiple sets of measurements are conducted to determine the influence of different confining pressures and different wetting ranges on the acoustic wave propagation speed and amplitude. This allows the wetting radius in the measurement direction to be calculated by observing changes in the received acoustic wave frequency at the start of the monitoring experiment, resulting in more accurate data.

[0024] Sound waves are a form of motion of matter, generated by the mechanical motion of matter. Vibrations propagate from near to far through the interaction between particles. Therefore, the elasticity between particles and the density of the particles significantly affect the propagation speed of sound waves. Thus, the water injection process increases the density of the coal sample, inevitably increasing the speed at which sound waves travel through the coal sample.

[0025] There are two main methods for determining the wetting range: one is to determine the wetting distance based on the different speeds of sound waves propagating in different media; the other is to determine the propagation distance based on the different energy transfers of sound waves absorbed by different media.

[0026] After a fissure opens and fills with water, it affects wave propagation. While an increase in water content can accelerate wave propagation with a constant volume, the change in wave propagation during the fissure's initiation and filling process is unknown. Does the wave undergo diffraction, altering its energy and propagation path, or does it pass directly through the fissure? Alternatively, diffraction may occur before the fissure is fully filled, followed by refraction once filled. If this is the case, a significant oscillation or discontinuity in wave reception will be observed macroscopically. By analyzing data from two consecutive monitoring sessions, a large difference in the measured P-wave velocity indicates significant anisotropy along the monitoring path, causing a marked deflection in wave propagation. In this case, the data set with the same P-wave velocity can be discarded, and the data set with the same P-wave velocity should be retained.

[0027] This invention employs a point-area array of acoustic emission transducers to monitor the wetting range of water bodies. It leverages the sensitive response of waves to changes in the density of the coal seam to pinpoint the wetting front. The propagation of waves in the water-bearing porous medium is non-destructively monitored using transducer self-excitation. A computational model with global anisotropy and local isotropy simplifies the problem and determines the calculation method. The similarity of monitoring results in both forward and reverse propagation directions is used as a data selection criterion to determine the homogeneity of the coal and rock structure along the ultrasonic wave propagation path. Ultimately, this invention achieves the monitoring and location of the wetting range of water-injected coal and rock bodies.

Claims

1. A method for monitoring the wetting range of coal and rock water injection based on ultrasonic point-surface array, characterized in that, The method includes: Step 1: Select a cubic coal and rock sample, drill a straight hole from the center of any face to the center point of the cube, seal the hole with sealing material, and dry at 105 ℃ for 24 hours; A piezoelectric ceramic assembly is arranged at the center point of a cube inside the borehole, and is coupled to the borehole wall. The piezoelectric ceramic assembly is composed of hemispherical and cylindrical piezoelectric ceramics. Acoustic emission transducers are evenly arranged in an equidistant array on the six faces of the cube, and are coupled to the coal and rock sample, forming an independent monitoring path with the piezoelectric ceramic assembly inside the borehole. Step 2: First, excite the piezoelectric ceramic array, and the transducer receives the data to determine the speed of ultrasonic wave propagation between them and the propagation time of P wave; then, excite the transducers on each surface in sequence, and the piezoelectric ceramic array receives the data, retaining the similar monitoring data from the same monitoring path in both forward and reverse directions. Step 3: Inject water into the borehole and repeat the operation in Step 2 until the water completely leaches out the cubic coal and rock sample. Record the data twice along the same monitoring path in real time, retaining the data sets with the same P-wave velocity, and record the P-wave propagation speed and time. Calculate the water wetting distance based on the obtained data. Based on the water wetting distance and the spatial information of the monitoring path, the spatial coordinates of the water transmission range on the monitoring path are determined, and the water transmission range is determined by data envelopment analysis. The calculation is performed using a computational model that is globally anisotropic and locally isotropic.

2. The method for monitoring the wetting range of coal and rock water injection based on ultrasonic point-surface array according to claim 1, characterized in that: The coal and rock sample has a size of 10×10×10mm, a drilling depth of 5cm, and a 2cm sealing area at the end of the water injection borehole.

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