Study on destabilization failure characteristics and energy evolution law of drilling holes

Borehole destabilization damage is an important factors affecting gas extraction efficiency. To derive the fracture development law, strain, and energy evolution characteristics of different borehole diameters, uniaxial compression experiments were conducted on prefabricated rock specimens with different borehole diameters. The results showed that the elastic energy and dissipation energy of the specimens with different pore diameters changed with the compression process, and the energy distribution inside the specimens. The results show that as the pore size of the specimen increases, the strain area damaged by tensile fracture gradually decreases and the strain area damaged by shear fracture gradually increases, and the damage of the dominant specimen gradually evolves from tensile fracture as the dominant factor to tensile fracture and shear fracture together as the dominant factor and finally to shear fracture as the dominant factor. For the whole compression process of pore size rock specimens, the total energy curve shows a nonlinear growth trend, and the elastic energy curve shows a nonlinear growth trend before the specimen damage and a cliff‐type decrease after the specimen damage. From the perspective of energy distribution, the elastic energy storage limit of the specimen decreases with the increase of the specimen pore size, the energy distribution law inside the rock specimen with pores changes, and the total energy absorbed from the outside gradually changes to the dissipated energy, resulting in an increasing proportion of dissipated energy, indicating that the larger the pore size, the larger the plastic deformation, and the more obvious ductile damage characteristics of the specimen with pores will be shown. The research analyzes the causes of destabilization of rock samples with different hole diameters and provides some theoretical guidance for the study of gas extraction drilling stability.


| INTRODUCTION
Under the background of carbon peak carbon neutrality, the green and clean utilization of fossil energy such as coal has become the primary task of sustainable development in China, 1,2 and with the continuous demand of coal mine resources, the depth of mine engineering construction is increasing, and the coal and gas protrusion disaster is becoming more and more serious, strengthening the green and clean extraction and utilization of coal mine gas is an important task for the high-quality development of coal industry. [3][4][5][6][7] Drilling prepumping gas technology can effectively reduce the occurrence of coal mine gas protrusion disaster, and is an important means of coal mine gas management. 8,9 The maturity of large-diameter borehole technology makes increasing the diameter of borehole one of the most direct and effective methods to improve gas extraction efficiency, but the increase of borehole diameter will be accompanied by the decrease of borehole stability, which leads to a large number of fissures around the borehole to provide flow channels for gas, thus reducing the efficiency of gas extraction. 10,11 Therefore, the destabilization of boreholes is one of the important factors affecting gas extraction efficiency. When the strain energy gathered around the hole reaches the energy storage limit of the coal and rock body, the large amount of strain energy gathered will be released suddenly, which will eventually show the physical phenomenon of large number of macroscopic fractures around the hole. 12,13 The analysis and study of borehole stability from the perspective of fracture development and energy evolution can summarize an effective solution to the destabilization damage of boreholes.
Based on the elasto-plastic damage theory, Ma et al. 14 concluded that the main cause of damage in boreholes is the expansion of primary microfractures during the redistribution of stresses around the borehole with the help of numerical simulation software. Wang et al. 15 analyzed the destabilization damage mechanism of slotted boreholes in high-porosity sandstones, and combined with the discrete element method-continuum mechanics modeling method to simulate the Duan et al. 16 took anisotropic microrock as the entry point, constructed a borehole model containing anisotropic microrock based on discrete elements, analyzed the characteristics of borehole fracture extension at different points of microrock grain size, borehole diameter, isotropy, and anisotropy, and obtained that the change of borehole diameter affects the fracture damage around the borehole He and other scholars 17 established the destabilization equation of gas extraction borehole perimeter damage considering various factors such as cohesion, internal friction angle, coal seam porosity, and ground stress, which revealed the law of borehole creep deformation and damage mode. For the energy evolution law in the process of borehole destabilization damage, Zhao et al. 18 scholars considered the effect of shear stress and adopted the multiaxial strength failure criterion, and concluded that borehole perimeter strain decay is an important characterization for borehole destabilization damage prediction. Feng et al. 19 used a combination of numerical simulations and uniaxial compression experiments to verify the accuracy of the established freeze-thaw damage model by comparing the stress-strain curves and energy laws of freeze-thaw rocks with experimental results, who conducted uniaxial compression experiments on white sandstone and analyzed the dissipation energy and elastic energy ratio of the specimens during loading and unloading to obtain equal elastic energy at each unloading point with equal loading. Hu et al. 20 used an elastic-viscoplastic ontological model and analyzed the stress distribution and evolution law around the damaged hole from an energy perspective, and obtained that the anisotropy of the stress field is the fundamental cause of shear damage in boreholes. Huang et al. 21 conducted uniaxial experiments on prefabricated granite specimens containing pores, analyzed the mechanical properties, acoustic emission characteristics, and damage modes of thermally damaged granite specimens, and obtained the fracture expansion law of the specimens. Gong et al. 22 conducted uniaxial compression experiments on 14 rock materials and analyzed that the peak dissipation strain energy during the loading process can be characterized by the linear energy storage law, and proposed a new characterization method for damage of rock materials.
The fracture, strain, and energy evolution characteristics of the specimens under load are helpful to summarize the causes of borehole destabilization damage, but most of the above studies on the fracture, strain, and energy evolution characteristics of the specimens during the compression experiments were conducted on the pore-containing specimens made of similar materials or with a single pore diameter. [23][24][25][26][27][28][29][30] The fracture, strain, and energy evolution characteristics of pore-bearing specimens during loading can show large differences as the pore size increases. Therefore, to find out the fracture development pattern, strain, and energy evolution characteristics of pore-bearing rock specimens with different pore diameters under load, uniaxial compression experiments were conducted on pore-bearing rock specimens, and the fracture and strain evolution characteristics of specimens under load were monitored based on digital image correlation (DIC) technology to summarize the causes of instability and damage of porebearing rock specimens with different pore diameters, and to provide some theoretical guidance for gas extraction drilling stability research. Theoretical guidance is provided for the study of the stability of gas extraction boreholes.
To reflect the influence of different pore sizes on the fracture and strain evolution characteristics around the borehole, the sandstone was processed into square samples with the size of 70 mm × 70 mm × 70 mm. There were no obvious cracks on the surface of the samples, and the samples were polished smooth and flat. In the middle of the samples, there were cylindrical boreholes throughout. Four groups of rock samples with different pore sizes were prepared. And the specific size and parameters of the rock samples are shown in Table 1.
The experimental equipment includes a microcomputer display hydraulic pressure test machine and digital image technology monitoring system (DIC for short), the experimental arrangement system is shown in Figure 1. The stress loading system consists of two parts: hydraulic pressure testing machine and data acquisition system, and the data such as time, pressure, and loading speed of the specimen under load are displayed on the microcomputer display of the data acquisition system in real-time. Due to the small size of the rock specimen, the surface of the specimen is prepared as a scattered specimen with white spots on a black background using paint treatment, which can effectively reduce the error of computer calculation of the displacement and strain change of the scattered spots on the surface of the specimen. The distance between the DIC system and the specimen, the brightness of the fill light, and the exposure of the camera are adjusted to the best condition. At the beginning of the experiment, the data acquisition system controls the constant loading speed of the hydraulic pressure tester, and the digital image acquisition system collects pictures of the specimen, with the acquisition frequency of once every 500 ms until the specimen is destroyed and the stress loading and picture acquisition are stopped. At the end of the experiment, the collected specimen pictures were processed with the help of graphic workstations to calculate the displacement and strain evolution characteristics of the loaded specimen. The strain evolution characteristics of the specimen under load can be accurately calculated by the displacement changes of the detected scattered points when the specimen is loaded by the DIC system, as shown in Figure 2, after the scattered point spraying process of the prefabricated rock specimen containing holes.

| Crack and strain evolution of 8 mm diameter samples
For intact rock specimens loaded until the process of destruction, under the influence of Poisson effect, the increase of the load on the specimen will cause the specimen on both sides of the tensile force, the direction is perpendicular to the direction of the load and extended outward from the specimen, so that both sides of the specimen will appear smaller tensile fractures. As the load continues to increase, the tiny cracks in the specimen will continue to extend and expand under the action of tension, and new cracks will continue to be generated inside the specimen until the specimen is destroyed. The fracture and strain evolution of the porous rock specimens is different from that of the intact rock specimens to a large extent. Due to the presence of pores in the specimen, the strength of the specimen is reduced to a certain extent, and the magnitude of the strength reduction increases with the increase of the pore size, and the strength reduction factor is defined by Equation (1). 31 The stress-strain curves of the 8 mm pore size rock specimens with pores are shown in Figure 3 by combining the load variation from uniaxial compression experiments with the strain variation calculated by the DIC image acquisition system.
where ω is the stress reduction factor, σ si is the uniaxial compressive strength of the intact specimen, MPa; σ sh is the uniaxial compressive strength of the specimen with holes, MPa.
As shown in the picture of the specimen collected at the point a in Figure 3, the fracture evolution of the 8 mm pore-size rock specimen is similar to the fracture evolution of the intact square specimen at the beginning, when the load on the pore-size rock specimen is small, a small expansion of the side of the specimen occurs and is accompanied by the emergence of small fractures. When the load is increased to about 36 MPa, the specimen picture collected from point b in Figure 3 shows that a small part of rock spalling occurs on the side of the 8 mm pore-bearing rock specimen, and the tiny fissures have expanded to the visible state, and the fissures are located in the middle of the side of the specimen, and the development direction is roughly parallel to the direction of loading of the specimen, which indicates that the specimen is dense and complete inside. When the load increased to about 41 MPa, the specimen side appeared obvious and large range of rock spalling, the main fissures on the side of the specimen developed to penetrate the top and bottom of the specimen, and several partially visible macroscopic fissures were produced on the side of the specimen under the influence of the increased load and the development of the main fissures, the fissures were all tensile fissures, this part of the fissures developed later, but the fissures expanded and extended fast, because the tensile fissures developed to when the form visible to the naked eye, there are already a lot of microscopic cracks inside the specimen so that the strength of the specimen is low. In addition to the tensile fracture on the side of the specimen, the shear fracture appears in the stress concentration area around the hole and extends along the weak surface inside the specimen from the stress concentration area under the action of the load. The ultimate compressive strength σsh of the perforated rock specimen was reached when the load was increased to 44 MPa, while the uniaxial compressive strength σ si of the intact rock specimen was about 48 MPa, resulting in a stress reduction factor ω of 8.3% for the 8 mm perforated rock specimen. As shown in the picture of the specimen collected at point c in Figure 3, the specimen disintegrates into two parts on the left and right along the shear fracture direction starting from the stress concentration area around the pore. the fracture evolution characteristics of the 8 mm pore-sized rock specimen with pores can be divided into four main stages. In the first stage, the specimen expands and develops small tensile fractures on the side of the specimen when the load is small. In the second stage, F I G U R E 2 Speckle patterns of rock samples with pores.
F I G U R E 3 Stress-strain curve of 8 mm pore size specimen with pore. the tensile fractures develop and expand to be visible to the naked eye. In the third stage, the shear fracture is developed in the stress concentration area around the hole and expands along the weak surface inside the rock. In the fourth stage, the shear fracture penetrates the rock specimen and finally leads to the destruction of the porebearing rock specimen, the shear fracture and tensile fracture penetrate each other, and the specimen as a whole is extremely broken.
Since the fracture development of the 8 mm porous rock specimen is the main strain concentration area, the strain evolution characteristics of the specimen calculated by DIC acquisition can predict the damage characteristics of the specimen. The strain evolution characteristics were analyzed according to the above fracture development stages, and the strain field evolution cloud diagrams of the specimens at each stage are shown in Figure 4.
From Figure 4A, it can be seen that the specimen of 8 mm pore-bearing rock expands laterally by the load, and the maximum strain at the lateral expansion is about 1.1%, and the average strain is about 1%. As shown in Figure 4B, the fracture development is located in the second stage, and the strain increases gradually with the increase of load. As shown in Figure 4C, some of the strain field clouds monitored by DIC are missing because some of the rock blocks on the side of the specimen have been spalled by the tensile fracture. The shear fracture development location is the marked area in Figure 4C, which is the strain concentration area, therefore, the damage characteristics of the specimen can be clearly seen from the strain field cloud, the maximum strain in the strain concentration area is about 3.1%, and the average strain is about 2.9%. From Figure 4D, it can be seen that the expansion of the shear fracture, which plays the main destructive role, leads to the destruction of the specimen, but the specimen still has a certain load-bearing capacity. The shear fracture height development is in the strain concentration area, the maximum strain is about 5.1%, and the average is about 3.7%. As the load continues to increase, the strain at the shear fracture around the hole will be suddenly released and the specimen will be damaged along the shear fracture, and the damage characteristics of the specimen are shown in the picture of the specimen collected at point c in Figure 3.

| Crack and strain evolution of 10 mm pore-size specimens
The uniaxial compressive stress-strain curve and fracture development, when the hole diameter of the porecontaining specimen increases to 10 mm, are shown in Figure 5. Compared with the pore-containing rock specimen with a hole diameter of 8 mm, the uniaxial compressive strength σ sh of the 10 mm pore-containing rock specimen decreases to 39 MPa, the stress reduction factor ω increases to 18.7%, and the fractures develop earlier at each stage. The F I G U R E 5 Stress-strain curve of 10 mm pore size specimen with pore. main development stage of tensile fracture is during the process of load gradually increasing to 33 MPa, the specimen side first expands, the naked eye visible tensile fracture begins to sprout at the side edge, then new tensile fracture is generated along the specimen side to the internal direction, the fracture extension direction is parallel to the stress loading direction. When the loading amount exceeds 33 MPa, the tensile fracture expands in the vertical loading direction, leading to the spalling of the rock mass on the side of the specimen and the shear fracture sprouting near the perimeter of the hole, at this time the specimen is more broken inside, and the shear fracture extends rapidly from the perimeter of the hole along the internal weak surface of the specimen until the specimen is destroyed in the direction of the shear fracture. However, due to the increase of the internal pore size, the fracture development rate of each stage is faster than that of the 8 mm porebearing rock specimen, especially in the shear fracture development stage when the specimen strength is lower leading to the short shear fracture development time and fast fracture development rate, and the main evolution stage is reduced from 36-44 MPa to 33-39 MPa in the 8 mm porebearing rock specimen. Figure 6 shows the strain field evolution cloud diagram for each stage of fracture evolution of 10 mm pore-sized rock specimens. In Figure 6A, when the side of the specimen expands, the maximum strain on the side is about 1.2%, and the average strain is about 0.8%. The strain plunge area presents a straight line parallel to the side of the specimen, which confirms the trend of tensile fracture evolution in the picture at point b in Figure 5. The position marked in Figure 6B is the extended development area of tensile fracture, the maximum strain is about 1.78%, the average strain is about 1.35%, and the curve presented in the strain plunge area is the breakage curve of the specimen damaged by the influence of tensile fracture development. When shear fracture is generated in the specimen, part of the rock spalling on the side of the specimen, the strain is concentrated in the highly developed area of tensile fracture, the maximum strain is about 4%, the average strain is about 2.9%, and the shear fracture is concentrated in the strain plunge area in the strain field. At the fourth stage of crack development, the shear fracture, which plays a major role in damage, leads to the destruction of the specimen. As shown in Figure 6D, the strain concentration phenomenon appeared in the highly developed area of shear fracture, the maximum strain is about 1.78%, and the average strain is about 1.35%, the rock in this area is more fragmented and easy to produce damage as the main factor leading to the overall damage of the specimen.
3.3 | Fracture and strain evolution of 12 and 14 mm pore-size specimens The fracture types of 8 and 10 mm specimens appear in the order of tensile fracture and shear fracture, and the damage of the specimens is characterized by the action of tensile fracture flanked by partial spalling of rock masses and then the overall damage of the specimens along the shear fracture under the action of shear fracture expansion. The difference is that with the increase of pore size, the appearance time of shear fracture will be earlier and the development speed will be accelerated. When the pore size increases to 12 mm, shear fracture, and tensile fracture appear almost simultaneously, and when the pore size increases to 14 mm, shear fracture appears earlier than tensile fracture, and the damage characteristics of 12 and 14 mm pore size rock specimens with pores are mainly by the action of shear fracture specimens lateral and overall damage, and the fracture evolution characteristics of the two pore sizes are similar. Figure 7 shows the uniaxial compressive stress-strain curves of the 12 and 14 mm pore-size rock specimens, and Figure 8 shows the strain field evolution of the 12 and 14 mm pore-size rock specimens corresponding to the position around point a in Figure 7. The uniaxial compressive strengths σ sh of the 12 and 14 mm pore size rock specimens are 34 and 30 MPa, respectively, and the stress reduction coefficients ω are 29.2% and 37.5%, respectively. 37.5%. As shown in Figure 7A, partial shear, and tensile fractures are produced in the 12 mm pore size specimen when the load is about 25 MPa, and the tensile fracture develops gradually through the upper and lower part of the specimen in the vertical direction, and the shear fracture develops through to the interior of the pore. As shown in Figure 7B, a large number of shear cracks and partial tensile cracks were produced in the 14 mm aperture specimen when the load was about 20 MPa, and most of the shear cracks were developed through to the inside of the specimen cavity. As the load increases, compared with the small pore size specimens, the large pore size specimens produce tensile fractures and shear fractures when the load is small, and the number of shear fractures produced is more, and most of the shear fractures are penetration fractures from the side of the specimen to the inside of the specimen cavity, so the specimen strength is low and the fracture development speed is fast. When the load increases to the specimen damage, the deformation generated by the damage is mainly developed along the direction of shear fractures, and the larger the specimen aperture damage, the more likely to produce X-shaped damage characteristics, as shown in Figure 7B.
Compared with the strain evolution of the former smaller aperture specimen, the strain concentration area on the side of the 12 mm aperture specimen becomes smaller, but the strain area near the perimeter of the aperture becomes larger, that is, the larger the aperture of the specimen, the smaller the range and number of tensile fractures, and the larger the aperture of the specimen, the smaller the range and number of tensile fractures, and the larger the range and number of shear fractures. From Figure 8B, it can be seen that the main form of fracture produced by the loading of 14 mm pore size specimens is shear fracture, and it is concentrated near the perimeter of the pore, and the tensile fracture produced is almost negligible. It can be seen that, with the increase of the pore diameter of the specimen containing holes, the strain area of damage by tensile fracture gradually decreases, and the strain area of damage by shear fracture gradually increases, and the damage of the dominant specimen evolves from tensile fracture as the dominant factor to tensile fracture and shear fracture together as the dominant factor and finally evolves to shear fracture as the dominant factor.

| ENERGY EVOLUTION ANALYSIS
Energy transformation is an essential feature of the uniaxial compression process of rock specimens containing pores; therefore, the damage law of rock specimens containing pores can be better explained from the perspective of energy transformation. 32 The specimen is loaded as a process of deformation until destruction, and the two parts of the specimen and the external environment with which the specimen generates energy exchange are treated as a system in which the energy exchange in the system is dynamically conserved. In the compression of the test machine under the action of compression generated by the test machine mechanical energy in the form of elastic deformation energy stored in the specimen, with the increasing compression load, the internal specimen in addition to elastic deformation energy gradually generated plastic deformation energy, damage energy, and heat, and this part of the energy is irreversible, this part of energy is released to the external environment in the form of dissipated energy. When the external energy input is too large, the energy stored inside the specimen is too much and reaches the storage limit of the specimen, it will trigger the specimen to prevent the internal energy from continuing to gather, so that the elastic deformation energy stored inside the specimen will be released to the external environment, and the process will show the physical phenomenon of destabilization and damage of the specimen from outside. The specimen-loaded system contains many forms of energy and energy conversion between each other, and the specimen loaded until the damage has undergone the transformation process between the specimen and the external environment such as energy absorption, energy storage, energy dissipation, and energy release, 33 and it is difficult to apply the verification of the frictional heat energy generated between the specimen and the testing machine and the radiation energy released into the external environment in the process, so it is necessary to simplify the energy conversion process of the specimen loaded system. The energy conversion process of the loaded system needs to be simplified to some extent. Assume that the mechanical energy generated by the compression tester in the loaded system is all stored in the specimen, and the energy absorbed by the specimen u is all converted into elastic deformation energy u e and dissipation energy u d by the law of conservation of energy, as shown in Equation (2). 34 where u is the total energy absorbed by the specimen, kJ/ m 3 ; u e is the elastic energy stored in the specimen, kJ/m 3 ; u d is the dissipated energy generated during the loading of the specimen, kJ/m 3 .
The total energy absorbed by the rock in the complex stress state is affected by the stress in each direction, the background of this paper is the energy evolution of the specimen in uniaxial compression, so only the axial stress does work, the total energy absorbed by the specimen u and the elastic energy stored in the specimen u e can be expressed in Equations (3) and (4).
where σ 1 is the axial stress, MPa; ε 1 is the axial strain; E 0 is the modulus of elasticity of the specimen, GPa.

| Energy evolution law
The experimental parameters of uniaxial compression can be obtained in the experiment, and the total energy and elastic energy obtained during the loading of specimens with different pore diameters are plotted as energy-strain curves and fitted to each curve, and the curve plotting and fitting results are shown in Figure 9. By fitting the total energy-strain curve and elastic energy-strain curve of specimens with different pore diameters, the fitting results are shown in Table 2, and the relationships between the total energy, elastic energy, and strain generated by the specimens during the loading process are obtained, and the R 2 of the fitted curves are all greater than 0.98, which indicates that the fitted curves have high accuracy, and the trend of the curves can be used to analyze and predict the energy evolution of the pore-containing rock of each pore diameter. The energy evolution of the specimens with various pore sizes can be analyzed and predicted by the trend of the curve. As can be seen from Figure 9A, the total energy absorbed during the loading of each pore size rock specimen shows a nonlinear growth characteristic, with the increase of compression load, the growth rate of the total energy absorbed also increases continuously. When the compression load is small, the specimen is gradually compacted with tiny cracks inside the specimen, which does not produce elastic deformation, and the total energy absorbed by the specimen and the growth rate of elastic energy is almost zero. When the specimen is deformed elastically, the energy growth trend is exponential, and the total energy absorbed by the specimen at this stage is equal to the elastic energy generated by the elastic deformation, therefore, whether the specimen is compacted or deformed elastically, the total energy-strain curve and the elastic energy-strain curve trends are consistent, and the energy growth trend is nonlinear growth. When plastic deformation of the specimen occurs, the growth rate of total energy continues to increase, but the growth rate of elastic energy remains constant and the growth trend is close to linear growth, and the gap between total energy and elastic energy increases continuously. The growth of elastic energy reaches the peak when the specimen is damaged, and then the energy evolution curve decreases with the increase of load, but the total energy absorbed by the specimen still keeps growing for a short time because the specimen still has a certain bearing capacity after the damage. In general, the total energy of the specimens containing pore rock shows a nonlinear growth with increasing load under uniaxial compression, and the growth rate of the total energy growth process also accelerates; when the total energy-strain curve absorbed by the specimen gradually deviates from the elastic energy-strain curve generated, that is, the plastic deformation generated inside the specimen increases rapidly, the growth rate of elastic energy slows down and the growth rate of dissipative energy accelerates.
As can be seen from the figure, for specimens with different pore diameters, the absorbed total energy and elastic energy evolution curves show a similar pattern with the increase of pore diameter. Eight millimeter pore diameter rock specimens with pores have greater peak total energy or peak elastic energy than other pore diameters, and the peak total energy and peak elastic energy are decreasing during the gradual increase of pore diameter from 8 to 14 mm. In the total energy-strain and elastic energy-strain curves of each hole diameter, the change trend of each hole diameter specimen in the compaction stage is almost the same. In the elastic deformation stage, the growth rate of the total energy curve of each hole diameter increases with the increase of hole diameter, but the growth rate of the elastic energy curve of each hole diameter is the same, because the elastic energy u e is only related to the elastic modulus of the specimen material and the loaded load, not related to the morphology of the material. During the plastic deformation stage, the growth rate of the total energy curve of each pore diameter increases, but the change of the growth rate of the curve with the increase of the pore diameter is small, and the growth rate of the elastic energy curve of each pore diameter remains unchanged. For the whole compression process of pore-size rock specimens, the total energy curve showed a nonlinear growth trend, the elastic energy curve showed a nonlinear growth trend before specimen destruction and a cliff-type decrease in the curve after specimen destruction, the peak total energy and peak elastic energy decreased with the increase of pore diameter of pore-containing specimens, the growth rate of total energy curve increased with the increase of pore diameter, and the growth rate of elastic energy curve did not change with the change of pore diameter. The growth rate of the total energy curve increases with the pore diameter, and the growth rate of the elastic energy curve does not change with the pore diameter.

| Energy distribution law
Comprehensive analysis of the stress-strain curve of the specimens with pore size under uniaxial compression shows that the deformation of the specimens is elastic when the specimens are loaded, and the plastic deformation of the specimens gradually occurs with the increase of the load. Table 3 shows the peak total energy, elastic energy, and dissipation energy of the specimens with pore size. As can be seen from Table 3, with the increase of the pore diameter of the rock specimens containing pores under uniaxial compression, the total energy absorbed by the pore-containing specimens and the peak elastic energy and dissipation energy generated by the specimens of each pore diameter have a large difference, but the peak energy changes are on a decreasing trend, in which the total energy and peak elastic energy of the 8 mm pore-containing rock specimens are the largest, 37.74 and 23.61 kJ/m 3 , respectively, and the peak energy and peak elastic energy of the 14 mm pore-containing The peak total energy and peak elastic energy of 14 mm pore-bearing rock specimens are the smallest, 23.66 and 10.07 kJ/m 3 , respectively, and the peak total energy and peak elastic energy of 10 and 12 mm pore-bearing rock specimens are 31.03 and 16.97 kJ/m 3 , 27.76 and 13.93 kJ/ m 3 , respectively, compared with the peak dissipation energy. Compared with the peak dissipation energy, the peak total energy and elastic energy change more with the increase of pore diameter, when the pore diameter increases from 8 to 14 mm, the peak total energy and elastic energy decrease by 37.31% and 57.35%, respectively, but the peak dissipation energy only decreases 3.82%, which is almost negligible compared with the peak total energy and elastic energy. For the rock specimens with pores under uniaxial compression, the elastic energy storage limit of the specimens decreases more as the pore size of the specimens increases, so it can be concluded that the main reason affecting the strength of the rock specimens with pores is whether the specimens as a whole have a large elastic energy storage limit, and the larger the elastic energy storage limit, the higher the strength of the specimens. From the energy ratio of peak elastic energy and dissipated energy, the mechanical energy generated by the uniaxial compression tester when the 8 mm pore size rock specimen is close to destruction is overwhelmingly transformed into elastic energy stored inside the specimen, of which the peak dissipated energy of the 8 mm pore size rock specimen accounts for 37.44% of the total peak energy, but as the pore size of the specimen increases, the strength intensity of the specimen decreases, and the total energy absorbed from outside, the peak elastic energy also further decreases. When the pore diameter of the pore-containing rock specimen increased from 8 to 10 mm, the peak total energy and elastic energy decreased by 17.78% and 28.12%, respectively, and the peak dissipation energy increased from 37.44% to 45.31%. As the pore diameter of the specimen with pores continued to increase to 12 mm, the peak total energy and elastic energy decreased by 26.44% and 41%, respectively, compared with the 8 mm pore diameter specimen, and the peak dissipation energy increased to 49.82%, with about half of the total energy absorbed from outside transformed into elastic energy and the other half into dissipation energy. When the pore diameter continues to increase to 14 mm, the peak total energy and elastic energy decrease by 37.31% and 57.35%, respectively, compared with the 8 mm pore diameter specimens, and the peak dissipation energy increases to 57.44%, it can be seen that with the increase of the pore diameter of the specimens, along with the increase of the pore diameter of the rock specimens containing pores, the total energy and elastic energy of the specimens containing pores nonlinear change trend is basically the same, but from the perspective of energy distribution. However, from the perspective of energy distribution, the maximum total energy absorbed by the specimen and the peak elastic energy decrease at the same time, the peak elastic energy also decreases, and the difference between the maximum elastic energy and the minimum elastic energy is about 20%. The pore size of the pore-containing rock specimen increases from 8 to 14 mm, the total energy absorbed from the outside world and the elastic energy storage limit decreases continuously, and the percentage of peak elastic energy also decreases continuously, although the difference between the peak dissipation energy under each pore size is not large, but the percentage of peak dissipation energy increases continuously, most of the energy absorbed from the outside world by the 8 mm pore size pore-containing rock specimen is converted into elastic energy, and part of the energy is converted into dissipation energy, with as the pore size of the specimen increases, the energy distribution law inside the pore-bearing rock specimen changes, and the total energy absorbed from the outside gradually changes to dissipative energy, resulting in an increasing proportion of dissipative energy.
where brittleness index modification (BIM) is the correction value of brittleness index; u e max is the peak elastic energy of the sample under load, kJ/m 3 ; u max d is the peak dissipated energy of the sample under load, kJ/m 3 .
The ratio of peak elastic energy to peak dissipation energy of the pore-bearing rock specimen under uniaxial compression can be defined as the BIM, 35 as shown in Equation (5), and the larger the BIM value, the larger the plastic deformation produced. From the data in Table 3, we can calculate the BIM values of 8-14 mm pore-size rock specimens with pores of 0.59, 0.83, 0.99, and 1.35, respectively, indicating that the larger the pore size, the larger the plastic deformation produced, and the more obvious the ductile damage characteristics of the porecontaining specimens will be.

| CONCLUSION
In this paper, uniaxial compression experiments were conducted on prefabricated pore-bearing rock specimens with different pore diameters by combining uniaxial compression experiments and DIC technology, and the strain field and energy evolution characteristics of porebearing rock specimens with different pore diameters were studied when they were loaded, and the destabilization damage law of pore-bearing rock specimens with different pore diameters was analyzed, and the following conclusions were obtained.
(1) The relationship between the specimen pore size and the specimen-loaded fracture development was obtained by combining the DIC technique to observe the compression-loading process of the pore-bearing rock specimen. With the increase of pore size, the strain area of damage by tensile fracture gradually decreases and the strain area of damage by shear fracture gradually increases, and the damage of the dominant specimen gradually evolves from tensile fracture as the dominant factor to tensile fracture and shear fracture together as the dominant factor and finally to shear fracture as the dominant factor. (2) The relationship between the energy change law inside the specimen and the pore size of the specimen when the specimen is loaded is obtained by analysis. The total energy change curve absorbed by the specimen when compressed and loaded showed a nonlinear growth trend, and the elastic energy curve showed a precipitous decrease after the specimen was damaged, while the growth rate of the total energy change curve increased with the increase of the pore diameter and the growth rate of the elastic energy change curve did not change with the change of the pore diameter. (3) The energy distribution law in the process of compression and loading of rock specimens with pores is analyzed, and the connection between the pore size of the specimen and the damage characteristics of the specimen is obtained, with the increase of the pore size of the specimen, the elastic energy storage limit of the specimen decreases more, and the energy distribution law inside the rock specimen with pores changes, the total energy absorbed from the outside gradually changes to the dissipated energy, resulting in an increasing proportion of dissipated energy, indicating that the larger the pore size, the greater the plastic deformation generated, the larger the pore size, the larger the plastic deformation, and the more obvious the ductile damage characteristics of the pore-bearing specimen will be.