Coal rock brittleness index evaluation method based on acoustic emission energy

Abstract

The application discloses a coal rock brittleness index evaluation method based on acoustic emission energy, which comprises the following steps: step 1, selecting a standard core of a target area coal reservoir to carry out a rock triaxial compression experiment under different confining pressures, obtaining a stress-loading time curve of the rock, and synchronously recording acoustic emission energy of the coal rock test piece in a loading process; step 2, accumulating acoustic emission energy values of the rock sample to be measured at each time from the start of loading to the yielding and failure period, and obtaining an acoustic emission cumulative energy-loading time curve of the coal rock test piece; and step 3, establishing a brittleness evaluation model based on acoustic emission cumulative energy according to energy dissipation and accumulation before and after a peak value, and further determining the brittleness index of the coal rock. The coal rock brittleness index is effectively determined, and a reference basis is provided for coal reservoir fracturing reconstruction.

Description

Technical Field

[0001] This invention belongs to the technical field of coal and rock brittleness index evaluation, specifically relating to a method for evaluating coal and rock brittleness index based on acoustic emission energy. Background Technology

[0002] Rock brittleness is a crucial parameter for evaluating the development of natural fractures in reservoirs, predicting the effectiveness of hydraulic fracturing, and assessing wellbore stability. Rock brittleness significantly impacts wellbore stability, which is essential for improving drilling efficiency and safety. Brittleness is also a key parameter for predicting the extent of natural fracture development and the effectiveness of hydraulic fracturing. Generally, greater reservoir rock brittleness indicates more developed natural fractures, a more complex fracture system during hydraulic fracturing, higher fracturing fluid usage, and smaller proppant particle sizes, all of which are more conducive to shale gas extraction from matrix pores, thus leading to higher gas production.

[0003] There are currently six points of consensus regarding the brittle characteristics of rocks: (1) Rocks fail immediately under low strain conditions; (2) Brittle failure of rocks is mainly dominated by internal microcracks; (3) Rocks have a high compressive to tensile strength ratio; (4) Rocks have high resilience; (5) Rocks have a large internal friction angle; (6) Rocks have fully developed cracks during hardness testing.

[0004] Domestic and international scholars have proposed numerous brittleness assessment methods to meet research needs. However, these methods are only meaningful for specific rocks, especially shale and sandstone. The applicability of these methods to coal reservoirs, which have low strength and well-developed natural fractures, remains unknown. Therefore, it is of great significance to establish a brittleness index assessment method based on the cumulative acoustic emission energy by recording the changes in acoustic emission energy of coal and rock throughout the entire process from the application of load to yield failure, and by analyzing the energy dissipation and accumulation before and after the peak value. Summary of the Invention

[0005] The technical problem to be solved by this invention is to provide a method for evaluating the coal and rock brittleness index based on acoustic emission energy, so as to effectively determine the brittleness index of coal reservoir rocks and provide a reference for coal reservoir fracturing.

[0006] The technical solution adopted in this invention is as follows:

[0007] A method for evaluating the brittleness index of coal and rock based on acoustic emission energy, the method comprising:

[0008] Step 1: Select standard core samples from the coal reservoir in the target area to conduct triaxial compression tests on the rock under different confining pressures, obtain the stress-loading time curve of the rock, and simultaneously record the acoustic emission energy of the coal and rock specimens during the loading process;

[0009] Step 2: Accumulate the acoustic emission energy values ​​of the rock sample under test at each time from the start of load application to yield failure, and obtain the acoustic emission cumulative energy-loading time curve of the coal and rock sample;

[0010] Step 3: Based on the energy dissipation and accumulation before and after the peak, establish a brittleness evaluation model based on the cumulative energy of acoustic emission, and then determine the brittleness index of coal and rock.

[0011] Methods for establishing a brittleness assessment model based on the cumulative energy of acoustic emission include:

[0012] Step 3.1: Based on the evolution law of acoustic emission accumulated energy, the acoustic emission accumulated energy of the coal and rock specimen fracture process is divided into two zones, and the boundary between the zones is the dividing point between the elastic segment and the plastic segment.

[0013] Step 3.2: Before the boundary point, the microcracks inside the specimen close and the specimen undergoes elastic deformation. The energy consumed in this stage represents the energy dissipation of rock fracture, that is, the accumulated acoustic emission energy is the dissipated energy.

[0014] Step 3.3: After the boundary point, the rock specimen fractures and tends to completely break down as the load increases, releasing a large amount of energy. At this point, the accumulated acoustic emission energy is elastic strain energy.

[0015] Step 3.4: Obtain the crack initiation start time t1 and the complete failure end time t2 from the stress-loading time curve;

[0016] Step 3.5: Obtain the cumulative acoustic emission energy E1 before rock crack initiation and the cumulative acoustic emission energy E2 after complete destruction from the acoustic emission cumulative energy-loading time curve.

[0017] Methods for determining the brittleness index of coal and rock include:

[0018] Step 3.6: Considering the energy changes before and after the peak, a new brittleness index was established, specifically including:

[0019]

[0020] In the formula: E1 is the cumulative acoustic emission energy before crack initiation, and E2 is the cumulative acoustic emission energy of the coal and rock specimen after complete destruction;

[0021]

[0022] In the formula: t1 is the crack initiation start time, and t2 is the complete failure end time of the coal and rock specimen;

[0023] B = B1B2 (Formula 3)

[0024] In the formula, B1 is the brittleness index based on dissipated energy, B2 is the brittleness index based on strain energy, and B is the brittleness index based on the cumulative energy of acoustic emission.

[0025] The beneficial effects of this invention are:

[0026] This invention involves conducting triaxial tests on coal and rock samples under different confining pressures, simultaneously recording acoustic emission data during the loading process, and accumulating the acoustic emission energy values ​​of the rock sample at each moment from the start of load application to yield failure to obtain the cumulative acoustic emission energy value of the rock sample. Based on the energy dissipation and accumulation before and after the peak value, a brittleness evaluation model based on cumulative acoustic emission energy is established to determine the brittleness index of coal and rock. This invention's method is based on the energy release throughout the rock fracture process, using cumulative acoustic emission energy to characterize the brittleness of coal and rock, thereby improving the accuracy and reliability of rock brittleness evaluation. This method is applicable to deep reservoir environments and can effectively determine the brittleness index of coal reservoir rocks, providing a reference for coal reservoir fracturing.

[0027] This invention is a method for evaluating the brittleness of coal and rock based on the acoustic emission cumulative energy of rock failure mechanism. It considers the energy evolution before and after the peak and establishes a coal-rock brittleness index by applying the acoustic emission cumulative energy, which can effectively explain the brittleness of coal and rock under different confining pressure conditions. The rock brittleness index established by this invention is reasonable and has certain advantages in evaluating rock brittleness compared with similar methods. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the process of the present invention;

[0029] Figure 2 This is a schematic diagram of the partitioning of the acoustic emission cumulative energy curve according to an embodiment of the present invention;

[0030] Figure 3 The graphs show the triaxial compression test curves of coal and rock under different confining pressures according to embodiments of the present invention.

[0031] Figure 4 This is the acoustic emission cumulative energy-stress-loading time relationship curve under a confining pressure of 2 MPa according to an embodiment of the present invention;

[0032] Figure 5 This is the acoustic emission cumulative energy-stress-loading time relationship curve under a confining pressure of 4 MPa according to an embodiment of the present invention;

[0033] Figure 6 The curve showing the relationship between acoustic emission cumulative energy, stress, and loading time under a confining pressure of 6 MPa, according to an embodiment of the present invention. Detailed Implementation

[0034] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. Obviously, the specific embodiments of the present invention are not limited to those provided.

[0035] Example

[0036] A method for evaluating the brittleness index of coal and rock based on acoustic emission energy, characterized by comprising the following steps:

[0037] Step 1: Select standard rock cores (50mm in diameter and 100mm in length) from the coal reservoir in the target area, conduct triaxial compression tests on the rock under different confining pressures, obtain the stress-loading time curve of the rock, and simultaneously record the acoustic emission energy of the coal and rock specimens during the loading process.

[0038] Step 2: Accumulate the acoustic emission energy values ​​of the rock sample under test at each time point from the start of load application to yield failure, and obtain the acoustic emission cumulative energy-loading time curve of the coal and rock sample;

[0039] Step 3: Based on the energy dissipation and accumulation before and after the peak, establish a brittleness evaluation model based on the cumulative energy of acoustic emission, and then determine the brittleness index of coal and rock;

[0040] The brittleness evaluation model based on cumulative acoustic emission energy is established according to the following method:

[0041] Based on the evolution law of acoustic emission accumulated energy, the acoustic emission accumulated energy during the fracture process of coal and rock specimens is divided into two zones, and the boundary of the zone is the dividing point between the elastic segment and the plastic segment.

[0042] Before the dividing point, the microcracks inside the specimen close and the specimen undergoes elastic deformation. The energy consumed in this stage represents the energy dissipation of rock fracture, that is, the accumulated energy of acoustic emission is dissipated energy.

[0043] After the boundary point, the rock specimen fractures and tends to completely break down as the load increases, releasing a large amount of energy. At this point, the accumulated acoustic emission energy is strain energy.

[0044] The crack initiation time t1 and the complete failure time t2 were obtained from the stress-loading time curve.

[0045] The cumulative acoustic emission energy E1 before rock crack initiation and the cumulative acoustic emission energy E2 after complete destruction were obtained from the acoustic emission cumulative energy-loading time curve.

[0046] It is generally believed that the higher the proportion of dissipated energy to total energy and the lower the proportion of elastic strain energy to total energy, the higher the brittleness of the rock. Therefore, the brittleness index based on the relative magnitude of elastic strain energy can be calculated as follows:

[0047]

[0048] In the formula: E1 is the cumulative acoustic emission energy before crack initiation, and E2 is the cumulative acoustic emission energy before complete destruction;

[0049] The energy accumulation and dissipation of rock samples during loading are time-dependent. A faster energy release indicates a faster stress decrease, meaning there is a correlation between strain energy release rate and brittleness. Therefore, the brittleness index based on strain energy release rate can be calculated as follows:

[0050]

[0051] In the formula: t1 is the crack initiation start time, and t2 is the complete failure end time of the coal and rock specimen;

[0052] The brittleness index based on the cumulative energy of acoustic emission is defined by the ratio of elastic strain energy to total energy and the elastic strain energy release rate.

[0053] B = B1B2 (Formula 3)

[0054] In the formula, B1 is the brittleness index based on dissipated energy, B2 is the brittleness index based on strain energy, and B is the brittleness index based on the cumulative energy of acoustic emission.

[0055] Below, this embodiment will provide an application example of determining the brittleness index based on the cumulative acoustic emission energy, and verify the brittleness evaluation method provided by the embodiment of the present invention under different confining pressure conditions.

[0056] Step 1: Drill three core samples from the coal sample to be tested and cut them into standard specimens with a height of 100 mm and a diameter of 50 mm.

[0057] Grind with a grinding wheel until both ends are parallel, with a height error not exceeding 0.2mm and a diameter error not exceeding 0.2mm;

[0058] Step 2: Conduct triaxial compression tests and acoustic emission tests on the rock core under different confining pressures (2MPa, 4MPa, 6MPa) to obtain the stress-loading time curve of the rock and simultaneously record the acoustic emission energy of the coal and rock specimens during the loading process.

[0059] Step 3: Accumulate the acoustic emission energy values ​​of the rock sample under test at each time from the start of load application to yield failure, and obtain the acoustic emission cumulative energy-loading time curve of the coal and rock sample;

[0060] Step 4: Obtain the crack initiation start time t1 and the complete failure end time t2 on the stress-loading time curve; the crack initiation start time t1 corresponds to the starting point of the surge in the cumulative acoustic emission energy on the acoustic emission cumulative energy-loading time curve, and the complete failure end time t2 corresponds to the ending point of the surge in the cumulative acoustic emission energy.

[0061] Step 5: Obtain the cumulative acoustic emission energy E1 before rock crack initiation and the cumulative acoustic emission energy E2 after complete destruction from the acoustic emission cumulative energy-loading time curve;

[0062] Step 6: Based on the data in the table, substitute the calculated cumulative acoustic emission energy E1 before crack initiation of the coal and rock specimen, the cumulative acoustic emission energy E2 before complete failure, the crack initiation start time t1, and the complete failure end time t2 of the coal and rock specimen into formulas (1)-(3) to obtain the coal and rock brittleness index B. The specific results are shown in the table.

[0063] Table 1

[0064]

[0065] The figure shows the triaxial test curves of coal and rock under different confining pressures (2MPa, 4MPa, and 6MPa). At a confining pressure of 2MPa, it can be observed that the stress in the coal and rock specimen drops rapidly after reaching its peak, exhibiting obvious brittle characteristics. As the confining pressure increases, the stress drop trend after the peak in the coal and rock specimen slows down, and its resistance to deformation also increases.

[0066] To verify the reliability of the brittleness index based on cumulative acoustic emission energy proposed in this invention, we selected the coal and rock brittleness obtained by the stress-strain curve method for comparison. The specific experimental data are shown in the table below.

[0067] Table 2

[0068]

[0069]

[0070] As the confining pressure increases, the brittleness values ​​calculated by both methods show a decreasing trend. At a confining pressure of 2 MPa, the coal-rock brittleness values ​​calculated using the acoustic emission cumulative energy method and the stress-strain curve method are the highest, at 4.61 and 0.56, respectively. At a confining pressure of 4 MPa, the values ​​are 2.95 and 0.49, respectively. At a confining pressure of 6 MPa, the values ​​are the lowest, at 2.74 and 0.43, respectively. By comparing the calculated results and experimental curves, it can be determined that the brittleness index B based on acoustic emission cumulative energy can reflect the differences in brittleness of coal-rock samples under different confining pressures, and has certain advantages in evaluating coal-rock brittleness.

Claims

1. A method for evaluating the brittleness index of coal and rock based on acoustic emission energy, characterized in that: The method includes: Step 1: Select standard core samples from the coal reservoir in the target area to conduct triaxial compression tests on the rock under different confining pressures, obtain the stress-loading time curve of the rock, and simultaneously record the acoustic emission energy of the coal and rock specimens during the loading process; Step 2: Accumulate the acoustic emission energy values ​​of the rock sample under test at each time from the start of load application to yield failure, and obtain the acoustic emission cumulative energy-loading time curve of the coal and rock sample; Step 3: Based on the energy dissipation and accumulation before and after the peak, establish a brittleness evaluation model based on the cumulative energy of acoustic emission, and then determine the brittleness index of coal and rock. Methods for determining the brittleness index of coal and rock include: Step 3.6: Considering the energy changes before and after the peak, a new brittleness index was established, specifically including: (Official 1); In the formula: E1 is the cumulative acoustic emission energy before crack initiation, and E2 is the cumulative acoustic emission energy of the coal and rock specimen after complete destruction; (Official 2); In the formula: t1 is the crack initiation start time, and t2 is the complete failure end time of the coal and rock specimen; (Official 3); In the formula, B1 is the brittleness index based on dissipated energy, B2 is the brittleness index based on strain energy, and B is the brittleness index based on the cumulative energy of acoustic emission.

2. The method for evaluating the coal and rock brittleness index based on acoustic emission energy according to claim 1, characterized in that: Methods for establishing a brittleness assessment model based on the cumulative energy of acoustic emission include: Step 3.1: Based on the evolution law of acoustic emission accumulated energy, the acoustic emission accumulated energy of the coal and rock specimen fracture process is divided into two zones, and the boundary between the zones is the dividing point between the elastic segment and the plastic segment. Step 3.2: Before the boundary point, the microcracks inside the specimen close and the specimen undergoes elastic deformation. The energy consumed in this stage represents the energy dissipation of rock fracture, that is, the accumulated acoustic emission energy is the dissipated energy. Step 3.3: After the boundary point, the rock specimen fractures and tends to completely break down as the load increases, releasing a large amount of energy. At this point, the accumulated acoustic emission energy is elastic strain energy. Step 3.4: Obtain the crack initiation start time t1 and the complete failure end time t2 from the stress-loading time curve; Step 3.5: Obtain the cumulative acoustic emission energy E1 before rock crack initiation and the cumulative acoustic emission energy E2 after complete destruction from the acoustic emission cumulative energy-loading time curve.

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