Review and prospect of directional hydraulic fracturing rock breaking technology in underground coal mine

Abstract

To address the application limitations of traditional weakening techniques under complex geological conditions such as hard coal mine roofs, directional hydraulic fracturing (DHF) technology has become a key technical measure to ensure safety production by virtue of its core advantages of directional rock breaking. This paper systematically reviews the research status and development trends of underground DHF technology in underground coal mines, focusing on an analysis of the three key dimensions: directional fracturing methods, processes, and equipment. Regarding fracturing methods, three mainstream technologies based on manual slotting, linear arrangement drilling, and high-pressure water jet slotting have been sorted out. The paper compares their principles, advantages, and applicable scenarios, pointing out that a linear synergistic fracturing method using multiple fracturing holes with high-pressure water jet slotting demonstrates both precision and scalability, making it the most promising technological path at present. For fracturing processes, it elaborates on the standardized progress of the four core procedures: drilling construction, pre-treatment, high-pressure water injection, and effect verification, and analyzes the key bottlenecks in process optimization under complex geological conditions. In terms of fracturing equipment, technical characteristics and existing issues of drilling, slotting, high-pressure water injection, and monitoring devices are summarized. Aligning the development trends of mining engineering technology, the paper proposes that future directional hydraulic technology will evolve towards intelligent directional fracturing, multi-field coupled fracturing, and miniaturized precision fracturing. At the process level, it will develop towards integrated efficiency, adaptive dynamics, and green low-carbonization, while equipment will focus on breakthroughs in intelligent automation, high efficiency and reliability, and miniaturization and integration. These research results provide a reference for theoretical study, equipment development, and engineering applications of underground DHF technology, contributing to safe, efficient, and sustainable coal mining practices.

Keywords

Underground coal mine directional hydraulic fracturing fracturing method fracturing process fracturing equipment development trend

Introduction

For hard, intact, and cohesive coal seam roofs, both primary and cyclic pressures are relatively high, and the mining-induced pressure manifests strongly. This can lead to roof falls and roof caving caused by step subsidence in the working face. In severe cases, it may even trigger impact pressure hazards, posing a serious threat to safe production of coal mine. Traditional methods for weakening hard roofs primarily include deep-hole blasting and water injection softening. However, deep-hole blasting is only applicable in low-gas areas and faces challenges such as charging, hole sealing, and misfire disposal. Meanwhile, due to the density of the rock formations and poor permeability, water injection for softening the coal-rock mass shows limited effectiveness (Fan et al., 2014). At this time, there is an urgent need for a technology that can directionally fracture hard roofs, disrupt their integrity, and enable timely layered and block-wise collapse, which is exactly what directional rock fracturing technology for coal-rock mass comes into play.

Directed hydraulic fracturing (DHF) technology employs high-pressure water to create controlled directional fractures on the rock mass, thereby enhancing the mechanical properties of the rock and reducing mining difficulty. Through decades of development, this technique has gradually matured its application in underground coal mine, establishing diverse fracturing methods, standardized process systems, and specialized equipment configurations. At present, DHF technology has been applied in engineering practices such as weakening hard roofs during coal mining (Huang et al., 2018; Klishin et al., 2018; Sun et al., 1999; Yang et al., 2025; Zhang et al., 2023), preventing rock bursts (Du et al., 2012; Guo, 2011; Li et al., 2025; Liu et al., 2024; Wu et al., 2011), enhancing coal seam permeability (Chen et al., 2020, 2023; Ge et al., 2019a, 2019b; Li, 2016; Wang, 2024; Zhao et al., 2023), eliminating coal seam gas outbursts (Li et al., 2023; Liu et al., 2025; Zhang et al., 2025), improving release capacity in massive coal seams with gangue inclusions (Gao et al., 2017; Yang et al., 2025), and controlling surface movement (Feng et al., 2019; Zheng et al., 2023). By enabling directional rock fracture and layered cutting of hard roofs, DHF effectively reduces structural integrity and strength. This technology demonstrates remarkable effectiveness in mitigating dynamic hazards caused by hard roof failures, guiding advancements in roof caving and impact pressure control. Consequently, its application for directional rock fracture in coal mines has gained increasing attention from experts and engineers. While existing research on underground fracturing techniques has been systematically reviewed by scholars, there is still limited literature addressing the current status and progress of DHF technology in coal mines.

The DHF technology underground in coal mine primarily focuses on three key aspects: DHF methods, fracturing processes, and equipment. This paper systematically reviews the current research status of these technologies and outlines their future development trends, aiming to provide researchers and engineers with a comprehensive understanding of this technique.

Status of DHF technology in underground coal mine

At present, the DHF technology in coal mine mainly adopts some measures to pre-treat the fracture holes, arranging the fracture holes linearly along the expected direction of hydraulic fracture propagation, or using mechanical blades or high-pressure water jet cutting, to guide the spatial fractures generated by hydraulic fracturing to extend along the direction of the pre-formed fractures.

DHF technology in underground coal mine

In hydraulic fracturing operations, relying solely on high-pressure water jets for directional fracturing yields limited effectiveness. To achieve directional fracturing propagation, three primary methods are employed: using mechanical tools or high-pressure water jets to create wedge-shaped slots that guide fracturing to extend axially or radially (Bai et al., 2020; Deng et al., 2018); arranging fracture-inducing boreholes (or pilot holes) linearly along the pre-fracture direction (Cheng et al., 2020a; Liu et al., 2019a, 2019b); and combining the first two approaches (Cheng et al., 2020b; Wang et al., 2020; Zhong et al., 2020). These three approaches are used to achieve the goal of directional rock breaking. The classification of the three mainstream DHF technologies is based on the core criterion of “directional control mechanism for fracture initiation and propagation,” which is widely recognized in coal rock hydraulic fracturing research. Each approach achieves directional rock breaking through distinct control mechanisms: manual slotting relies on geometric constraints of prefabricated slots, linearly arranged drilling utilizes borehole-induced stress superposition, and hydraulic slotting-based linear synergistic technology integrates both mechanisms. It should be noted that other technical variants (e.g., pulse hydraulic fracturing, CO₂ phase-transition fracturing) are not included herein, as they focus on energy input or medium optimization rather than directional control mechanism innovation, and have limited engineering application in underground coal mines.

DHF technology based on manual slotting

The DHF method based on manual slotting is one of the earliest directional fracturing technologies applied in coal mine underground operations. Its core principle is to mechanically cut slots in specific directions on the borehole wall in advance, and use stress concentration effect at these slots to guide high-pressure water to generate directional fractures. The key lies in matching the geometric parameters of the slots (depth, width, and angle) with the mechanical properties of rock mass. The manual slotting-based DHF method (Figure 1(a)) requires specialized mechanical tools to manually create one or multiple annular wedge-shaped slot surfaces at predetermined positions (Figure 1(b)). Subsequently, continuous high-pressure water injection is performed through sealed holes. Under stress concentration at the slot tips (with the tensile stress ratios exceeding 10 times those in uncut conditions), hydraulic fractures extend directionally along the wedge-shaped slot surfaces, achieving the purpose of directional fracture propagation guided by wedge-shaped slots. In practical applications, dedicated slotting tools are typically used to complete slotting operations within boreholes, with slotting directions precisely designed according to engineering requirements (such as roof pressure relief or the guidance direction of gas channels).

Figure 1.

Schematic of DHF process with mechanical cutters to cut guide slot: (a) schematic of a wedge-shaped slot at the bottom of the hole (Kang and Feng, 2017); (b) schematic diagram of the annular wedge slot.

The method demonstrates high directional precision and mature technology, making it suitable for mining scenarios involving medium-hard rock masses and relatively simple geological conditions. However, the wedge-shaped annular slot drill bit used in directional rock-breaking technology requires the rock at the hole bottom to serve as a base for to press the tool and complete the operation of cutting the wedge-shaped slot, resulting in two major drawbacks: (1) The necessity to withdraw the drill pipe for regular bit and slot cutter replacement complicates the process, particularly in deep-hole operations where such procedures become time-consuming and labor-intensive; (2) Multiple directional guide slots in the same borehole require multiple drill bit and cutter changes. The more guide slots there are, the more frequent the drill bit and tool changes. However, using high-pressure water jet cutting to create annular wedge-shaped slots can effectively avoid these issues. Nevertheless, the method has notable limitations: manual slot cutting proves inefficient, especially in deep holes and hard rock conditions, where tool wear out quickly and the operation cycle is long; the depth of slot cutting is constrained, making it difficult to form sufficiently long directional fractures in high-stress, thick rock layers, which affects the fracturing effectiveness; additionally, rock powder generated during slotting may clog drill holes, increasing resistance to subsequent high-pressure water injection.

Hydraulic fracturing method of directional propagation of water pressure fracture guided by linearly arranged boreholes

The hydraulic fracturing method with linearly arranged boreholes for directional water pressure fracture propagation is based on the mechanical characteristics of rock fracture propagation. By evenly arranging multiple fracturing boreholes in a linear manner and utilizing the stress superposition effect between adjacent boreholes, it guides the high-pressure water-generated fractures to propagate directionally along the borehole alignment. The core technology lies in optimizing borehole arrangement parameters, including spacing, depth, and inclination angle, which require precise calculations based on parameters such as rock compressive strength, tensile strength, and ground stress distribution. Currently, this method includes two techniques: linearly arranged guide holes and linearly arranged fracturing holes.

1. Control method of DHF with linearly arranged guide holes

This method employs manually created guide holes, which is equivalent to adding artificial weak planes within the coal-rock mass. During hydraulic fracturing process, these guide the hydraulic fractures to propagate directionally along the guide holes, thereby achieving DHF. The technique involves first arranging two fracturing boreholes linearly along the pre-fracture direction. Then, one (or more) guide holes are drilled along the line connecting the centers of the two fracture holes (Figure 2(a)), or two (or more) guide holes are placed on both sides of a single fracturing hole (Figure 2(b)). During fracturing, water is injected into the fracturing holes while the guide holes remain dry, enabling directional propagation of hydraulic fracturing (Fu, 2013; Yang, 2015). The number of guide holes directly correlates with the effectiveness of directional propagation and the extent of directional expansion (Li et al., 2011; Xu et al., 2011a).

2. Linear arrangement of DHF technology

Figure 2.

Control method of DHF with linearly arranged guide holes: (a) the fracturing holes are on both sides, and the guide holes are in the middle; (b) the fracturing holes are in the middle, and the guide holes are on both sides.

Recently, based on the DHF of linearly arranged guide holes, researchers have proposed a linearly arranged DHF method (Wang, 2014; Zhao et al., 2018). This method first arranges multiple hydraulic fracturing holes in a straight line (along the direction of targeted rock fracturing), then simultaneously injecting high-pressure water into all holes to achieve crack propagation along the central line connecting the holes. However, the fracture propagation direction is influenced by the angle between the center line of the borehole and the direction of maximum principal stress. When the angle falls within a specific range, the hydraulic fractures propagate along the borehole's central line. These patterns have been validated by theoretical derivations, physical experiments, and numerical simulation results (Figure 3(a) and (b)) (Cheng et al., 2020a; Jia, 2018; Lu et al., 2020).

Figure 3.

The DHF technology with linearly arranged fracturing holes: (a) results of physical experiments (Zhao et al., 2018); (b) numerical simulation results (Lu et al., 2020).

However, when the angle between the borehole centerline and the direction of the principal stress exceeds a certain critical value, the hydraulic fracture propagation shifts toward the maximum principal stress direction rather than the borehole centerline. This prevents linearly arranged fracture holes from effectively guiding directional fracture propagation. Furthermore, excessive space between hydraulic boreholes limits the directional propagation length and spatial extent of individual fracture hole, resulting in engineering issues such as increased boreholes and high workload.

Compared to manual slotting, this method does not require additional slotting operations, simplifying the construction process and significantly improving work efficiency. It is suitable for fracturing medium-hard rock masses and thick rock layers, capable of forming long directional fracture zones. This makes it widely applicable in projects such as large-scale roof pressure relief and coal seam permeability enhancement. However, the method also has certain limitations: directional accuracy is heavily influenced by the precision of borehole arrangement. If the positioning deviation exceeds the allowable range, it may cause the fracture propagation direction to deviate from the designed trajectory. Additionally, its adaptability to geological conditions is poor. In heterogeneous rock masses or geological environments with faulted fractured zones, fractures tend to deflect, making precise directional control challenging. Furthermore, the complex stress interference mechanism between adjacent boreholes has not yet been fully addressed by theoretical calculations, and parameter design still largely relies on engineering experience.

DHF method with linearly coordinated multi-fracturing holes based on hydraulic slotting

To overcome the limitations of DHF methods with linearly arranged fracturing holes, a DHF method with linearly coordinated multi-fracturing holes based on hydraulic slotting has been proposed. This method integrates the advantages of high-pressure water jet technology and multi-hole coordinated fracturing. The technical principle involves: first, using high-pressure water jets to create uniformly oriented slots on the inner walls of multiple fracturing holes for precise directional guidance; then, simultaneously injecting high-pressure water through multiple holes to leverage stress synergy between adjacent slots, promoting synchronized fracture propagation along both slot orientations and hole alignments, thereby achieving directional fracture in coal-rock formations (Figure 4). This technique increases fracture hole spacing, reduces distribution density, and expands the range of DHF (Cheng, 2018; Cheng et al., 2018; Lu and He, 2020). Compared with mechanical tool-guided wedge-shaped slot technology, this method first creates directional fracture guide slots at predetermined spatial positions using high-pressure water jets, followed by simultaneous sealing of multiple holes before directional fracturing (Fan et al., 2014; Li, 2016; Wang and Li, 2012; Yan et al., 2000). Thus, it can be considered a combination of water jet slotting and multi-hole linearly coordinated fracturing, as it also utilizes the principle of stress concentration at the slotting tip. Compared with the technology of cutting annular wedge-shaped guide grooves with mechanical tools, this method can cut axial guide slots with better effects on slot depth and width. Additionally, the integrated drilling and cutting bit allows creating multiple axial or radial guide slots without changing the drill bit (Li et al., 2020; Xiao, 2014).

Figure 4.

DHF method with linearly coordinated multi-fracturing holes based on hydraulic slotting.

The outstanding advantages of this method include: high-pressure water jet slotting offers high efficiency and deep slot depth (50–100 mm), particularly effective for hard rock (uniaxial compressive strength > 50 MPa) and deep-hole (depth > 8 m) operations. During cutting, rock powder is flushed away with water flow, preventing borehole blockage. Multi-hole coordinated fracturing forms a denser and longer directional fracture network (fracture length 3–5 m, density 2–3 fractures/m), significantly enhancing rock mass modification efficiency. It is suitable for large-scale fracturing projects under high stress (ground stress > 20 MPa), hard rock, and relatively homogeneous geological conditions. The directional precision combines the accuracy of manual slotting (deviation ≤ ±3°) with the scalability of multi-borehole coordinated collaboration, meeting the directional requirements of complex engineering scenarios. However, this method also has inherent limitations and operational risks: (1) High equipment and cost thresholds, with initial investment and energy consumption significantly higher than traditional methods; (2) Strict parameter matching requirements between slotting and water injection, and complex operation; (3) Risk of fracture deflection due to pressure imbalance or sealing failure under high pressure. Its effectiveness may be reduced in highly heterogeneous rock masses, extreme ground stress environments, or shallow thin rock layers.

Currently, this method remains in the stage of technical refinement. The key challenges, supported by quantitative data and engineering cases, are as follows: (1) High energy consumption of high-pressure water jet slotting equipment: the rated power is 150–220 kW, and the energy consumption per meter of slotting is 35–50 kWh/m, which is 60–80% higher than traditional methods, leading to a significant increase in engineering costs. The supporting water supply system requires stable pressure (300–500 MPa) and flow (10–20 L/min), further increasing construction investment (≈2.3 million RMB for a complete set of equipment); (2) Difficulty in coordinated control of multi-borehole simultaneous water injection: the pressure control accuracy needs to be within ±3 MPa and flow consistency ≥90%, but current industrial control systems can only achieve ±5 MPa and 80–85% consistency, resulting in a 25–35% reduction in synergistic effect in 30% of cases; (3) Unclear parameter matching relationship: the slotting pressure (300–500 MPa) should be 3–5 times the injection pressure (50–100 MPa), and the slotting duration (10–15 min/hole) must be coordinated with the injection interval (≤30 min); deviations beyond these ranges will reduce the fracture initiation success rate from 92% to 65%. Among these, the lack of dedicated coordinated control systems, high economic costs, and unclear parameter matching models are the main barriers to large-scale practical implementation. Extensive laboratory tests and on-site optimization are required to address these issues.

Development status of DHF technology

The DHF technology for coal-rock mass refers to injecting high-pressure water into boreholes to create one or more directional fractures in a defined horizontal plane or surface. These fractures divide the coal rock mass into blocks or layers of specific sizes and shapes, destroying the integrity of coal rock mass and reducing its strength. This mitigates risks from sudden roof collapse (or hard rock) and impact-induced ground pressure, ensuring safe production of coal mining. The manual slotting-based DHF procedure comprises: arranging fracturing boreholes in the coal-rock mass→creating wedge-shaped slots in the fracturing boreholes (using mechanical tools or high-pressure water jet)→sealing the fracturing boreholes near the wedge slot sections and injecting high-pressure water→directing the hydraulic fractures to propagate along the tip of the wedge slots within the coal-rock mass.

Figure 5 illustrates the process of controlled DHF using manual slotting in underground coal mines to address the challenge of hard and caving-resistant roof. The detailed procedure is as follows:

Construction-induced fracturing holes. By using conventional drilling equipment, fracture holes are drilled into the hard roof of the roadway or working face requiring hydraulic fracturing (Figure 5(a)). Parameters such as the number of holes, spacing, depth, and diameter are designed based on the rock layer's hardness, thickness, working face length, and fracturing range.

Replace the slotting to perform an artificial annular wedge slot. The slotting bit replaces the drilling bit to cut an annular wedge slot at the predetermined depth (hydraulic fracturing section) (Figure 5(b)).

Install the hydraulic fracturing packer. Tightly connect the packer to the high-pressure hose and lower it to the bottom of borehole, ensuring the annular wedge slot is positioned between the packer and the bottom of hole (Figure 5(c)).

Seal the fracture hole. Manual water injection causes the sealing capsule to expand and contact the fracture wall, thereby fixing the fracturing packer (Figure 5(d)).

Hydraulic fracturing. Start the high-pressure pump and continuously inject high-pressure water. Determine whether the hydraulic fractures have initiated and propagated in a directional manner by monitoring the turbidity of water flowing out of the observation borehole and the changes in the pressure readings on the pipeline (Figure 5(e)).

Figure 5.

Schematic of the DHF process for hard roof based on artificial slotting (Ma et al., 2016).

The DHF technology is the key to guarantee the fracturing effect, which mainly includes four core processes: drilling, pre-treatment, high-pressure water injection and effectiveness monitoring.

In terms of drilling construction technology, current operations primarily utilize equipment such as roof bolters and geological drilling rigs, combined with laser guidance and GPS-inertial navigation integrated positioning technology (adapted to underground environments) to achieve precise control of drilling position, inclination angle, and depth. For laser guidance: an explosion-proof, dust-proof laser transmitter is fixed near the drilling face, and a laser receiver integrated on the drill rig captures real-time deviation data (precision ±0.01°), which is transmitted to the control system to automatically correct the drilling direction-reducing average deviation to ±0.3–0.5°. For positioning: a ground GPS base station (accuracy ±2 cm) transmits data to the underground drill rig via leaky coaxial cable, and a high-precision IMU compensates for underground GPS signal attenuation; the two sets of data are fused via the EKF algorithm to ensure hole spacing control within ±5 cm. To address the confined spaces, high dust, and humid working conditions in underground coal mines, drilling equipment is gradually developing toward miniaturization, intelligentization, and explosion-proof. However, challenges persist in hard rock and deep-hole drilling operations, including slow drilling speed (0.5–1 m/h for hard rock with uniaxial compressive strength>60 MPa), rapid drill bit wear (service life≤50 m under hard rock conditions), and difficulty in controlling drilling deviation, which adversely affect the effectiveness of subsequent fracturing processes.

The pre-processing technology primarily involves two steps: borehole cleaning and sealing. Borehole cleaning employs high-pressure water flushing or compressed air blowing to remove rock power and debris, ensuring unobstructed water injection channels. Sealing techniques utilize cement slurry, polymer materials, or mechanical sealing to prevent leakage and pressure loss during high-pressure water injection. Current trends in sealing technology emphasize efficiency and durability. Polymer sealing materials, with their advantages of rapid sealing, excellent sealing performance, and high-pressure resistance, are seeing expanding applications. However, under high stress or dynamic load conditions, the sealing structure remains vulnerable to failure, and this issue has not yet been effectively resolved.

The high-pressure water injection process is the core procedure of directional fracturing, involving precise control of parameters such as injection pressure, flow rate, and time. Current practices primarily rely on rock mechanics testing and engineering experience to determine these parameters, employing a staged approach: initial low-pressure injection to verify sealing integrity, followed by gradual pressure escalation to design values, and sustained pressure injection until full fracture propagation. While some advanced mines have implemented intelligent water injection control systems enabling real-time monitoring and automatic adjustment, the high costs of such systems limit their adoption in small and medium-sized coal mines. Additionally, the lack of robust dynamic monitoring technology for fracture propagation during injection makes it challenging to track crack expansion in real time, resulting in insufficient scientific basis for parameter adjustments.

Effect inspection techniques are primarily used to assess the modification effects on the fractured rock masses. Common detection methods include acoustic testing, borehole inspection, and stress monitoring, with their performance, limitations, and applicability systematically compared as follows: (1) Acoustic testing method: Evaluates fracture development by measuring acoustic wave velocity changes (relative error ±5%–8% for homogeneous rock masses) with an effective detection radius of 1–3 m per sensor. It features high efficiency (10–15 min/zone) and low cost but is susceptible to rock heterogeneity (error increases to ±10%–12% in heterogeneous strata) and cannot distinguish natural and hydraulic fractures. (2) Borehole inspection method: Uses downhole TV probes to directly observe fracture morphology (visual resolution 0.1 mm) within 0.5–1 m of the borehole wall, but it is limited by borehole diameter (≥50 mm) and blockage, with low efficiency (30–60 min/hole) for large-scale projects. (3) Stress monitoring method: Measures stress release with a precision of ±0.1 MPa and a detection range of 5–8 m per sensor, accurately characterizing pressure relief effects, but it has high cost (≈50,000 RMB/sensor) and complex installation, with poor anti-interference to underground vibration and humidity. In practical applications, acoustic testing is often used for preliminary screening, borehole inspection for local detail verification, and stress monitoring for pressure relief-oriented projects-their combined use can achieve comprehensive and accurate effectiveness evaluation.

Development status of DHF equipment

DHF equipment serves as the material foundation for technical implementation, primarily including four categories: drilling equipment, slotting equipment, high-pressure water injection equipment, and monitoring equipment. In recent years, significant advancements have been made in equipment performance, intelligentization, and adaptability. Similar to traditional non-DHF techniques, DHF in coal rock masses requires equipment such as high-pressure pumps, vacuum pressure gauges, packers (or sealing tools), high-pressure hoses, and high-pressure shut-off valves. Additionally, it also needs some specialized equipment (Rybalkin et al., 2018; Serdyukov et al., 2016).

1. Drilling equipment

Currently, the primary directional fracturing drilling equipment used in underground coal mines includes pneumatic roof bolter, hydraulic geological drilling rig, and crawler integrated drilling rigs. Pneumatic roof bolters, with their small size, light weight, and ease of operation, are ideal for shallow and medium borehole drilling. Hydraulic geological drilling rig, featuring robust power and deep drilling capabilities, are suited for deep boreholes and hard rock operations. Crawler integrated drilling rigs, combining drilling, powder discharge, and positioning functions, have a high automation and operational efficiency, making them ideal for large-scale, high-intensity drilling operations. However, existing drilling equipment still faces challenges: low efficiency in hard rock drilling and short drill bit lifespan; insufficient intelligent capabilities, lacking features like automatic deviation correction and real-time positioning feedback; and some equipment is relatively large, with poor maneuverability in narrow tunnels.

2. Slotting equipment

To overcome the limitations of mechanical slotting drill bits, engineers have developed integrated drilling-cutting bits that combine hole drilling with high-pressure water jet slotting. This type of bit can switch between high and low-pressure water flows, enabling simultaneous drilling and slotting operations. The Model GFQ73-132/100 high-low pressure conversion slot cutter (Figure 6) exemplifies this technology. By adjusting water pressure to open or close the conversion control valve, it seamlessly switches between high-pressure and low-pressure water flows, achieving integrated drilling and slotting operation. This innovation eliminates the need to replace drilling bits and slitting tools, significantly improving slitting efficiency. Similar devices exist in various configurations, all sharing the same core principle of water pressure-controlled drilling-slitting systems (Lin et al., 2008; Zhang, 2014), with only minor structural variations.

Figure 6.

GFQ73-132/100 model high-pressure and low-pressure conversion slot cutter (Zhang and Lu, 2020).

Slot-cutting equipment is primarily categorized into mechanical cutting systems and high-pressure water jet cutting systems. Unlike conventional hydraulic fracturing equipment, DHF requires specialized cutting tools capable of creating annular wedge-shaped guide slots (Figure 7(a)). While these slots can be formed using bottom-wedge bit technology, the limitations of mechanical cutting tools restrict their application to shallow boreholes where only a single slot is created at the bottom (Figure 7(b)).

Figure 7.

Physical photo of the mechanical cutting tool and the schematic of the slot: (a) a wedge-shaped ring slot drill bit at hole bottom (Kang and Feng, 2017); (b) opening a circular wedge-shaped slot (Bao, 2012).

Mechanical slotting equipment features a simple structure and low cost, but suffers from low efficiency and severe wear, making it suitable for soft rock and shallow hole slotting. In contrast, high-pressure water jet slotting equipment utilizes a high-pressure pump to generate a high-pressure water flow forming a high-speed jet through a nozzle, achieving efficient slotting for hard rock and deep hole slotting. Currently, such equipment operates at pressure levels of 300–500 MPa, with slotting speeds reaching 0.5–1 m/min and a depth of 50–100 mm. However, it faces challenges including high energy consumption, complex water supply systems, and nozzle wear, while also requiring further improvements in underground safety features like explosion-proof, waterproof, and dust-proof performance.

3. High-pressure water injection equipment

High-pressure water injection equipment is the core power equipment for directional fracturing, primarily comprising high-pressure pumps, accumulators, control valve assemblies, and pipeline systems. Currently, the most commonly used high-pressure pumps in coal mine operations include plunger pumps and screw pumps, with pressure ratings reaching 50–100 MPa and flow rates adjustable range10–50 L/min. To meet the demands of multi-hole coordinated fracturing, some systems now feature independent multi-channel pressure and flow control with remote operation capabilities. However, existing high-pressure water injection equipment still faces several limitations: significant pressure fluctuations that affect fracture propagation stability; excessive vibration and noise during operation, adversely affecting underground working conditions; and pipeline systems requiring enhanced pressure resistance and wear resistance, as prolonged high-pressure operation increases leakage accidents.

4. Monitoring equipment

The monitoring equipment primarily includes stress monitoring devices, crack monitoring devices, and water injection parameter monitoring devices. Stress monitoring devices utilize components such as stress sensors and strain gauges to monitor real-time stress variations in rock masses. Crack monitoring devices employ acoustic wave detectors and borehole cameras to assess crack development. Water injection parameter monitoring devices employ pressure sensors and flow sensors to provide real-time feedback on injection pressure and flow data. Currently, the precision of these monitoring devices has met engineering application requirements, but challenges such as unstable data transmission, weak anti-interference capability, and limited monitoring range hinder the realization of comprehensive, real-time monitoring throughout the entire cracking process.

Development trend of DHF technology in coal mine

The DHF technology in underground coal mine plays an irreplaceable role in ensuring safe production of coal mine. Currently, it has been widely applied in engineering practices such as controlling hard roof directional caving, preventing impact ground pressure, and enhancing coal seam permeability (Klishin et al., 2017; Lekontsev and Sazhin, 2008, 2014; Pan et al., 2014; Vladimir et al., 2018; Xu et al., 2011b). Additionally, the technology holds significant potential for future applications in practical scenarios like the “N00” or “110” mining methods, leaving roadways along goafs, and mining with small (or no) coal pillar, which awaits further exploration by experts and engineers. Although some achievements have been made with this technology, more theoretical, laboratory, and field experimental research are still required. The author analyzes the development trends of DHF methods, fracturing processes, and fracturing equipment in coal mine underground operations as follows.

Development trend of DHF

Among the three existing DHF techniques, mechanical blade slotting has notable limitations. High-pressure water jet slotting effectively addresses these drawbacks. Furthermore, using high-pressure water jets for multiple fracture holes can better achieve the effect of multi-hole linear coordinated DHF.

The method of DHF based on the linear synergistic multi-hole fracturing technique is a fusion of manual slotting and linear coordinated multi-hole fracturing, which has the advantages of both methods. Therefore, this DHF method is the most effective and has the greatest development potential.

In addition, during coal mining production, it is possible to encounter situations where the hard roof rock contains natural fractures that affect hydraulic fracturing pressure retention, resulting in consistently low pressure within fracture holes and ineffective DHF. Since high-pressure water jet cutting does not need to consider roof integrity and allows for the presence of primary fractures or other permeable structures within the roof, this method can be employed for directional roof cutting. Although its directional expansion range may be smaller compared to the combined use of high-pressure water jet cutting and hydraulic fracturing, and its directional fracturing effect may not match that of the combined approach, it can compensate for geological conditions where hydraulic fracturing technology is ineffective.

Intelligent directional fracturing method

With the deep integration of technologies like artificial intelligence, big data, and the Internet of Things into mining engineering, intelligent directional fracturing methods will become a core development direction in the future. By integrating multi-dimensional sensors (such as ground stress sensors, fracture monitoring sensors, and water injection parameter sensors) during fracturing, real-time data on rock mechanics parameters, fracture propagation status, and equipment operation parameters can be collected. Machine learning algorithms are then used to establish fracture propagation prediction models, enabling dynamic optimization and precise control of fracturing parameters. For instance, based on rock heterogeneity data, water injection pressure, flow rate, and timing can be automatically adjusted to guide fractures to propagate precisely in the designed direction. Additionally, digital twin technology is employed to create virtual models of underground fracturing scenarios, simulating fracture propagation processes to provide decision support for actual fracturing operations.

Multi-field coupling synergistic fracture method

Current directional fracturing methods primarily rely on hydraulic pressure from high-pressure water injection. Future advancements will focus on multi-field coupled fracturing, integrating hydraulic, stress, temperature, and chemical fields to improve fracturing efficiency and directional accuracy. For instance, while injecting high-pressure water, preheating the rock mass through microwave or electromagnetic heating reduces tensile strength and brittleness, facilitating fracture propagation. Chemical agents (like expansive agents and corrosives) can weaken rock structures through chemical reactions, improving fracture stability and propagation. Crustal stress adjustment technology modifies stress distributions to create favorable stress environments for directional fracture growth. This multi-field coupled synergistic fracturing approach effectively addresses fracturing challenges under high-stress, hard rock, and complex geological conditions, significantly improving rock mass modification effect.

Miniaturized and precision-directed directional fracturing method

For engineering needs such as local rock mass modification and small-scale pressure relief in underground coal mine tunnels, miniaturized and precision-directed directional fracturing methods will be further developed. Develop compact and portable fracturing equipment to achieve precise fracturing in confined spaces and localized areas; optimize fracturing parameter design to control fracture length, width, and density of cracks finely, avoiding excessive fracturing that could affect the stability surrounding rock; develop micro-fracture directional fracturing technology to create a dense network of micro-fracture in coal seams, enhancing gas extraction efficiency while maintaining the overall stability of the coal seam.

Development trend of DHF technology

The current DHF process using mechanical cutters followed by hydraulic fracturing is relatively complex. Future development of this process should focus on integrating drilling and cutting operation within a single fracturing hole, while strategically arranging multiple fracturing holes along predetermined directions. This approach would allow for simultaneous high-pressure water injection into these holes to achieve synergistic fracturing effects.

Integrated and efficient construction techniques

Future directional fracturing technologies will evolve toward integration and efficiency, combining multiple processes such as drilling, slotting, water injection, and monitoring to achieve seamless connection and coordinated operations. Integrated fracturing equipment will be developed, incorporating functions such as drilling, high-pressure water jet slotting, water injection, and real-time monitoring, reducing equipment transportation, installation, and commissioning time. Optimized construction processes, such as concurrent operations including sealing preparations during drilling and effectiveness tests during water injection, can significantly boost efficiency. Key technologies like rapid sealing and efficient powder discharge will shorten construction timelines and lower project costs.

Adaptive and dynamic process optimization

In response to complex and variable geological conditions, adaptive and dynamic process optimization will become the mainstream. A matching database between geological conditions and fracturing process parameters will be established. Based on four categories of critical monitoring signals (stratum stress, fracture propagation, injection process, equipment operation) and strict response time requirements, drilling layout parameters, slotting parameters, and water injection parameters are adaptively optimized. Critical monitoring signals include: (1) Stratum stress (resolution ±0.1 MPa, stress change rate threshold 0.3 MPa/min); (2) Microseismic/AE signals (microseismic event count >30 events/min, AE amplitude <40 dB as warning thresholds); (3) Injection parameters (pressure accuracy ±0.5 MPa, flow variation ≤5%); (4) Equipment status (pump temperature <85 °C, packer sealing pressure ≥90% of injection pressure). System response time is differentiated by scenario: ≤2 s for routine parameter adjustment (adapting to stratum changes) and nse ms/1 s for abnormal emergencies (critical failures/general abnormalities), guaranteed by high-speed data acquisition (1000 Hz sampling) and optimized PID algorithms. Using real-time monitoring data of fracture propagation, injection pressure, flow rate, and injection duration can be dynamically adjusted to ensure that fractures extend stably along the designed directions. Emergency handling processes will be developed to automatically activate contingency plans for anomalies during fracturing operations, such as fracture deflection, drilling leakage, or sudden pressure drops, allowing timely parameter adjustments to ensure both safety and efficiency of fracturing operations.

Green and low-carbon processes

Under the context of the dual carbon goals, green and low-carbon development will become a key focus for directional fracturing technology. This involves optimizing water injection systems with water-saving equipment and recycling technologies to reduce water consumption, developing eco-friendly sealing materials and chemical agents to prevent groundwater and soil contamination, and adopting energy-efficient motors and hydraulic systems to improve operational efficiency. Additionally, it aims to minimize dust and noise pollution during construction, thereby enhancing underground working conditions. These green and low-carbon processes not only meet environmental standards but also reduce operational costs, delivering significant economic and social benefits.

Development trend of DHF equipment

The advancement of any technology is inseparable from the development of supporting equipment, and DHF technology is no exception. During DHF, it is essential to ensure that the water pressure within the pre-fracturing holes continues to rise and remains stable until reaching the rock's fracture pressure, thereby ensuring effective directional fracturing. When fracturing hard roof rocks, the fracturing holes are typically upward and have high internal water pressure, often posing a risk of the packer being blown out. Therefore, it is necessary to develop packers or their supporting devices with strong anti-punching performance.

In addition, high-pressure water jet pre-slotting technology in coal mines still predominantly utilizes clear fracturing fluids, which limits slots depth. To develop integrated drilling and cutting devices capable of employing high-pressure abrasive water jets for slotting, it is essential to explore the application of high-speed abrasive water jets for creating deeper wedge-shaped slots. This approach would enhance slotting efficiency, increase fracture spacing, and expand the propagation range of DHF.

Intelligent and automated equipment

Intelligent and automated equipment represents the core development trend of DHF systems. The development of fracturing equipment with autonomous decision-making and automatic control functions, integrated with PLC control systems and remote operation modules, enables full-process automation including borehole positioning, slotting operations, high-pressure water injection, and parameter monitoring. Machine vision technology is employed to automatically identify and accurately locate drilling positions, combined with automatic deviation correction technology to enhance drilling accuracy. An intelligent diagnostic system is developed to monitor equipment operation in real-time, predict potential failures, and provide early warning and automated maintenance to reduce equipment failure rates and maintenance costs.

High-efficiency, high-reliability equipment

In light of the engineering demands such as hard rock, deep holes, and large-scale fracturing, equipment will develop towards high efficiency and reliability. Structural designs will be optimized by using high-strength, wear-resistant, and corrosion-resistant materials to extend service life in harsh underground conditions. High-pressure water injection systems will increase pressure ratings and wider flow regulation ranges of high-pressure pumps, develop high-pressure water injection equipment with large flow and high pressure to meet large-scale fracturing demands. Nozzle designs for high-pressure jet slotting equipment will incorporate advanced wear-resistant materials to improve slotting efficiency and nozzle lifespan. Furthermore, safety features including explosion-proof, waterproof, dust-proof, and shock-resistant will be reinforced to ensure stable operation in complex underground environments.

Miniaturized and integrated equipment

In order to meet the requirements of confined underground spaces and complex working environments in coal mines, miniaturized and integrated equipment will see widespread adoption. The development of portable fracturing devices featuring compact size, lightweight design, and high mobility enables efficient operation in narrow areas and corners of roadways. By adopting a modular design concept, the equipment integrates functions of drilling, slotting, water injection, and monitoring into a unified system, facilitating rapid assembly and disassembly to enhance versatility and flexibility. Additionally, wireless transmission modules are being developed to replace traditional wired systems, reducing cabling requirements and improving mobility and operational convenience.

Conclusions

As an efficient means of rock mass modification, DHF plays an important role in coal resource mining. This paper analyzes the current situation and development trends of DHF method, fracturing process and fracturing equipment, and draws the following conclusions:

Three mainstream methods (manual slotting, linearly arranged drilling, and hydraulic slotting-based multi-hole linear synergy) have established mature engineering application systems. Among them, the “hydraulic slotting + multi-hole linear synergy” method is widely recognized as the most promising technical path due to its balanced directional accuracy of ≤±3° and scalability for large-scale fracturing, which has been verified by multiple field applications. Meanwhile, there are two unresolved controversies: first, the fracture propagation mechanism under multi-field coupling (hydraulic-stress-temperature-chemical) lacks sufficient experimental verification (accounting for less than 30% of relevant studies); second, there is no unified standard for parameter matching (e.g., slotting/injection pressure ratio, borehole spacing/rock tensile strength ratio), leading to significant variations in engineering effectiveness across coal mines with a coefficient of variation exceeding 30%. Additionally, key technical gaps persist: intelligent control systems are disconnected from complex geological conditions (heterogeneous rock formations, natural fractures), resulting in a 25–35% reduction in directional accuracy in practical applications; monitoring equipment faces a cost-performance dilemma-acoustic testing has an error of ±10–12% in heterogeneous rock masses, while stress monitoring costs approximately 50,000 RMB per set-failing to meet the demand for large-scale real-time effectiveness evaluation. This synthesis lays the foundation for subsequent targeted research and engineering optimization.

Aligning with the coal industry's development needs and cutting-edge technological trends, future underground coal mine DHF technology will evolve toward intelligentization, multi-field coupling coordination, and precision. In terms of fracturing methods, intelligent directional fracturing, multi-field coupling coordinated fracturing, and miniaturized precision fracturing will become core development directions. Regarding processes, integrated efficient construction, adaptive dynamic optimization, and green low-carbon development will be primary trends. For equipment development, intelligent automation, high-efficiency and reliability, and miniaturized integration will be key areas for breakthrough.

With continuous technological innovation and refinement, DHF and rock breaking will play a more significant role in coal mining under high-stress, hard-rock, and complex geological conditions, providing robust technical support for safe, efficient, and environmentally sustainable coal mining. Future research should prioritize fundamental studies on fracturing mechanisms to refine theoretical frameworks, increase the development of intelligent and high-efficiency equipment to enhance equipment capabilities, and strengthen the integration of engineering practice with theoretical research to accelerate the transformation and application of technological achievements.

Footnotes

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by Key Research and Development of Lvliang City (2025GY20), the Research Fund of State and Local Joint Engineering Laboratory for Gas Drainage & Ground Control of Deep Mines (Henan Polytechnic University) (SJF202503), Fundamental Research Program of Shanxi Province (202303021222249), Key R&D Program for the Introduction of High-level Scientific and Technological Talents in Lüliang City (2023RC19), National Natural Science Foundation of China Project (52504133) and Engineering Research Center for Digital Risk Control of Underground Engineering of Jiangxi Province (East China University of Technology) (JXDFJJ2025-003).

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

ORCID iDs

Weiyong Lu

Heng Zhang

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