Coal is China's dominant energy source, accounting for approximately 57.7% of China's energy consumption in 2019 (Li, 2014; Lou et al., 2024; Lu et al., 2011). In the process of mine production, coal and gas outbursts and gas explosion accidents often cause heavy casualties and economic losses, which restrict the development of coal mines. Gas extraction is an effective method to eliminate gas accidents (Zhou et al., 2020; Lou et al., 2025). However, the permeability of coal bodies is typically low, ranging from (1–100) × 10^–3 μm², which poses challenges and inefficiencies in gas extraction (Zhang et al., 2023; Chen H et al., 2017; Huang et al., 2014). To enhance gas extraction efficiency effectively, corresponding measures to improve coal seam permeability are essential (Gao et al., 2015; Lu et al., 2017; Chen L et al., 2017). These include CO₂ or N₂ gas-phase fracturing (Busch and Gensterblum, 2011; Yang et al., 2022; Zhou et al., 2023; Chen et al., 2020), high-pressure water jet techniques, hydraulic fracturing, and hydraulic slotting, among others (Kong et al., 2016; He et al., 2021; Zhang et al., 2017; Liu et al., 2025; Zhang H et al., 2019). In engineering applications, the low-permeability characteristic of Chinese coal seams means gas drainage using these technologies still falls short of meeting actual safe production requirements (Gao et al., 2015; Li et al., 2024; Li et al., 2025).

Zhang et al. (2017) proposed a novel in-seam borehole hydraulic flushing (ISBHF) technology for gas extraction. The consistency between field test results and simulation outcomes validated the reliability of the corresponding numerical model. After applying this new technology, the number of gas extraction boreholes at the heading face decreased by 50%, while the gas extraction concentration and flow rate increased by 10-fold and 6-fold, respectively. Consequently, the gas extraction duration was shortened by 65%, and the monthly excavation length of coal mine roadways doubled. For structural coal seams, Zhang R et al. (2019) conducted research on bedding hydraulic cavitation to strengthen gas drainage. The results indicated that this technology reduced the number of boreholes by 66.7% and increased the gas extraction rate and concentration by 1.33 times and 3 times, respectively, demonstrating significant advantages over traditional extraction methods.

Beyond hydraulic flushing and cavitation technologies, Lin et al. (2015) developed a cross-borehole hydraulic slotting technique. Field application data showed that after half a month of gas extraction, the average gas extraction concentration of slotted boreholes reached 26%, compared to only 7% for conventional boreholes—meaning the concentration of slotted boreholes was approximately 3.7 times that of conventional ones, highlighting the remarkable effectiveness of slotting technology in improving extraction concentration. Zhang H et al. (2019) presented a hydraulic-flushing gas drainage technology. This technology efficiently flushed out more tectonic coal from thick coal seams, and the large-diameter cavities formed in the tectonic coal sub-layers significantly expanded the scope of stress relief and permeability enhancement, thereby remarkably improving gas extraction efficiency. To address the core demands of technological optimization and construction cost reduction, Chen et al. (2020) established a multi-physical coupling model. Through this model, they determined the optimal spacing of hydraulic flushing boreholes (HFB), providing precise parameter support for field tests and filling the gap in quantitative design within this field. However, existing hydraulic enhanced extraction technologies all have obvious limitations, a consensus widely recognized by numerous scholars (Li et al., 2023; Liu et al., 2023; Wang et al., 2021). For instance, hydraulic slotting technology exhibits poor sustainability in permeability enhancement effects. Under high in situ stress conditions, the fractures generated by slotting are prone to closure, leading to a rapid decline in extraction efficiency. Hydraulic fracturing technology, on the other hand, tends to cause local stress concentration in coal masses, which in turn restricts the permeability improvement effect. More critically, both technologies demonstrate poor performance in gas extraction from soft tectonic coal seams. Soft tectonic coal inherently has low strength and disorganized fracture development. Fractures from hydraulic slotting are liable to collapse and close, while hydraulic fracturing may result in overall coal fragmentation that clogs boreholes. This has become a key bottleneck limiting gas control in soft coal seams.

In contrast, borehole hydraulic cavitation has emerged as an innovative pressure relief and permeability enhancement technology, attracting growing attention and extensive research in recent years. The core process of this technology involves three steps: first, drilling conventional boreholes into the coal seam; second, fracturing the coal surrounding the boreholes using high-pressure water jets; and finally, flushing the fragmented coal out of the boreholes to form large-diameter cavities. Compared with competing technologies such as hydraulic slotting and fracturing, its unique advantage lies in achieving long-term stress relief by forming physical cavities rather than relying on fracture structures that are prone to closure. This provides a novel solution to the challenges of gas extraction in soft tectonic coal seams.

Gas cavitation plays a crucial role in the safe and efficient extraction of coalbed methane. Although previous studies have predominantly focused on seepage characteristics, gas cavitation exerts a significant impact on coal mass by altering its stress distribution: through stress relief and concentration effects, this stress change promotes gas desorption from coal matrix pores, increasing coal permeability, and thereby achieves effective gas extraction. Meanwhile, gas cavitation damages the coal surrounding the cavitation hole, leading to the formation of a plastic zone in the affected area. Notably, gas cavitation disrupts the original stress balance of the coal mass, resulting in the formation of a two-zone annular structure within the coal, which is composed of a stress concentration zone and an original stress zone. As the core disturbed region, the stress concentration zone undergoes the redistribution of radial and tangential stresses, accompanied by corresponding variations in coal permeability, and can be further subdivided into two sub-zones. In the fluidized zone, tangential compressive stress drives the opening of coal fractures, leading to a sharp increase in permeability; in the compaction zone, radial compressive stress dominates the coal compaction process, resulting in a temporary reduction in permeability. In contrast, the original stress zone remains in an undisturbed state, with its physical–mechanical properties and seepage characteristics maintaining their initial conditions. Numerical simulations have been conducted to analyze the stress distribution around the cavitation hole and the formation characteristics of the plastic zone. Relevant indicators for evaluating the effectiveness of gas cavitation have been proposed based on field practice, which provide a solid theoretical foundation for the practical application of gas cavitation technology in coalbed methane extraction.

Existing research has not yet clearly studied the mechanism of borehole hydraulic cavitation. Therefore, based on the redistribution of coal stress caused by the excavation of the hole, this article attempts to explain the mechanism of pressure relief and permeability enhancement of the hole from the perspective of stress. In this article, based on the simulation of engineering practice, limited by the field parameters, only simplified models are used instead of complex models. The innovation is that it is not limited to the conventional gas seepage analysis, but instead offers a new way to analyze the stress change and coal damage in the process of gas cavitation from the mechanical failure mechanism.

Initially, the mechanism of the distressed zone and permeability increase is examined based on the stress distribution law surrounding the cavitation hole. Subsequently, the finite-difference FLAC3D numerical analysis software is utilized in the context of the gas extraction project at the Wulihou Coal Mine in Shanxi, China, to investigate the optimal radius of the cavitation hole. This investigation is conducted by comparing and analyzing variations in the area of the pressure relief zone, the pressure relief area ratio, the plastic zone area, and the plastic zone area ratio for different cavitation hole radii. On this basis, combined with practical engineering, a set of quantitative indicators has been carefully established to accurately evaluate the effect of gas cavitation, injecting new vitality into engineering practice.

The Wulihou Coal Mine is situated in Zuoquan County, Shanxi Province. The coal seam has a thickness of approximately 6 m, with an absolute gas outflow of approximately 18 cubic meters per minute and a relative gas outflow of 11 cubic meters per ton, indicating it is a high-gas mine. The maximum principal stress of the lower coal group in Wulihou Coal Mine is horizontal stress, with an average value of 18.5 MPa and a direction angle of N31.65 E ∼ N36.53 E; the minimum principal stress is still horizontal stress, with an average value of 10.5 MPa and a direction angle of N121.32 E ∼ N125.32 E; the intermediate principal stress is vertical stress, with an average value of 11.5 MPa and a direction angle of is N214.95° E ∼ N219.83° E. The lower coal seam has a large dip angle, and the strength of the roof and floor mudstone and coal body is small; they are unstable rock strata. Furthermore, the coal seams exhibit limited permeability and diminished compressive strength, complicating gas extraction and heightening the associated risks of coal and gas outbursts.

To guarantee the secure excavation of the belt highway and the track roadway, alternating cavitation technology is employed for gas extraction, as seen in Figure 1. The technical principle is as follows: first, ordinary boreholes are drilled into the coal seam in the belt bottom drainage roadway, subsequently employing high-pressure water to generate current as the energy source to construct a series of cylindrical cave-building chambers within the coal seam. This causes the coal body to unload and increase permeability, thereby facilitating the extraction of coal seam gas. To increase production efficiency and decrease costs, the interval is alternately utilized to position standard drilling holes and cavitation holes. Specifically, a row of acupoint holes is situated between two rows of standard drilling. However, existing research is insufficient to understand the pressure relief and permeability enhancement effects around the cavitation hole in the coal body. The radius (R) of the cavitation hole may influence the coal unloading and permeability increase, necessitating more investigation.

The primary coal structure is composed of pores, cracks, coal matrix blocks, and gas, as shown in Figure 2. It is assumed that the coal matrix, pores, cracks, and gas are subjected to the original rock stress (σ₁, σ₂, σ₃) before the cave is formed; that is, in the original rock stress state. Then the coal body is caved, the original rock stress balance of the coal body is broken, and the coal body stress is redistributed around the cavitation. After the stress equilibrates, three zones will be formed around the cavitation hole: a stress-decreasing zone (referred to as the stress-relaxation area), the stress concentration zone, and the initial stress area. Within the stress-decreasing zone (the pressure relief shown in Figure 2), the coal body expands the acupoints around the cavitation hole, driving the pores and cracks to deform. The pores and cracks also expand under the effect of the gas pressure (such as the gas pressure in Figure 2) inside the hole, propagating the pores and cracks, which leads to an increase in the permeability of the coal body and a decrease in the pressure of the reservoir. As a result of the changes, the gas in the coal body desorbs, diffuses, and seeps into the cavitation. When the stress exceeds the strength limit of the coal body in the stress concentration area, the coal matrix block around the cavitation hole is damaged, and new cracks sprout and connect with the original pores and cracks, which increases the permeability of the coal body. The gas in the coal body diffuses and seeps into the cavitation hole. Within the primary rock stress zone area, the coal body is not affected by the disturbance of the cavitation hole, and it has no effect on the gas in the coal body.

The above analysis shows that the essence of pressure relief is to form a stress reduction zone and a stress concentration area around the cavitation hole, which causes internal changes in the coal body and increases the permeability of the coal body. For this reason, the issue will be analyzed from the perspective of stress distribution. It is assumed that the coal rock mass, overburden rock, and floor are isotropic linear elastic bodies (Ma et al., 2023; Xue et al., 2013). There is an excavation in the infinite elastic body to create acupuncture holes, simplifying the space problem into a plane strain problem. The force diagram of the cavitation hole is shown in Figure 3. The vertical and horizontal original rock stresses are p and q, respectively; the radius of the cavitation hole is a; the angle of horizontal axis from the horizontal t to the right to the right time to the unit body is θ ; the radial stress is σ r , the ring stress is σ θ , the shear stress is τ r θ , and the side pressure coefficient k = p/q. The stress on the cavitation hole is the same as that on the horizontal to the right horizontal axis. According to the theory of elasticity, the stresses acting on any tiny cell are (Equation 1). σ r = 1 2 p 1 + k 1 − a 2 r 2 − 1 2 p 1 − k 1 − 4 a 2 r 2 + 3 a 4 r 4 cos ⁡ 2 θ σ θ = 1 2 p 1 + k 1 + a 2 r 2 + 1 2 p 1 − k 1 + 3 a 4 r 4 cos ⁡ 2 θ τ r θ = 1 2 p 1 − k 1 + 2 a 2 r 2 − 3 a 4 r 4 sin ⁡ 2 θ (1)

Based on the vertical stress of the primary stress state on most global coal bodies, the vertical stress of the original rock stress is greater than the horizontal stress, that is, k < 1 . The distribution of cutting stress σ θ along the periphery of the hole is shown in Figure 3. The shape of the circular stress distribution curve is related to the value of K. This curve shape has a value of k < 1 . If k < 1 3 , the top (B-B line direction) of the cavitation hole will occur. According to the principle of pressure as “+” and “−” according to rock mechanics, the ring stress distribution curve at the top will be moved. The shape of the acupoint is roughly similar. Therefore, in order not to lose the general nature, the shape of the hoop stress σ θ distribution curve in Figure 4 is analyzed.

Figure 4 shows that the hoop stress σ θ around the periphery of the cavitation hole has a stress reduction in the direction of the B-B line; that is, it is the stress of the force release or decompression effect. Therefore, it is proposed to analyze the decompression effect of the cavitation hole in the B-B line by the hoop stress in the B-B line direction σ θ In addition, in order to quantify the scope of influence of the decompression effect, the decompression area ratio is used as a judgment index, which is calculated by the following formula: η s = S r S , (2)

where η_s denotes the pressure relief area ratio; S_r represents the area where the hoop stress σ_θ is diminished following cavitation; S signifies the area of the cavitation hole.

The hoop stress around the cavitation hole is σ θ . There is an increase in stress in the direction of the A-A line; that is, it is a stress concentration, and the stress should be the force concentration that will lead to shear damage of the coal matrix, sprouting new cracks, and increasing the permeability of the coal body. In order to quantify the scope of influence of the increased permeability of the coal body due to the stress concentration, the area ratio of the plastic zone is used as a judgment index, and its calculation formula is as follows: η p = S F P S , (3)

where η_p denotes the plastic zone area ratio; S_FP represents the plastic zone area after cavitation.

The No. 15 coal seam in Baiyangling Coal Mine (Yan, 2022) has an average thickness of 5 m, and the 15108 track drift area is a low-permeability and difficult-to-drain coal seam. The original gas content ranges from 4.52 m³/t to 6.5 m³/t, the inverse maximum gas pressure is 0.45 MPa, and the maximum initial gas desorption velocity Δp is 38.5 mmHg. The maximum firmness coefficient f is 1.22, the coal mass failure types are mainly Class III and IV, and the permeability coefficient is 0.079 m²/(MPa²·d). The cavitation hole radius has a significant impact on drainage effect and borehole stability: when the radius is 0.4 m, the permeability of the plastic zone around the hole reaches 300 times that of the elastic zone, the gas diffusion channel is unobstructed, and the gas pressure around the hole drops below 0.1 MPa after 150 days of drainage. The initial single-hole gas flow rate is nearly twice that of the 0.2 m radius, the stable drainage concentration is 21.49% (with 7 m borehole spacing), which is 3.36 times higher than that of non-cavitation holes (6.39%–8.64%), and the flow attenuation coefficient is 0.015 d⁻¹, only 1/4.27 of that of non-cavitation holes (0.118 d⁻¹). For the 0.2 m radius, due to the limited plastic zone and insufficient permeability improvement, local high-gas areas still exist after 150 days of drainage. For the 0.6 m radius, the plastic zone doubles, the hole collapse risk is more than three times that of the 0.4 m radius, the borehole spacing must be reduced to 5 m, the construction cost increases by more than 30%, and additional investment in hole collapse control is required.

The optimal parameters are 0.4 m cavitation hole radius + 7 m borehole spacing. At this time, there is no blank zone in the drainage area, and the total drainage volume is 1.41 times higher than that of the 5 m spacing. This practice verifies that when the cavitation hole radius is selected between 0.4 m and 0.5 m, it not only establishes a parameter benchmark for the drainage of low-permeability coal seams but also achieves the dual goals of drainage efficiency and borehole stability. Its applicability has been effectively verified under different coal quality conditions.

Based on the stress distribution characteristics around the cavitation hole, three distinct stress zones are identified: the stress relief area, the stress concentration region, and the original rock stress area. Both the stress relief area and stress concentration region increase gas permeability within the coal mass, while the original rock stress area does not affect coal gas permeability.