The development of shale gas is widely regarded as a revolution in the global energy sector, not only increasing natural gas production, but also having significant impacts on global energy supply, climate change, and related policies. China is blessed with abundant shale gas with geological resources reaching 123.01 × 10¹² m³ (Sun et al. 2021), and the exploration and exploitation of shale gas is an important way to alleviate the energy and environmental pressure of China (Zhang et al. 2022b). Shale gas is an in-situ reservoir gas that is self generated and self stored, characterized by low porosity, low permeability, and high adsorption. With the increase of the exploration degree of shallow and middle shale gas systems, it is an inevitable trend for shale gas exploration to explore deeper(3500 ~ 4500 m) (Wang et al. 2022). Due to abundant micropores and microcracks in shale formation, the fow mechanisms in shale reservoir are much more complex than that in the conventional gas reservoir (Shen et al. 2017). Relevant studies have shown that enrichment and high production of deep shale gas are mainly influenced by lithology, thickness, physical properties, total organic carbon mass fraction, maturity, gas content, pressure coefficient and brittleness index (Hu et al. 2023; Saberi and Hosseini-Barzi 2024; Zhang et al. 2022c). The current mainstream shale gas extraction technology is to use hydraulic fracturing to create fractures. Although the technology is relatively mature, its shortcomings are also prominent, such as rapid production decline, low recovery rate, high water consumption, and resource waste. In addition, although carbon dioxide fracturing technology is also developing, this method has a large filtration vector of fracturing fluid, which limits the scale of fracturing construction. Laser (Bazargan et al. 2012), a highly concentrated “pure” energy, has been successfully applied in industrial production, medical treatment, technological research and development due to its excellent monochromaticity and directionality, extremely high brightness, and strong coherence, which provides new possibilities for breaking through the bottleneck of existing drilling and completion technologies.

In recent years, research on laser rock breaking technology has gradually entered the public eye, mainly focusing on indoor experiments and numerical simulations. In terms of indoor experiments, Li et al. (2022) found that under laser irradiation, the melting zone has the highest temperature, and the damage exhibits a stepped fracture, while the damage form in the damage zone and heat affected zone is mainly crack cracking. Wang et al. (2020a) examined the rate of perforation, temperature gradient, and specific energy of limestone irradiated by laser under different laser powers. Guo et al. (2022a) studied the weakening degree of rock strength by laser power, irradiation time, and irradiation distance. Research has shown that laser irradiation time has the greatest impact on the degree of rock strength weakening, followed by laser irradiation distance, while laser power has the least impact on the degree of rock strength weakening. Hu et al. (2018) determined the maximum thermal expansion coefficient of different rock types irradiated by high laser power. Yang et al. (2023) conducted mechanical and drillability tests on sandstone under laser irradiation of different durations, and the results showed that as the laser irradiation time increased, the number and area of surface cracks on sandstone significantly increased. Reasonable setting of laser parameters helps to improve rock breaking efficiency and reduce drilling costs, and can achieve faster and more economical rock breaking goals. Yang et al. (2020a, b) believed that the strength of rock is the main factor affecting laser perforation efficiency, and the lower rock strength resulted in deeper perforation depth, higher ROP, and lower SE.The characteristics of rock temperature during laser perforation process affect the efficiency and energy consumption of rock breaking. Liu et al. (2023) studied the effects of immersion conditions on temperature, pore size, rock fragmentation efficiency, and macroscopic fracture after laser irradiation of rock fragmentation. Their research has shown that the perforation depth of soaked samples is significantly lower than that of dry samples. Pan et al. (2022) studied the characteristics of circumferential strain, temperature, pore size, and corrected specific energy of shale under laser irradiation with different powers, frequencies, and focal lengths. The results indicate that there is a significant positive correlation between laser power under aperture effect and laser frequency, while the corrected specific energy shows a negative correlation. Li et al. (2017) analyzed the influence of laser process parameters on rock fragmentation specific energy and perforation size under different media conditions. The results indicate that as the power and irradiation time increase, the perforation size of granite and sandstone gradually increases. Wang et al. (2020b) investigated whether the removed mass, cracking quality, open porosity, and grooving depth of samples increased nonlinearly with the decrease of the spot diameter by laser irradiation on hot dry rock. Kuang et al. (2022) used laser irradiation cracking method to study the effect of laser irradiation with different power, diameter, and movement speed on the distribution of granite cracks. The study showed that the crack angle and crack area at both ends were related to laser parameters, and the cracks mainly occurred around the slotted incision generated by the laser beam. Guo et al. (2022b) found that when the laser power is 1300 W, the scanning speed is 6.7 mm/s, and the radiation distance is 51.6 mm, the laser rock breaking efficiency is highest and the energy consumption is lowest. Yan et al. (2013) studied the interaction mechanism between laser and rock during perforation using optical microscopy, high-speed video imaging, X-ray diffractometer, and other methods. The results indicate that the vapor/plasma of laser perforation is in a sudden change process, indicating that complex physical and chemical reactions are taking place.

In terms of numerical simulation, Ndeda (2017), Li et al. (2018) and others used ANSYS finite element numerical software to simulate the temperature, stress distribution, and pore formation characteristics of laser irradiated granite under different parameters. San Roman Alerigi et al. (2016, 2017) achieved thermal mechanical coupling simulation of laser rock interaction using COMSOL Multiphysics and 2D-FLAC numerical simulation software. They used the periodic distribution of mass density in axisymmetric structures to characterize heterogeneous rocks and pointed out that the mechanical damage caused by rock excitation varies with the direction and number of cycles of heterogeneity. Chen et al. (2022) revised the energy density distribution function model, and innovatively constructed a numerical calculation method for the thermodynamic coupling problem by laser drilling into sandstone. The simulation results show that the efficiency of laser drilling can be greatly improved by controlling the efficient removal of the melting rock particles during the phase transformation process so that the diameter and depth of the drilling hole are in the S-shaped growth stage. Chen et al. (2019) established the laser-induced damage growth model in optics under high-power laser irradiation based on the Weibull distribution model. Through numerical simulation and experimental tests, Li et al. (2019) determined the stress distribution and fragmentation mechanism of granite during laser perforation. Liu et al. (2015) established a three-dimensional finite element model for laser rock breaking. Research has found that under set conditions, for rocks with high intensity, the laser gasification plays a major role in the laser rock breaking process. For rocks with lower strength, the main effect is the fragmentation caused by thermal stress. Mu et al. (2017) applied the method of temperature field distribution combined with thermal stress equation to simulate the thermal stress distribution inside rocks with and without inclusions. The research results showed that under the same laser beam for the same time, rocks with inclusions generated greater thermal stress near the edge of inclusions than those without inclusions. Rui et al. (2021) established a mathematical model to describe the temporal and spatial evolution of rock damage under a given laser scanning path. By comparing numerical and experimental results, it was demonstrated that the multi parameter damage model is suitable for numerical simulation of laser-induced rock damage.

In summary, scholars have started with experimental and numerical simulation research on laser assisted rock breaking, and have conducted research on rock strength reduction, crack propagation law, and multi field coupling effect after laser irradiation, and have achieved a lot of valuable results. However, it is worth noting that there is no research on the bedding effect of shale under the irradiation of laser at present. The fracturing effect of shale under laser irradiation is still unclear, and the mechanism of the influence of laser on rock bedding structure needs further investigation. At the same time, the correlation between experimental research and engineering applications needs to be further improved. This manuscript intends to investigate shale with different bedding angles and systematically conduct laser irradiation thermal fracturing experiments. Starting from three perspectives: the distribution characteristics of shale temperature field under laser irradiation, the spatial distribution of fractures, and the mechanism of failure in shale under laser irradiation, this research comprehensively explores the response law of laser irradiation time, spot diameter, and other factors to shale thermal fracturing, aiming to provide a basis and reference for the theory and technology of laser assisted shale gas extraction.

2.1 Experimental preparation

2.1.1 Sample preparation

The sample was taken from the Longmaxi Formation in Yibin, China, and is black in color with a layered structure (as shown in Fig. 1). The complete rock sample density and wave velocity are shown in Table 1. The experimental group selected 21 standard samples with a size of ø50 mm × 100 mm, 3 blocks each at 0°, 15°, 30°, 45°, 60°, 75°, and 90°, with bedding angles as shown in Fig. 2. According to the experimental results of X-ray diffraction (XRD), the main components of the sample were quartz, calcite, dolomite, pyrite, plagioclase, and clay minerals. Among them, quartz, calcite, and dolomite had the highest content, with mass fractions of 31.0%, 29.6%, and 28.3%. The content of pyrite and plagioclase was relatively low, with mass fractions of 2.8% and 1.4%, respectively, and the mass fraction of clay minerals was 7.0%, as shown in Fig. 3. According to the experimental results of X-ray fluorescence spectrometer (XRF), the mass fraction of SiO₂ in the sample was 46.84%, and the mass fraction of CaO was 29.41%, which were the two highest content components, as shown in Table 2.

Shale sample

Schematic diagram of sample bedding angle

XRD pattern and mineral composition of shale samples

2.1.2 Experimental system and composition

The experimental system is a laser weakening hard rock testing device system designed and developed by our research group (Yang et al. 2023). It can simulate the laser-assisted rock-breaking test in an engineering in-situ environment and monitor the evolution of drilling temperature and crack development characteristics in real-time. The system can capture the entire process of rock surface heating and fracture under different laser powers in real-times, as shown in Fig. 4. The system mainly consists of a JPT-QCW-R-B-W-2000-2500-52 quasi-continuous fiber laser, a CCV-030AATL49-B-2500 dual temperature dual control laser water cooler, a PIXConnect-MA-E2020-05-A infrared thermometer, an A7A20CU30 industrial camera, a protective gas tank (nitrogen), etc. The power range of quasi continuous fiber laser is 250–2500 W. The temperature control range of the dual temperature and dual control laser water cooler is 8–35 °C. The infrared thermometer has a temperature measurement range of 0 ~ 926 °C (low temperature) and 926 ~ 2450 ℃ (high temperature), with a temperature measurement accuracy of ± 2 °C. The maximum resolution of industrial cameras is 4096 × 3000 pixels, with a maximum frame rate of 30 frames.

Laser assisted rock breaking equipment system

2.2 Experimental steps

Through preliminary exploration experiments, it was found that this batch of samples reacted violently under laser irradiation. Under irradiation conditions with a power of 1000 W and a spot diameter of 4mm, the crack growth and development speed was very fast. It was completely ruptured by 22 s, producing a considerable number of cracks and detachment of rock blocks. The laser rock breaking effect was excellent. The experimental procedures are as follows:

1. (1)

   Select rock samples and completed grouping, numbering and basic physical property testing according to requirements. The numbering convention followed the format of “group number—bedding angle”.

2. (2)

   Conduct laser irradiation tests on selected rock samples, with the laser irradiation scheme parameters set as shown in Table 3.

   Bedding angle α (°)	Irradiation power (W)	Spot diameter (mm)
   0/15/30/45/60/75/90	250	4/6/8

3. (3)

   Perform 3D laser scanning on the rock sample after laser irradiation to obtain images of the fracture surface of the rock sample.

4. (4)

   Select the area near and relatively far from the molten area of the sample after laser irradiation for SEM, XRF, and XRD testing.

5. (5)

   Summarize the data and observe the crack propagation and related failure characteristics of rocks before and after laser irradiation, as the bedding angle and laser spot diameter change. The experimental process is shown in Fig. 5.

   

   Experimental flowchart

3.1 Evolution characteristics of temperature field

The morphological characteristics of the shale sample labeled with 1–45 after 24 s of laser irradiation are shown in Fig. 6. Based on the surface temperature values and the changing of the sample morphology, it was divided into the pore forming zone (I), the high heat affected zone (II), and the low heat affected zone (III). The temperature rising characteristics of the irradiation surface of the sample are shown in Fig. 7. With the increase of irradiation time, the temperature decreases from zone I to the surrounding area in a circular pattern. At the beginning of laser irradiation, the temperature in the central perforation area can instantly exceed 926 °C. The temperature around zones II and III was relatively low, with a temperature range of about half of the temperature at the center of the perforation. With the increase of time of laser irradiation, heat was continuously transferred from region I to regions II and III, and the influence range of region I tended to be stable until the time of irradiation time was 15 s. However, with the continuous increase of irradiation time, the influence range of heat still expanded slowly. In conclusion, the area of the rock sample’s transverse heat affected zone rose as the irradiation period increased and stabilized after 15 s. At this time, the molten material in the central pore area hindered the downward penetration of the laser. The heat transfer in air convection and reflection on the rock surface squandered some of the total energy of the laser, and the remaining fraction was used to maintain the molten material in the sample hole at a boiling point.When the total energy provided by the laser is basically equal to the total energy consumed by boiling and convective heat transfer of the molten material in the perforation, a thermal equilibrium state will be reached, and the temperature change will also tend to stabilize, which is essentially consistent with the research conclusion of Yang et al. (2023)

Heat affected zone of the sample

Temporal variation of temperature on the irradiation surface

To analyze the temporal temperature variations at various positions on the sample surface, designated points A, B, and C were selected along three diameter directions on the top surface of the shale sample that was labeled as 1–45. These points were positioned at distances of 5 mm, 10 mm, and 15 mm from the center, as illustrated in Fig. 8.

Schematic diagram of positions A, B, and C

The point A1 is 5 mm away from the center of the circle. Because the diameter of laser beam is 4 mm, this position is very close to the laser irradiation position. After only 1 s of irradiation, the temperature had risen to 78.4 °C. According to Fig. 9, the temperature rise curve of point A is a natural logarithmic function within the irradiation time. The heating rate is very fast in the first 5 s, and although the rising rate slows down in the 5–10 s, it still rises rapidly. Afterwards, the rate of increase continued to slow down until it stabilized. The point B1 is 10 mm away from the center of the circle. Compared to point A1, it is further away from the center. During the process of conduction to point B1, the high temperature near the center is absorbed by rock minerals, resulting in some loss. Therefore, the temperature rise rate at this point is significantly slower than that at point A1. The fitting equation for the temperature change over time during irradiation is a quadratic polynomial, and the coefficient of the higher-order term is very small. The point C1 is 15 mm away from the center of the circle. When the central temperature extends to this position, the rock minerals at this point no longer have a large amount of heat to absorb, so the rate of temperature rise is the slowest. The fitting curve of temperature with time during irradiation is a linear equation with a slope k of 0.7904. The fitting curve equation is shown in Table 4, and the fitting curve is shown in Fig. 9.

Temperature variation fitting curve of sample irradiation surface at different positions from the center of the circle

Overall, the temperatures at points A2 and B2 are significantly higher than those at A1, A3, B1, and B3, which results in uneven temperature changes and a larger temperature gradient in the A2 and B2 directions. This is consistent with the development of cracks, where cracks first appear in the direction of A2 and B2. The reason why the temperature rise curve changes with distance from the irradiation center may be due to the high energy of the laser beam. When the laser is irradiated on the surface of the rock, the surface of the rock directly irradiated by the laser beam will rapidly heat up until it is about to reach the maximum temperature that the laser beam can provide. Rocks that are far from the center and have not been directly irradiated by the laser beam receive temperature from the diffusion heat transfer of the temperature at the center position, and the heat transfer rate is much slower than that of direct laser beam irradiation. Therefore, the temperature rise in this area is significantly lower than that in the central area, and the maximum temperature that can be reached is also significantly lower than that in the central area. Further, in the area farther away from the center, a portion of the heat is absorbed by the rock during the conduction process. So, less heat can be obtained, the temperature will rise more slowly, and the highest temperature can be lower.

According to the results shown in Table 4, the correlation coefficients (R²) of the temperature change fitting curves for points A2, B2, C2, A3, B3, and C3 are all above 0.95, indicating good fitting results. It is worth noting that although the B1, B2, and B3 fitting curves are quadratic function curves, the quadratic coefficients are very small, and the quadratic coefficients of the three fitting curves are almost all less than 0.05. Within the range of x ≤ 30 s, 0.05x² is almost negligible compared to the sum of the other two terms. Therefore, linear fitting was performed on the temperature changes of B1, B2, and B3, and a comparison was made with the quadratic fitting. The results are shown in Fig. 10. According to the fitting results, the R² of the linear fitting was all above 0.94, and the fitting results were still good. It is obvious that the quadratic fitting curves of points B1 and B3 in the horizontal and vertical directions can better reflect the actual temperature changes. Therefore, the quadratic fitting curves are used for points B1 and B3, and the linear fitting curves are used for point B2. The reason for the above phenomenon was that the point B2 rapidly reached a higher temperature than the other two points, which leaded to cracks and released energy, thus transforming into linear growth.

Comparison of temperature change fitting curves

Additionally, from the variation of the temperature distribution of irradiation surface over time shown in Fig. 7a, we can infer that although the temperature in the B2 direction rises faster in the early stage, the temperature distribution in the three directions becomes almost consistent later on. This also indicates that regions outside the laser irradiation center cannot be described by a single temperature rise curve.

From the analysis of the experimental temperature data, we found that the temperature rises fastest near the laser irradiation point and slowest near the outer edge of the rock sample. Qualitatively, due to heat absorption by the rock minerals during conduction, the temperature rise rate in the intermediate region between the two should lie between them, indicating the presence of different temperature rise curves.

In summary, the upward trend of temperature from the irradiation center along the radius direction on the irradiation surface is: logarithmic growth → square growth → linear growth.

3.2 Response characteristics of shale fracture field under laser irradiation

3.2.1 Growth pattern of cracks on the irradiation surface

The illustration in Fig. 11 depicts the process of crack propagation on the irradiated surface of shale with varying bedding angles (α) under laser irradiation, with a spot diameter of 4 mm. It is crucial to acknowledge that, owing to the post-production supplementation at 0° and 15°, there were inconsistencies in the lighting intensity of the supplementary lights, leading to variations in image colors. Additionally, to safeguard the testing equipment, the test was promptly halted upon the destruction of the rock sample.

Crack propagation process at the end of shale sample under laser irradiation with a spot diameter of 4 mm (the part marked in the figure is the crack)

When α was 0°, continuous laser irradiation caused a notable overflow of molten material from the irradiation hole at t = 5 s. At t = 10 s, the surface area covered with molten material on the irradiated surface broke away from the main body, and two radial cracks appeared along the approximately same diameter. The crack width continued to develop as the laser irradiation continued. At t = 20 s, a third radial crack, approximately perpendicular to the first two cracks, appeared.

When α was 15°, laser irradiation caused a continuous molten material overflow from the irradiation hole, accompanied by splashing. There were no apparent cracks within the first 7 s, and cracking occurred at t = 8 s. The image at t = 8 s (Fig. 11) displays the irradiated surface after the test, with the sample split into two pieces along the diameter.

When α values were 30°, 60°, and 75°, the failure process exhibited similarities to that observed at 15°. In the initial stages, no apparent cracks were discerned. However, at t = 7 s, 6 s, and 8 s, respectively, the sample suddenly burst and divided into two parts along the diameter.

When α was 45°, microcracks began to manifest at t = 3 s. With increasing irradiation time, three radial through cracks gradually developed, with two roughly aligned along the same diameter and vertically related to the third crack. At t = 19 s, a fourth crack emerged, approximately parallel to the two cracks on the same diameter, exhibiting rapid growth and becoming the primary crack at t = 30 s.

When α was 90°, continuous laser irradiation led to a continuous overflow of molten material from the irradiation hole. However, no obvious cracks were generated until t = 19 s. At this juncture, the region containing molten material on the irradiated surface cracked and propelled away from the main body, producing two radial cracks along the same diameter.

In summary, shale with different bedding angles showed significant similarities when exposed to laser irradiation. These similarities can be classified into two distinct types: the first category involves fracturing, and the second category involves bursting. When the bedding angle is 0° or 45°, it falls into the first category. When laser irradiation was initiated, cracks appeared near the irradiation point at 10 s and 3 s, respectively. These cracks then propagated along the bedding plane until they penetrated the surface. Continuous exposure to the laser caused the formation of new cracks that were perpendicular to the initial ones. These cracks extended until they reached the outermost surface. Afterward, arc-shaped cracks appeared near the irradiation point, and cracks that penetrated them gradually appeared between the aforementioned cracks, eventually forming a “Y”-shaped crack.

3.2.2 Surface crack growth pattern of shale with different bedding planes

Under laser irradiation with a spot diameter of 4mm, cracks propagated on the side of the shale samples with varying bedding angles, as shown in Fig. 12. The fractures generated on the rock samples after laser irradiation have been indicated with dashed lines in the figure. In Fig. 12a, at a bedding angle of 0°, following the initiation of the crack from the irradiation hole, it initially propagated downward perpendicular to the horizontal plane, propagated to half the height of the sample, then gradually propagated to one side, and finally penetrated to the surface.

Crack propagation on the side of shale sample under laser irradiation with a spot diameter of D = 4 mm

At a bedding angle of 15°, there were no noticeable cracks in the sample before burst failure (Fig. 12b). The crack propagation had an obvious tendency to propagate along the bedding angle. Similarly, at a bedding angle of 30°, no visible cracks were observed before burst failure (Fig. 12c), but crack propagation penetrated the entire sample in a nearly vertical manner.

In Fig. 12d, at a bedding angle of 45°, the initiation of the crack closely resembled α = 0°. It extended vertically downward before deviating towards one side, expanding roughly parallel to the bedding angle, and ultimately penetrating the surface. Simultaneously, it extended to the opposite side at the transition point. In Fig. 12e, when the sample with a bedding angle of 60° experienced catastrophic failure, the cracks strictly propagated from the irradiation surface along the bedding plane. Similarly, in Fig. 12f, at a bedding angle of 75°, the crack propagated from the irradiated surface, roughly following the bedding plane during sample bursting.

In summary, when bedding angles are lower than 45°, the crack consistently undergoes an initial stage of downward propagation perpendicular to the top surface. This stage may vary in length, either being brief or prolonged. Subsequently, the crack extends laterally until it penetrates the surface. However, the correlation between crack propagation direction and bedding direction is generally not readily apparent. At approximately a 45° bedding angle, there is also a stage of downward propagation perpendicular to the top surface, followed by continuous expansion along the bedding. For bedding angles surpassing 45°, the crack propagation direction closely aligns with the bedding direction, as nearly all cracks directly propagate along the bedding until penetrating the surface.

4.1 Macro fracture characteristics of shale under laser irradiation with different spot diameters

Photographs illustrating damage to shale with varying bedding angles under laser irradiation with different spot diameters were presented in Fig. 13. When the bedding angle was 0°, indicating a perpendicular relationship between the laser beam and the bedding plane, the depth of crack propagation decreased gradually as the spot diameter increased. This trend can be attributed to the greater energy accumulation per unit area for smaller spot diameters, which enhances the penetration capability into the rock sample.

Macroscopic failure characteristics of shale at different bedding angles under laser irradiation with different spot diameters (solid lines represent developed fractures, dashed lines represent fracture development trends) 4.2 Zoning characteristics of shale fracture morphology

At a bedding angle of 15°, the laser beam forms a 75° angle with the bedding plane. As the spot diameter increases, the depth and angle of crack propagation sustain stabilization. Similarly, at a bedding angle of 45°, where the laser beam forms a 45° angle with the bedding plane, the depth and angle of crack propagation also stabilize with an increasing spot diameter.

The cracks in their development process consisted of two stages: the first stage involved a vertical downward extension, and the second stage involved an extension along the bedding plane. To sum up, the sample showed the highest level of fragmentation when the spot diameter was 6mm. Conversely, the sample experienced the lowest degree of fragmentation when the spot diameter was 8mm. When the spot diameter was 4 mm, the sample’s fragmentation level fell between the other two.

When the bedding angle was 75°, the test result was similar to the result obtained when the bedding angle was 15°. For a bedding angle of 90°, where the laser beam is parallel to the bedding plane, the depth of crack propagation initially increased and then decreased as the spot diameter increased.

In summary, when the spot diameter is 4 mm, the high energy per unit area can cause an uneven distribution of thermal stress. This is due to the hindrance of heat lateral conduction caused by the presence of bedding planes. This results in rapid damage to the irradiated surface, with no visible cracks on the lateral side at this time. When the spot diameter is 6 mm, the energy per unit area is relatively reduced. While the irradiation hole extends downward, cracks also develop along with the bedding plane. With an 8 mm spot diameter, after the irradiation hole extends downward for a certain distance, the laser beam’s energy can only sustain the molten material in a high-temperature state without creating additional holes. At this time, heat accumulates at the bottom of the formed irradiation hole, and the temperature is much higher than that of the surrounding rocks. Eventually, under the action of thermal stress, volume fracturing occurs, and the crack deflects from the center side to the outside.

4.1.1 Macro zoning characteristics of fracture morphology

The morphological features of rock fracture surfaces post laser irradiation exhibit a close correlation with the rock failure mode. A quantitative characterization of the rough structure on various rock fracture surfaces subjected to laser irradiation can significantly enhance our comprehension of rock fracture patterns during laser irradiation. In this investigation, the fractal dimension has been introduced as a quantitative measure for characterizing the morphological features of the sample’s fracture surface. The steps involved in obtaining the rough morphological characteristics of the fracture surface are depicted in Fig. 14. Subsequent to the extraction of three-dimensional point cloud data from the fracture surface, the fractal dimension of the fracture surface was computed.

Fracture surface 3D scanning-reconstruction-data point cloud acquisition

Moreover, due to the presence of both melting and tensile or shear failures on the fracture surfaces of each sample, with the areas exhibiting melting failure predominantly concentrated near the perforations, the overall difference in the fractal dimension of the fracture surfaces is relatively subtle. To better capture the distinctive roughness characteristics within various failure regions of the fracture surface, this investigation computed the fractal dimension by employing a 5 mm × 5 mm square to delineate three areas from the bottom of the irradiation hole—ranging from near to far. These areas are designated as the proximal region (Q1), the intermediate region (Q2), and the distal region (Q3), as indicated in Fig. 14. The selecting method involved connecting the lowest point of the laser penetration hole to the lowest point on the fracture surface, forming a line segment. The center point of the square region lies on this line segment, with the upper boundary of Q1 near the bottom of the penetration hole, the lower boundary of Q3 near the lowest point of the fracture surface, and the center of Q2 near the midpoint of the line segment. To facilitate the process of truncation, the plane containing the square is first aligned parallel to the yoz plane, followed by subsequent truncation.

The results of the fractal dimension calculation for each region are presented in Fig. 15. Specifically, the fractal dimension of Q1 region near the bottom of the irradiation hole mostly exceeds 2.012, which is significantly higher than that of Q2 and Q3 regions far from the bottom of the irradiation hole. This discrepancy arises from the pronounced influence of high temperatures on the region near the bottom of the irradiation hole, primarily undergoing melting damage. The molten material adhered to the surface of the damage, which exhibits relatively heightened roughness, resulting in a larger fractal dimension. The fractal dimensions of the Q2 and Q3 regions on the fracture surface of samples with different bedding angles both fall below 2.006, with a small difference ranging between 0.002 and 0.004. Nevertheless, the fractal dimension of Q2 remains consistently greater than that of Q3. This discrepancy arises because the majority of thermal energy supplied by the laser was absorbed by the mineral components in close proximity to the irradiation hole, with rapid decay in energy diffusion towards the periphery. Consequently, the intermediate and distal regions experienced less heat influence, resulting in a significantly lower fractal dimension compared to the proximal region near the laser irradiation center. In summary, the consistent trend in the overall fractal dimension across different intercepted areas on the same fracture surface is as follows: Q1 > Q2 > Q3. The roughness of different areas on the fracture surface is correlated with the distance from that area to the perforation, and the closer the distance, the rougher the fracture surface.

Fractal characteristics of fracture surface zoning in samples with different bedding planes

4.1.2 Microstructural characteristics of fracture morphology

To further understand the microscopic characteristics of bedding shale fractures, scanning electron microscopy analysis was conducted on rock samples from different regions of the fracture surface. The fracture morphology, microscopic composition, microstructure, and fracture characteristics of rock samples in different regions of the fracture surface could be observed and identified by scanning electron microscopy. The microstructure of each region is shown in Fig. 16.

Schematic diagram and micro-structure of different regions on the fracture surface

Based on the scanning images of the electron microscope, it can be found that the melt debris on the test surface changed from more to less in the three areas below the bottom of the irradiation hole. The rock sample in the A1 test plane is mainly melted and damaged, with many holes and a large amount of debris left after the overflow of molten material visible in the test plane. There are visible fractures along the structural plane and a large amount of debris in the A2 test plane, indicating that there is not only melting failure but also failure along the bedding structure. A smooth area can be seen in the A3 test plane, with only a small amount of detached particles and blocks, indicating that the main sliding failure along the bedding plane occurs in this test plane. In summary, the roughness of different areas on the fracture surface is correlated with the distance from that area to the perforation hole. The closer the distance, the rougher the fracture surface. The results of the electron microscopy scanning analysis are consistent with the fractal dimension calculation results in Sect. 4.2.1.

It is possible to conclude that the roughness characteristics of the fracture surface are closely related to the fracture mode in the laser thermal field of the sample by thoroughly analyzing the fractal characteristics of fracture surfaces of shale with different bedding planes and the crack propagation on the surface of shale samples. When the sample exhibits high-temperature melting failure, the roughness of the fracture surface of the sample is higher, which is consistent with the research results of Gao et al. (2022). It can be found from previous research that the larger the energy consumed during sample fracture, the more the fractal dimension of the fracture surface of the sample will also increase synchronously (Xu et al. 2015). Through analysis of the fractal dimension statistics, the fractal dimension of the melt fracture surface increased by up to 2.0% compared to the burst fracture surface. Therefore, from the roughness characteristics of the fracture surface, the failure mode of the sample can be inferred. The failure modes with high-temperature melting are primarily found in areas with high fractal dimensions, whereas the failure modes with low-temperature bursting are primarily found in areas with low fractal dimensions.

5.1 Physical and chemical mechanisms of shale failure under laser irradiation

5.1.1 The relationship between the temperature evolution law of the sample irradiation surface and crack growth

It can be found from previous research that a strong correlation between the stress gradient and the temperature gradient during the crack initiation and coalescence process was observed (Yang et al. 2021). If the temperature distribution is non-uniform, different areas will expand or contract differently, leading to the generation and concentration of thermal stress. Consequently, the regions with non-uniform temperature distribution are more likely to develop cracks. As shown in Fig. 17a, under the laser irradiation, the irradiated surface of shale with a bedding angle of 0° produced three radial main cracks (F1, F2 and F3), which passed through the center of the circle. In the process of crack growth, F1 appeared at the earliest time, and the growth direction of the crack was consistent with the uneven distribution of temperature in Fig. 17b. As shown in Fig. 18a, under laser irradiation, shale with a bedding angle of 45° exhibited the formation of four main cracks on the irradiated surface. Among them, F1, F2, and F3 are radial cracks passing through the center, while F4 is a crack traversing the irradiated surface of the sample. During the growth process of the cracks, F1 appeared earliest, and the direction of crack propagation was consistent with the uneven temperature distribution shown in Fig. 18b. In summary, during the early development of cracks, the uneven temperature distribution plays a dominant role in the formation and progression of cracks.

Schematic diagram of irradiation surface damage of the shale sample labeled with 1–0. a Cracks on the irradiation surface; b Temperature distribution on the irradiation surface

Schematic diagram of irradiation surface damage of the shale sample labeled with 1–45. a Cracks on the irradiation surface; b Temperature distribution on the irradiation surface

5.1.2 Vaporization of shale oil in reservoirs under laser irradiation

Shale oil refers to the liquid hydrocarbon accumulating in shale reservoirs, which is typically generated during organic matter pyrolysis (Song et al. 2020). It mainly occurs in nano-sized pores and fractures parallel to the bedding direction of reservoirs (Yang et al. 2019). During the experiment of laser irradiation, it can be found that when the laser beam is irradiated on the surface of the sample to form a perforation, shale oil will overflow on the surface of the rock sample around the perforation due to the high temperature, resulting in a large area of moist area (as shown in Fig. 19). In the region near the perforation center, shale oil underwent vaporization and volatilization due to the high temperature in this area, forming a dry zone. The reason for the overflow of shale oil is that, under the influence of high temperature, pre-existing microcracks within the rock sample continually develop, resulting in cracks that penetrate to the irradiated surface. A schematic diagram illustrating the overflow and vaporization of shale oil in the reservoir is shown in Fig. 20. Shale oil stored in the shale undergoes volume expansion at high temperatures, making its original storage space insufficient to contain it. Consequently, it overflowed along the cracks to the surface of the sample. With the continuous irradiation of the laser beam, the rock sample was continuously heated, causing partial shale oil to evaporate at high temperatures. Simultaneously, during the process of shale oil overflow and vaporization, the stress generated by volume expansion further accelerated the development and extension of cracks, positively influencing the rock-breaking effect under laser irradiation.

Rock sample irradiation surface after laser irradiation

Schematic diagram of shale oil overflow and vaporization in the reservoir

5.1.3 Chemical reactions of shale minerals under laser irradiation

It can be found from previous research that the vapor/plasma of laser perforation was in a mutative process, indicating that complicated physical and chemical reactions were proceeding (Yan et al. 2013). To further explore the changes in mineral composition of shale during laser fracturing, X-ray diffraction tests were conducted on the shale near the perforation after laser irradiation. The test results are shown in Fig. 21a. According to the test results, the content of quartz has significantly decreased, while the content of calcite, dolomite, and pyrite has decreased. The main component of quartz is SiO₂. Calcite is a calcium carbonate mineral with a molecular formula of CaCO₃, which can be decomposed into CaO and CO₂ at high temperatures of 900 ℃. Dolomite, with a chemical composition of CaMg(CO₃)₂, decomposes into a mixture of carbon dioxide, calcium oxide, and magnesium oxide when heated to 700–900 ℃. The decomposition equations is as follows:

Changes in mineral and oxide content before and after laser irradiation

Pyrite undergoes thermal decomposition at around 400 ℃ during the roasting process, with approximately 1 mol of elemental sulfur released per mole of FeS₂. Free elemental sulfur immediately reacts with oxygen in the air to form sulfur dioxide gas. This thermal decomposition reaction is generally represented by the following equation (Schlumberger Logging Company 1998): \({\text{FeS}}_{2} \to {\text{FeS}}_{1 + x} + 0.5\left( {1 - x} \right){\text{S}}_{2}\).

During roasting, pyrite undergoes complex oxidation–reduction reactions, and possible reaction mechanisms include (the thermal effect at 500 ℃):

With the generation of SO₂, the possible reactions that may occur simultaneously and the corresponding thermal effects at 650 ℃ are (Yu et al. 1997):

Combined with the changes in oxide content in the XRF test results (as shown in Fig. 21b), we can speculate that: (1) under the high temperature provided by the laser beam, calcite and dolomite underwent decomposition, resulting in a significant increase in the content of CaO and MgO. (2) In the microwave field, the pyrite underwent the oxidation–reduction reaction (1) when roasting at 600 °C (Jin et al. 2020), while the laser in this study provided a high temperature of nearly 1000 °C. Therefore, it is inferred that the pyrite underwent the oxidation–reduction reaction (1) at high temperature, generating Fe₂O₃ and SO₂. The schematic diagram of the reaction process is shown in Fig. 22. According to the findings of Yu et al. (1997) and Zhang et al. (2019), at temperatures of 650 °C and above, if the concentration of SO₂ is low, pyrite can only react with oxygen to produce iron oxide and SO₂. In our study, the SO₂ in the experimental system solely originated from the aforementioned reaction (1), without additional input of SO₂ into the experimental system. Consequently, the concentration of SO₂ was very low, rendering it unable to proceed with reactions (3) or (4) and thus escaping through pores after formation. (3) Furthermore, SiO₂ reacted with metal oxides such as Fe₂O₃, CaO, and Al₂O₃ under high-temperature conditions to form silicate-like substances, and the progress of this reaction reduced the content of quartz. At the same time, the above reaction (2) and (3) released additional energy in the form of heat energy, which had a favorable impact on the crack expansion. This may be an important reason for the samples, which did not undergo burst fracture in the early stage, to experience volume fracturing in the later stages.

Schematic diagram of chemical reaction and energy release of pyrite

5.2 Mechanical mode of shale fracture under the synergistic effect of laser irradiation and bedding structure

By conducting a comparative analysis of crack’s propagation direction and spatial distribution in rock samples that exhibit different bedding structures under laser irradiation, we can categorize the mechanical fracture patterns of shale. These patterns emerge due to the combined influence of laser irradiation and bedding structures, and they can be classified into the following three modes:

Pattern 1—Thermal Failure (Pattern T): As shown in Pattern 1 of Fig. 23, this corresponds to the typical failure observed in rock samples with a bedding angle of 0° when exposed to laser irradiation with a spot diameter of 6 mm. The direction of crack propagation is primarily influenced by thermal stress, regardless of the orientation of the rock sample’s bedding structure. After the sample surface is irradiated by the laser beam, there is a rapid increase in temperature. Initially, micro-cracks appear near the irradiation hole. As the laser beam continues irradiation, the temperature gradient on the irradiation surface increases. Subsequently, due to thermal stress, the micro-cracks gradually develop into main cracks. As the laser beam continuously erodes downwards, the cracks also spread from the top to the bottom on the side surface.

Failure mode of shale under the combined action of laser irradiation and bedding structure

Pattern 2—Thermal and Bedding Synergistic Failure (Pattern T-B): As shown in Pattern 2 of Fig. 23, this pattern represents the typical failure observed in rock samples with a bedding angle of 45° when exposed to laser irradiation with a spot diameter of 6 mm. The direction of crack propagation is influenced by the orientation of the rock sample’s bedding structure. Cracks initiate under the influence of thermal stress and propagate in a top-down trend. Simultaneously, influenced by the bedding structure, the crack tends to extend along the bedding. The crack propagation scenario observed in Pattern 2 of Fig. 23 emerges. The reason for the cracks only propagated along the bedding structure may be that this region, where cracks propagate along the bedding structure plane, is farther away from the bottom of the irradiation hole, and therefore primarily influenced by the bedding effect, with weaker influence from thermal stress.

Pattern 3—Bedding Failure (Pattern B): As shown in Pattern 3 of Fig. 23, this pattern indicates the typical failure observed in rock samples that have a bedding angle of 75° when they are subjected to laser irradiation with a spot diameter of 6mm. In this scenario, the bedding effect primarily influences the direction of crack propagation and the direction of crack propagation is consistent with the direction of the rock sample bedding structure. All three rock samples, which had a bedding angle of 75°, experienced burst failure during the test. No discernible cracks were evident before the failure occurred. During the process of laser irradiation, the minerals surrounding the irradiation hole continuously absorb and accumulate heat energy, resulting in thermal expansion. When the stress exceeded the load-bearing capacity of the bedding plane, it underwent destruction along the bedding plane, releasing energy and resulting in a fractured surface.

5.3 Expectation on laser rock breaking technology assisted deep shale gas reservoir stimulation

With the continuous maturity of mining technology, hydraulic fracturing plays an important role in shale gas extraction. Although hydraulic fracturing could realize large-scale fracturing, it cannot avoid the problem of clay hydration swelling (Li et al. 2016), and there are other problems. On the one hand, the consequences of traditional hydraulic fracturing include surface water and groundwater contamination, soil pollution, air pollution, and health hazards to on-site workers (Thomas et al. 2019). On the other hand, during the mining process using the hydraulic fracturing method, potential hazards such as noise and induced earthquakes may occur (Frohlich et al. 2016; Zhang et al. 2022a). Furthermore, a significant decrease in soil permeability was observed, with potentially adverse impacts on crop production (Oetjen et al. 2018). Therefore, domestic and foreign scholars have made a lot of efforts to improve the hydraulic fracturing fluid and fracturing process used in hydraulic fracturing technology (He et al. 2023a, b; Hou et al. 2016; Hu et al. 2019; Marsden et al. 2022; Qu et al. 2022; Wang et al. 2019a, b; Wu et al. 2021; Zheng and Sharma 2021). This article proposed the idea of using a laser system to assist in rock breaking before using hydraulic fracturing, referred to as the combined application of “laser pre-fracturing—hydraulic fracturing” technology. The concept of laser-assisted rock-breaking applied in engineering practice is shown in Fig. 24. The initial step mirrors traditional hydraulic fracturing procedures, involving the use of a drilling rig to create vertical and horizontal wells at the designated site. The subsequent step employs a laser-assisted rock-breaking system to irradiate the surrounding rock layers of the borehole wall with lasers, facilitating pre-splitting of the rock layers. The analysis of the experimental results indicates that optimal laser rock-breaking efficacy and a comprehensive spatial distribution of fracture structures are achieved when the laser beam is approximately at a 45° angle with the bedding plane. When keeping the laser power unchanged, select the spot diameter with the best cracking effect obtained by laboratory calibration. For Longmaxi shale, a 6 mm spot diameter demonstrates superior fracturing effects on bedding shale. The final step entails the application of conventional hydraulic fracturing techniques to enhance permeability extraction in shale gas reservoirs. Under laser irradiation, rocks initially generate a certain degree of fractures. Subsequently, hydraulic fracturing technology is employed to systematically enlarge and prolong these fractures, aiming to diminish water consumption and pressure. This approach ultimately yields positive economic and ecological outcomes. With the increasingly serious global common problems such as the shortage of living land and the exhaustion of shallow energy, the development of deep underground resources has become the strategic focus of China’s future scientific and technological development (Zhang et al. 2024). Compared to traditional hydraulic fracturing techniques, this combined approach potentially reduces the reliance on hydraulic fracturing in the extraction of shale gas and other resources. It potentially offers environmental benefits in terms of water resource utilization, chemical usage, geological disruption, noise and vibration, and waste management. Additionally, it has the potential to lower the costs of shale gas extraction and enhance extraction efficiency. However, further comparative researches using physical models in the laboratory and in situ field experiments are necessary to conduct a systematic evaluation of these performance aspects.

Schematic diagram of laser assisted rock breaking applied in engineering practice

For laser-assisted rock-breaking technology to be effectively applied in future engineering practices, further research on shale samples with different compositions and bedding angles is required to refine the theoretical system. Additionally, foundational research and development in integrating lasers into drilling equipment need strengthening. Large-scale industrial experiments are also necessary to scientifically evaluate the effectiveness and adaptability of laser-assisted modification of deep unconventional energy reservoirs. Finally, field-testing of laser-assisted rock-breaking equipment is essential to comprehensively evaluate its environmental, cost, and benefit aspects.

The article systematically conducted research on the damage and fracture behavior and mechanism of shale with different bedding planes under laser irradiation, mainly discussing the temperature field and the distribution of the fracture field of shale after laser irradiation, and analyzing the failure mechanism of shale from a physical and chemical perspective after laser irradiation. The main conclusions are as follows:

1. (1)

   Based on the surface temperature values and the changes in the sample’s morphological characteristics, the irradiated surface of the rock sample was categorized into pore-forming areas, high-heat-affected areas, and low-heat-affected areas. Further division of the thermally affected area was carried out according to the temperature rise curve, resulting in three regions: the rise region with a logarithmic curve, the rise region with a quadratic curve, and the rise region with a linear curve.

2. (2)

   Drawing from both macroscopic and microscopic morphological analyses of shale, we observed distinct zoning characteristics in the roughness of the fracture surface in rock samples post-laser irradiation. Furthermore, the roughness of various regions on the fracture surface exhibited a correlation with the distance from each area to the perforation hole. Notably, the fracture surface tended to be rougher as the distance to the perforation hole decreased.

3. (3)

   Under laser irradiation, mineral constituents such as quartz, calcite, and pyrite present in shale experienced chemical reactions. These reactions release additional heat energy and accelerate the propagation of cracks. At the same time, the shale oil in shale reservoirs expands because of high temperatures, and then it overflows along existing cracks towards the irradiation surface. Throughout this process, the stress induced by the volumetric expansion plays a pivotal role in accelerating the development and expansion of cracks. This positively influences the effectiveness of laser-induced rock fracturing.

4. (4)

   Under the combined influence of laser irradiation and bedding structure, rock samples exhibit three distinct failure modes: Pattern T, Pattern T-B, and Pattern B. For bedding angles less than 45°, the relationship between crack propagation direction and bedding angle is not pronounced, with thermal stress primarily controlling crack expansion. At a bedding angle of approximately 45°, both thermal stress and bedding effects jointly influence crack development. The region near the irradiation center is notably impacted by heat, while the area farther from the center experiences a more prominent influence from bedding effects. At bedding angles greater than 45°, crack expansion is predominantly governed by bedding effects, closely following the bedding trend and essentially extending along the bedding plane.