煤层顶板 水力压裂 泄压 水力压裂参数 射孔参数

为解决由岩层界面和层间强度差异引起的煤系岩层水力压裂裂缝扩展轨迹及压裂范围有效控制的难题, 采用有限–离散元方法模拟研究了煤系岩层水力裂缝扩展规律, 发现多因素耦合作用下煤系岩层水力裂缝扩展轨迹有12种主要模式, 可归纳为穿过岩层界面扩展、穿过岩层界面扩展的同时沿着岩层界面扩展、沿着岩层界面扩展和在岩层界面处停止扩展4类; 建立了融合BP神经网络、差分进化算法和灰狼优化算法的混合人工智能模型, 用以预测煤系岩层水力裂缝扩展轨迹和压裂效果; 分析了不同地应力条件下岩性强度差异系数、岩层界面倾角、岩层界面强度系数、注入速率和定向射孔角度变化时水力裂缝在岩层界面处扩展轨迹的变化规律, 揭示了煤系岩层水力裂缝扩展轨迹的主要控制因素及重要程度; 提出了基于BP-DEGWO混合人工智能预测模型的煤系岩层水力压裂技术方案智能优化设计方法, 为利用人工智能算法进行水力压裂的关键技术方案优化设计和压裂效果预测提供参考。

… roof rockburst in domestic collieries, the mechanism and field application of directional hydraulic fracturing … The results show that the weighting span of the main roof and the released …

… hard roof. In order to make hard roof fracture in a directional way, a hydraulic fracture field test has been conducted in the third panel district of Tashan Coal Mine in Datong. First, …

… The roof hydraulic fracturing phenomena along the front and the back … the fracture water pressure of single-hole hydraulic fracturing is in the range of 22–45 MPa; the hydraulic fracturing …

The occurence of hanging roof commonly arises in the face end of longwall coal mining under hard roof conditions. The sudden break and subsequent caving of a hanging roof could result in the extrusion of gas in the gob to the face, causing gas concentrations to rise sharply and to increase to over a safety-limited value. A series of linear fracturing-holes of 32 mm diameter were drilled into the roof of the entries with an anchor rig. According to the theory that the gob should be fully filled with the fragmentized falling roof rock, the drilling depth is determined as being 3~5 times the mining height if the broken expansion coefficient takes an empirical value. Considering the general extension range of cracks and the supporting form of the entryway, the spacing distance between two drilling holes is determined as being 1~2 times the crack’s range of extension. Using a mounting pipe, a high pressure resistant sealing device of a small diameter-size was sent to the designated location for the high-pressure hydraulic fracturing of the roof rock. The hydraulic fracturing created the main hydro-fracturing crack and airfoil branch cracks in the interior of the roof-rock, transforming the roof structure and weakening the strength of the roof to form a weak plane which accelerated roof caving, and eventually induced the full caving in of the roof in time with the help of ground pressure. For holes deeper than 4 m, retreating hydraulic fracturing could ensure the uniformity of crack extension. Tested and applied at several mines in Shengdong Mining District, the highest ruptured water pressure was found to be 55 MPa, and the hanging roof at the face end was reduced in length from 12 m to less than 1~2 m. This technology has eliminated the risk of the extrusion of gas which has accumulated in the gob.

… The permeability of the coal seam can also be improved, … Roof hydraulic fracturing before mining can enhance the roof caving in the gob, thus avoiding a large area of hanging roof …

… roofs in a Chinese coal mine. A damage parameter (D) is proposed to assess the degree of hydraulic fracturing in the roof. The mechanisms and effects of MFHW for controlling coal …

… and coal are fractured in order to create more surface area and retrieve the gas or oil in it. Thus, hydraulic fracturing … In mining industry, hydraulic fracturing is also a crucial approach to …

… To precisely fracturing the far-field hard roof, this paper studies determination of the target stratum by referring to 8101 working face of Tashan coal mine, the most typical hard roof mine. …

Aiming at the characteristics of low permeability and large gas content of deep soft coal seam, hydraulic fracturing pressure relief and improvement permeability technology of the roof are proposed. FLAC3D was used to invert the roof hydraulic fracturing of the soft coal seams. On this basis, the dynamic evolution characteristics of permeability before and after fracturing were given by the COMSOL multiphysics software. Finally, roof hydraulic fracturing technology was applied on the site. The results show that after the hydraulic fracture of the roof, the peak stress on both sides of the roadway is reduced by 1.7 MPa, the peak stress concentration in front of the roadway is reduced by 2.2 MPa, the stress concentration range is reduced, and the stress is effectively transferred to the deep part of the roadway. After fracturing, the permeability of the stress concentration area increased by 2.18 times, the permeability of the original rock stress area increased by 1.84 times, and the gas fluidity improved. After roof hydraulic fracturing, the occurrence of a coal cannon decreases from 2 to 4 times per 1 m driving to 0 to 1 times, and the phenomenon of a coal cannon at the working face decreases significantly. The concentration of the return air flow is reduced from 0.5–0.9% to 0.2–0.4%. The results of this study have a guiding role in taking the corresponding measures for gas disaster prevention and control.

… of roof hydraulic fracturing, the three-dimensional stress in roof before and after hydraulic fracturing as … In this paper, a field test of roof hydraulic fracturing in a coal mine was conducted. …

… propagation pressures,while the broken soft coal seam is relatively plastic and has a … hydraulic fracture are performed in horizontal well of the roof strata, the hydraulic fractures in roof …

… Roof management in coal mines is … roof when encountering a thick hard roof above a coal seam. To solve this problem, directional hydraulic fracturing technology was utilized in the coal …

… coal, indirect hydraulic fracturing (HF) coal is a technology with great potential to increase coal … HFs can cross the coal–rock interface from the roof and propagate into the coal. To this …

… coal pillars and an 18.3 m thick-hard roof, categorizing six types of complex residual coal … , the horizontal directional long borehole staged hydraulic fracturing (LBSHF) technology is …

The extension mechanisms of penetrating fractures along the coal–rock interface during cross-layer fracturing in the roof of fragmented, soft, and low-permeability coal seams remain unclear. In this article, combined with the globally embedded cohesive elements technology, the cohesive zone model (CZM) is employed to investigate the primary controlling factors influencing the cross-material propagation of fractures in coal seam roofs. The results show that the vertical stress differential coefficient k controls the extension and steering of the crack, k ≤ 0.5, the cracks will extend along both sides of the interface or cease to extend when they reach the coal–rock interface, k > 0.5, and the crack can cross the interface and enter the coal seam. When the value of k is specific, the greater the strength of the coal rock interface, the longer the distance that cracks extend in the coal seam. Inject rate from 6 to 14 m3/min, the total length of the hydraulic crack increases by 3.05 m, an increase in about 57%. When the interface spacing between the horizontal well spacing and the coal–rock interface was increased from 0.5 to 3 m, the fracture length and width showed a tendency of increasing and then decreasing. This study proposes a globally embedded cohesive element method, and reveals the mechanism for complex cross-layer propagation behavior of hydraulic fractures within coal–rock composite structures. This obtained results display the scientific significance of the optimization of the cross-boundary penetrating fracturing in the coal seam roof.

… and can be used in high gassy coal seam 6., 7.. In directional hydraulic fracturing, the hanging hard roof is pre-split along a key plane. Then the hanging roof rotates and slips to the gob. …

In coal seam groups where the spacing between the upper and lower seams is small, the lower seam working face is significantly influenced by residual coal pillars from the upper seam and the void spaces created during mining. This presents considerable challenges for underground mining safety. Through field investigations, the layout of the coal seam quarry above the working face of the 3−1 coal seam in Yanghuopan Mine was examined, along with the distribution of the residual coal pillars. This allowed for the identification of the interlayer rock strata characteristics. Subsequently, we analyzed the mechanism of directional hydraulic fracturing and decompression to determine the key parameters of the 3−1 coal seam. Using the Rock Fracture Process Analysis 3D (RFPA 3D) numerical simulation, we evaluated the effects of various factors on the initiation and propagation of hydraulic fracturing-induced cracks, formulated the evolution law of these fractures, and incorporated the damage variables into the analysis. Additionally, we assessed the influence of different parameters on crack initiation and extension during hydraulic fracturing, using RFPA 3D simulations to derive the evolution law governing directional hydraulic fractures. This allowed us to define the hydraulic fracturing parameters for the 3−1 interbedded rock layers by integrating the process parameter calculations with the damage variables. Based on these findings, an on-site implementation plan was developed and executed, followed by a comprehensive evaluation of the construction results. The study concludes that directional hydraulic fracturing and decompression effectively contribute to the prevention and control of roof-related disasters in the mining of lower coal seams where seam spacing is minimal. This research offers valuable theoretical insights and practical reference for disaster prevention and control in similar geological conditions.

Using hydraulic fracturing for cutting roof pressure is a critical technology to protect coal pillars. In this paper, based on the engineering background of 18506 working face in the Xiqu Coal Mine, using the methods of theoretical analysis, numerical simulation, and field measurement, a reasonable coal pillar width and practical parameters of hydraulic fracturing are given. The results show that roof cutting can significantly increase the stress in goaf and relieve the advanced pressure of the working face. Taking 18506 working face as the research object, the industrial test is carried out, and the surrounding rock control scheme of hydraulic fracturing and roof cutting is put forward, the mine pressure monitoring results show that the auxiliary roadway of 18506 working face reaches a stable state within 20 days, the deformation and damage degree of roadway surrounding rock are small, and the integrity of surrounding rock is improved.

Narrabri Coal Operations is longwall mining coal directly below a 15 to 20 m thick conglom‐ erate sequence expected to be capable of producing a windblast upon first caving at longwall startup and producing periodic weighting during regular mining. Site characterisation and field trials were undertaken to evaluate hydraulic fracturing as a method to precondition the conglomerate strata sufficiently to promote normal caving behaviour at longwall startup and reduce the severity of periodic weighting. This paper presents the results of the trials and illustrates the effectiveness of hydraulic fracturing as a preconditioning technique. Initial work was directed at determining if hydraulic fractures were able to be grown with a horizontal orientation, which would allow efficient preconditioning of the rock mass by placing a number of fractures at different depths through the conglomerate from vertical boreholes drilled from the surface. The measurements and trials were designed to determine the in situ principal stresses, the hydraulic fracture orientation and growth rate, and whether the fractures could be extended as essentially parallel fractures to a radius of at least 30 m. Overcore stress measurements were used to determine the orientation and magnitude of the in situ principal stresses, a surface tiltmeter array was used to determine the hydraulic fracture orientation, and stress change monitoring, pressure monitoring and temperature logging in offset boreholes were used to establish the fracture growth rate, lateral extent, and that the fractures maintained their initial spacing to a radial distance of greater than 30 metres. The measurements and trials demonstrated that horizontal fractures could be extended parallel to one another to a distance of 30 to 50 m by injection of 5,000 to 15,000 litres of water at a rate of 400 to 500 L/min. Results from the trial allowed a preconditioning plan to be developed and successfully implemented. © 2013 Jeffrey et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract A strong and hard hanging roof causes high underground stress in underground mines, leading to rock burst, coal and gas outburst, or large deformations in roadways. To address these problems, hydraulic fracturing can form hydraulic cracks in strong hanging roofs and promote fractures in the hanging-roof. However, it is difficult to ensure that hydraulic cracks propagate vertically through rock strata. Therefore, a novel method has been proposed that uses hydraulic fracturing to generate vertical fractures in hard hanging roofs for-roof cutting and stress relief. The method exploits the additional horizontal tensile stress produced by the bending deformation of the roof so that the fracturing point occurs in a desirable tensile-stress area, and therefore to promote the vertical expansion of hydraulic cracks that cut off the rock strata. In this paper, the position of the maximum additional horizontal tensile stress is taken to be the reasonable fracturing position given by a mechanical model. Solving this model indicates that a shorter hanging roof equates to a thicker hanging roof, a larger elastic modulus, a smaller elastic cushion coefficient, and a fracturing position that is deeper in the coal wall. The various factors that affect the reasonable fracturing position may be ordered in terms of their impact as follows: hanging-roof length > hanging-roof elastic modulus > hanging-roof thickness > elastic cushion coefficient > horizontal stress. The proposed method has been applied to roadway 5103 of the Majiliang Coal Mine of Datong Coal Mine Group Co., Ltd. The results show that hydraulic fracturing forms vertical hydraulic cracks at the point of maximum additional horizontal tensile stress, which can effectively control the deformation in the surrounding rock. The fracturing position calculated by using this model is reasonable and reliable and can be used to guide the on-site construction.

… of fracturing parameters on fracture propagation behavior and RSR stress mechanisms … hydro-mechanical coupling model for the fracturing in roadway roof based on phase-field theory. …

… immediate roof may not substantially affect front abutment pressures. Continuous collapse of the immediate roof can cause timely fracture and collapse of the overlying main roof layers,…

A hard roof can cause serious issues corelated with the stability of the panel including large deformation of the roadway in the gob, rock burst, coal and gas outbursts, etc. Currently, hydraulic fracturing has ever-increasingly been used to help control these above-mentioned issues in many engineering cases. This paper presents a series of numerical simulations for hydraulic fracturing with a recently developed model to examine the weakening effect of this pre-conditioning measure on a hard roof. The results show that large deflections in the principal stress direction occur above the coal seam after mining, which can be progressively enhanced as the working face continuously advances. This further could significantly affect the hydraulic fracturing pattern and result in arc-shaped fracture propagation, especially for the hydraulic fracture in stress-descending areas. The obtained results suggest that the hydraulic fracturing operation in a hard roof is preferable in the areas close to the middle of the gob where the created fractures would be deflected more. At last, sensitivity analysis shows that geological conditions have great influence on the hydraulic fracturing pattern. Among the factors analyzed in this paper, the difference between the maximum and minimum stress has the largest influence and should be fully considered. This study could provide a theoretical basis for the practical hydraulic fracturing operation in a hard roof.

Under complex deep mining conditions, crosscuts adjacent to goafs often face severe surrounding rock stability issues due to the hanging of hard roofs and mining disturbances. Taking the large deformation of the 261 cross‐cut at Huoshaopu Coal Mine as an example, hydraulic fracturing for roof cutting and pressure relief was applied for control. Through integrated field observation, theoretical modeling, numerical simulation, and engineering practice, it was found that the depth of roof fractures reached 6.23 m, with an integrity coefficient as low as 0.4–0.5. A cantilever beam model was established and stress formulas were derived, while UDEC simulations verified the effectiveness of hydraulic fracturing in cutting off the roof and redistributing stress. On‐site implementation of bolt‐grouting reinforcement combined with the “retreat‐style single‐borehole multi‐stage fracturing” technique successfully severed the main roof cantilever, leading to a significant reduction in abutment pressure: coal pillar stress decreased from 39.35 to 32.35 MPa (a reduction of 17.8%), and solid coal side stress decreased from 31.05 to 27.02 MPa (a reduction of 13.0%). Roadway convergence rates were reduced by 28%–38% without any collapse. The study demonstrates that hydraulic fracturing is an effective method for mitigating stress and deformation in crosscuts, providing a critical engineering strategy for controlling thick and hard roof strata.

… fracturing generates distinct pressure-relief (PR) and high-water-pressure (HWP) zones, with tensile failure dominating within the roof … In-situ stress ratio and borehole spacing are …

Lateral cantilever beam structure is easy to be formed after extracting coal from working face under hard top plate, resulting in the superposition of the dynamic pressure of mining and the high hydrostatic pressure caused by the lateral overhanging rock layer and transferring to the mine roadway through the key layer, which causes the destabilization and large deformation of the peripheral rock structure of the roadway in the neighboring working face. In order to improve the rock structure of roof plate, adjust the breaking and transporting law of hard rock layer, and then control the strong mine pressure in the adjacent roadway, this paper analyzes the distribution law of mine pressure in the mine roadway, adopts numerical simulation means to study the distribution of the support pressure in the adjacent roadway under the hard roof plate, investigates the mechanism of the peripheral rock structure transferring loads under the conditions of different roof-cutting parameters, and puts forward a joint method of cutting top plate to remove the pressure by the abrasive jet ultra-high pressure hydraulic cutting - hydraulic fracturing. The method of roof cutting and pressure relief is proposed. The results of the study show that the mine pressure in the goaf roadway is the result of the coupling of dynamic and static loads, which is mainly manifested in the coupling of the high dynamic pressure caused by mining and the static load pressure of the lateral overhanging roof of the goaf roadway. The coupling effect is transmitted to the bottom plate by the surrounding rock in the limit equilibrium area on both sides of the roadway, and then produces a large range of plastic deformation in the bottom plate. Through the abrasive jet ultra-high-pressure hydraulic slitting technology, the slots are preset in the hard top plate, which can play a guiding role for the hydraulic fracturing cracks to drag and expand, and increase the influence range of hydraulic fracturing cracks by opening up the original cracks in the rock body, forming a continuous weak surface in a hard roof to weaken the stress transmission of the roof, and the depth of the slits is positively correlated with the slit pressure and the sand mixing ratio. The roof strata will interact with each other during the collapse process, and the changes in fracture angle and height will lead to changes in the sliding speed of the fractured rock mass and significant differences in the support force of the high-level rock strata. In the range of 66°～86°, the tensile stress on the rock layer at the top of the cut-off is increasing with the decrease of the top-cutting angle, and the rate of tensile stress increase is faster with the increase of the top-cutting height. When the range of the top-cutting height is 26～56 m, the fracture angle at which the tensile stress of the rock formation at the fracture point is higher than the ultimate tensile strength increases with the increase of the top-cutting height. Based on the above research, an industrial test was carried out at the 81403 working face of the Huayang mine. The on-site monitoring results with the loads of supporting structure and deformation of surrounding rock as the indexes show that the anchor cable stress of the roof plate of the roadway was reduced by 35%, the stress of the roadway wall was reduced by 44%, the amount of roadway top and bottom plate migration was reduced by 57%, and the migration amount of the roadway two walls was reduced by 42%, which has a good effect of pressure relief. The research results can provide a new technical means for controlling the ground pressure in the gob roadway under the condition of hard roof.

… , the roof breaking structure of PLRR is analyzed. It is concluded that the roof cutting with vertical hydraulic fracture (HF) at a specified position, that is, fixed-length roof cutting, can …

To address the technical challenge of large-area roof hanging and induced strong strata behaviors in deep mines with hard roof strata, a study on pressure relief using hydraulic fracturing technology was conducted, taking the 1012006 working face in the Yuanzigou Coal Mine as the engineering background. Through geological survey and key stratum theory analysis, a low-position key stratum located 23 m above the roadway roof was identified as the target layer for fracturing. True triaxial hydraulic fracturing experiments coupled with acoustic emission (AE) monitoring revealed a synchronous response characterized by a sudden drop in injection pressure and a rapid increase in AE counts. This established a quantitative correlation between rock mass fracturing and AE characteristics, providing a theoretical basis for field microseismic monitoring. Based on the “dual-borehole synergy” borehole layout principle, a fracturing network comprising 6 drilling fields and 12 directional long boreholes was designed, with a total drilling length of 5727 m and 120 planned fracturing stages. Specialized equipment was selected for implementation. Field monitoring results demonstrated: a maximum fracturing influence radius of 27.8 m; that the average daily frequency and total energy of microseismic events decreased by 50.65% and 27.73%, respectively; and that the stress in the deep part of the roadway decreased by 17.69%. These results confirm the effective improvement of the roof stress environment and the successful achievement of the expected pressure relief and rockburst prevention effect.

… thick and hard roof strata. To address this issue, this paper proposes a composite pressure relief method (CPRM) based on roof cutting and fracturing expansion for thick hard roof in a …

With the increasing depth of coal mining, thick-hard overlying strata (THOS) often induce dynamic disasters such as rockbursts, posing significant threats to mine safety. This study focuses on the application of directional hydraulic fracturing roof pressure relief technology (HFRPRT) as a key disaster prevention technology in the Hongqinghe Coal Mine’s 3-1302 longwall face. An integrated monitoring system combining microseismic (MS) and acoustic emission (AE) data was established to quantitatively evaluate the fracturing process through multi-indicator analysis, including support pressure response, energy distribution, and surface subsidence. The results demonstrate that HFRPRT effectively weakened THOS integrity, reducing periodic weighting intervals by 25% and peak pressure intensity by 21.95%. Daily AE energy and event count increased by 154% and 636%, respectively, indicating enhanced microfracture propagation. MS events shifted to lower-energy patterns, with second-order events predominating (59.16%), highlighting the technology’s role in mitigating elastic energy accumulation and dynamic hazards. This research provides a theoretical foundation for optimizing hydraulic fracturing parameters in similar geotechnical conditions, advancing coal mine disaster prevention strategies.

… of thick and hard roof in 632 working face of Huangling No. 1 Coal Mine, a hydraulic fracturing-drilling roof cutting collaborative pressure relief technology for hard roof is proposed. …

… of directional hydraulic fracturing in hard roof is introduced, and the fracturing schemes are further … for roof fracturing and bridge the gaps of hydraulic fracturing under abutment pressure. …

Abstract Multilayers-commingled fracturing is the vital step to achieve gas co-exploration in coal measures. Hydraulic fracture is required to penetrate vertically through multilayers in Multilayers-commingled fracturing. However, investigation on the propagation of commingled crack in coal measures which is soft-hard interlaced strata is less studied and not well understood. Therefore, based on the rock seepage-stress coupling effect, a three-dimensional (3D) hydraulic fracture model of multilayers-commingled fracturing in coal strata was established in combination with ABAQUS finite element software. The influence of geological factors and construction factors on the hydraulic cracks expansion in sandstone-coal interbedded reservoirs of X well in Linxing area in China was investigated. We found that the minimum horizontal stress difference is an essential geological factor to contain the vertical extension of commingled fractures; The high elastic modulus of neighboring reservoirs promotes the longitudinal penetration of fractures in the multilayers; The low tensile strength contrast between multiple layers increases the possibility of vertical expansion of combined fractures, while the high permeability of adjacent layers restrains the longitudinal propagation of fractures. The critical displacement of fracturing fluid exists in the combined fracturing process. The commingled cracks breakthrough the stratigraphic interface and penetrate into the adjoining layers when reaching the critical displacement; The high-viscosity of fracturing fluid facilitates the extending behaviors of hydraulic cracks in the vertical direction. The research results provide theoretical guidance for optimization of commingled fracturing layers and design of construction parameters in coal strata.

… designing hydraulic fracturing pumping parameters in coal measure strata. Laboratory true-triaxial hydraulic fracturing … natural coal and mudstone interlayers. A three-dimensional (3D) …

The No.8 deep coal seam in the Ordos basin is situated at a burial depth of over 2 000 meters and has a complex geological structure. The roof and floor layers of the target coal seam are composed of limestone and mudstone, respectively. Horizontal well fracturing is used on-site as a development method for the No.8 deep coalbed methane. A finite element simulation implementation process has been developed to help clarify the propagation path law of hydraulic fractures in the No.8 deep coal seam. This process takes into account the reservoir geological structure, wellbore trajectory, and mechanical characteristics of reservoir rock to simulate hydraulic fracturing with different perforation positions at the coal seam and roof/ floor rock. Firstly, the rock specimens from the reservoir are processed, and their mechanical properties are tested using triaxial compression experiments. The characteristics of the matrix pores and fracture structures are analyzed by using CT scanning. Then, the cohesive model with pore pressure nodes is used to understand the impact of weak surfaces and pore fractures on the fluid seepage within deep coal seam. A three-dimensional finite element model of deep coal seam hydraulic fracturing is established which considers the seepage - stress - damage coupling. The parameters of the model are inverted and verified based on on-site fracturing testing and construction technology. Finally, the simulation of fracturing are conducted with different perforation positions at deep coal and roof/ floor rock. According to the research findings, the following results are obtained: The coal rock has a lower elastic modulus and a higher Poisson’s ratio than the limestone and mudstone rocks, and the pores and fractures are developed. When perforating in the coal seam, the hydraulic fractures are completely limited to the expansion of the coal seam. It is difficult for hydraulic fractures to pass through the interface to enter the limestone and mudstone. The hydraulic fractures are wholly closed before reaching the peak injection rate. When perforating in the limestone, the hydraulic fractures propagate into the coal seam by passing through the interface between the coal and limestone, and they expand further into the coal seam. It is worth noting that the length of hydraulic fractures in the coal is greater than that in the limestone. When perforating in the mudstone, the hydraulic fractures propagate into the coal seam by passing through the interface between the coal and mudstone. The hydraulic fractures in both mudstone and coal are extended in length. The perforation in limestone and mudstone requires a higher initiation pressure compared to the perforation in coal. During the perforation in the limestone and the mudstone, the hydraulic fractures in the coal tend to close first, due to the filtration of a significant amount of fracturing fluid in the coal. The research results provide some ideas for modeling the expansion of hydraulic fractures in deep coal seams and guide the efficient development of deep coalbed methane.

To solve the problems that gas outburst accidents are prone to occur in the mining process of broken and soft outburst coal seam, and the existing research on the directional roof hydraulic fracturing of broken soft outburst coal seam lacks the analysis of roof bedding and perforation design, this paper uses the method of combining experiment with particle flow code (PFC), investigates the influence of perforation on the directional hydraulic fracturing of roof, and defines the optimal perforation layout scheme under the joint action of bedding and perforation. The results reveal that hydraulic fracture propagation is significantly influenced by the bedding angle and in situ stress conditions of the rock mass. During roof fracturing, once fractures propagate to the coal-rock interface, they induce the formation of a complex fracture network within the coal seam, effectively enhancing its permeability. PFC has accuracy and engineering adaptability in hydraulic fracturing numerical simulation. Under the conditions of different bedding angles, the directional hydraulic fracturing perforation should be arranged in different ways, a vertical perforation configuration is recommended for bedding angles of 0°–15°, while for angles of 30°–60°, perforations should be oriented perpendicular to the bedding direction. At 45°–60°, perforations should avoid alignment with the bedding plane and instead adopt a large-angle orientation. Under the parameter combination of “three perforations-optimized angle-suitable bedding,” the effect of directional hydraulic fracturing of bedding rock broken soft coal seam roof is the best. This perforation scheme offers an effective strategy for gas control in outburst-prone soft coal seams.

… effects of horizontal wellbore location, perforation strategy, roof lithology, and vertical stress … the soft coal seam roof. The results indicate that bilateral downward perforation with a phase …

Hydraulic fracturing experiments were conducted to study the change of initial fracture pressure and roof stress under repeated hydraulic fracturing in coal mines and to investigate the actual penetration enhancement effect on coal seams. The results show that as the fracturing is repeated, a lower initiation pressure is required. The roof stress changes consistently with water injection pressure. First, the stress in the area around the water injection hole gradually increases, and then stress transfers to the surroundings. With the re‐implementation of fracturing, the area of increased stress expands, and the scope of influence of hydraulic fracturing also becomes larger. When fracturing in a coal mine, for the same total fracturing time, the total water injection volume of repeated fracturing is 35% higher than that of single fracturing, and the affected area is increased by 31%. The concentration of repeated hydraulic fracturing is about 36% higher than that of a single hydraulic fracturing, and the pure flow of gas drainage is increased by about 1.3 times. The results of this study can guide the efficient implementation of hydraulic fracturing underground in coal mines, so as to achieve a larger impact range of coal seam hydraulic fracturing and better gas drainage effects.

… perforation and fracturing in the roof of crushed soft coal … through the roof to communicate with the coal seam [4]. Xu et … parameters with the actual coal seam and sandstone roof layer, …

To address the problems of strong roof integrity, severe energy accumulation, and difficult caving in thick and hard roofs, a three-dimensional numerical study on fracture propagation and pressure-relief control durisng segmented hydraulic fracturing was carried out based on the engineering geological conditions of the 6125-1 working face at the Haishiwan Coal Mine, Shaanxi Province, China. using the ABAQUS finite element platform coupled with Ins-coh cohesive elements. A systematic analysis was conducted to elucidate the effects of elastic modulus, Poisson’s ratio, injection rate, and fluid viscosity on fracture initiation, stress evolution, and fractured volume. The results show that for every 10 GPa increase in elastic modulus, the average fractured volume decreases by 8%, and the fracture width exhibits a marked reduction; increasing Poisson’s ratio enhances the lateral deformation compatibility of the rock mass, raising the fracture width and volumetric growth rate by approximately 3% and 5%, respectively, although an excessively high Poisson’s ratio induces stress diffusion and reduces fracture stability. When the injection rate increases from 0.01 m3/s to 0.025 m3/s, the fractured volume increases by about 160%, and the maximum fracture width increases by 43%, whereas increasing fluid viscosity exerts a limited influence on volumetric growth but is conducive to stabilizing fracture morphology. Field observations via borehole imaging and seepage confirm full fracture connectivity within the roof and the formation of a continuous rupture zone, promoting timely roof breakage and caving along the dip direction and thereby creating favorable conditions for reducing rockburst hazards at the working face. This study clarifies the mechanical mechanisms and multi-parameter coupling laws governing hydraulic fracture propagation in thick and hard roofs, providing a theoretical basis and engineering reference for roof pressure-relief control and rockburst-resistant design under similar geological conditions.

The staged multi-cluster hydraulic fracturing technique of horizontal well in roof strata of coal seam is a new method for effective developing the broken-soft coal seam. Therefore, based on cohesive model and Bernoulli equation, considering the friction of fracturing fluid flowing through wellbore, perforation and the stress interference during fracture propagation, a finite element numerical model of multi-cluster fracturing of coal seam roof coupling wellbore and formation is established. The effects of perforation diameter, perforation number and injection rate on fracture extension were studied. The results indicated the fracture tip is always located in the roof during fracture dynamic propagation, and coal seam fractures are wider than roof fractures. During multi-cluster perforation in the fracturing section, the fractures initiated from each perforation cluster have competitive propagation. The stress interference between the fractures and the friction of the fracturing fluid make the distribution of the fracturing fluid between the perforation clusters uneven. Increasing injection rate, reducing number of single perforation cluster, and decreasing diameter of perforations hole can facilitate the even propagation of fractures in each cluster, and increasing the stimulated reservoir volume. The influence of the perforation diameter is significantly greater than the injection rate and perforations number. China has a wide distribution of broken soft coal seams that have low permeability, and hydraulic fracturing is the key to its efficient development. However, due to their low strength(S), low elastic modulus(E), and high Poisson's Ratio(ν), broken-soft coal seams have high plasticity compared to hard coals or other rocks (Li et al., 2021; Lyu et al., 2020). Long fractures are difficult to form in coal seams and the fracturing sand is severely inlaid when conduct hydraulic fracturing in the plastic broken soft coal seam directly. The stimulation effect is generally poor. In response to this problem, scholars have proposed indirect fracturing technology for coal seam roof. (Li et al., 2014; Olsen et al., 2007; Olsen et al., 2003; Zhang et al., 2018). The technology realizes the efficient extraction of coalbed methane in broken soft coal seam by arranging horizontal wells in the roof strata adjacent to the coal seam, and the fracture extends from the coal seam roof to the coal seam. The field test indicates that the application of this approach to broken soft low-permeability coal seam can significantly improve the efficiency of coalbed methane exploitation (Zhang and Bian, 2015).

Thin interbedded coal seams exhibit characteristics of numerous layers, minimal thickness, and a clustered distribution. Horizontal well cross-seam fracturing technology represents an effective approach for coalbed methane exploration and development. However, the adaptability of construction parameters for longitudinal cross-seam fracturing in thin interbedded coal seams remains a critical factor constraining the seamless integration of horizontal wells with coal seam clusters and hindering efficient gas production. This study experimentally analyzed the mechanical characteristics and mineral content of coal and rock based on mechanical properties, mineral composition, and pore structure. A fully three-dimensional, structured grid geological model of the longitudinal multithin interbedded formation was constructed using the GOHFER platform. Crack propagation simulations were conducted, and the parameters for longitudinal cross-bedding fracturing operations were optimized through a five-factor, four-level orthogonal experimental design. Research findings indicate that the compressive strength of the roof and floor strata in coal seams 10# and 12# exceeds that of the coal mixture, exhibiting higher brittleness. The crack pressure for coal seams 10# and 12# ranges between 13.22 and 13.35 MPa. The roof and floor strata contain a high proportion of clay minerals, reaching 62.4–67.7%. When perforation points are located within the roof strata of the lower coal seam, vertical cracks can effectively communicate between the two coal seams. However, when perforation points are situated within the lower coal seam itself, cracks encounter significant difficulty penetrating upward through the intercalated layer, thereby limiting their ability to communicate with the overlying coal seam. Taking into account both the longitudinal layer-penetrating capability of cracks and the convenience of construction organization, the recommended combination of construction parameters is flow rate 14 m3/min + single-stage sand volume 60 m3 + single-stage fluid volume 1700 m3 + perforation count 25 + crack spacing 50 m + two clusters per stage. The research findings provide theoretical reference for optimizing construction parameters in horizontal fracturing operations targeting thin, interbedded, multilayer coalbed methane reservoirs.

To address the challenges of source-based rock burst elimination and efficient mining in ultra-thick coal seams prone to rock bursts due to hard roofs, a safety assurance technology was developed to create a favorable environment for intelligent and efficient coal caving. Building on this foundation, breakthroughs in intelligent caving technologies were achieved, resulting in the development of an intelligent top-caving mining system for ultra-thick coal seams with hard roofs. The research process includes the following: For safety assurance in mining ultra-thick coal seams with hard roofs, the Timoshenko beam theory was applied to establish an elastic energy accumulation model for the periodic breakage of hard roofs. This enabled the analysis of energy density distributions under different uniaxial tensile strengths and revealed the advanced rock burst elimination mechanism based on pre-fabricated artificial fracture networks from ground. Accordingly, horizontal well fracturing and liquid explosive blasting techniques were developed, forming ground-based advanced rock burst elimination technology. Furthermore, using Reissner’s thick-plate theory, mechanical models for roof behavior before and after directional fracturing were constructed. These models analyzed the effects of artificial directional fractures on roof elastic energy density and coal static load increments, uncovering the rock burst elimination mechanism of underground artificial directional fractures. Then, a directional composite blasting technology was invented, leading to underground-based advanced rock burst elimination technology. In intelligent mining, several innovations were introduced, including radar-based coal thickness detection, near-infrared spectroscopy for coal-rock identification, vibration and audio-based coal-rock identification, and laser 3D scanning for real-time coal extraction monitoring. These developments formed an intelligent perception and identification technology for top-caving working faces with ultra-thick coal seams. A multi-source information database for intelligent top-caving longwall panels integrating human, machine, and environmental data was established, along with coordinated mining and caving decision-making models and intelligent decision-making technologies for caving processes. High-precision inertial navigation for equipment positioning and a remote communication and control platform were developed, enabling remote intelligent control for top coal caving. Key findings include: ① For tensile strengths of 0.76, 1.57, 2.68, 3.95 and 5.68 MPa, the corresponding peak elastic energy densities of hard roofs were 6.5, 25.4, 71.6, 168.2 and 340.1 kJ/m, respectively, with a quadratic relationship between peak elastic energy density (\begin{document}$U_{\max}^{{\mathrm{e}}} $\end{document}) and tensile strength (σ0): \begin{document}$U_{\max}^{{\mathrm{e}}} $\end{document} = 10.715σ02−0.7182σ0. ② Artificial fracture networks altered the boundary conditions of hard roofs. For instance, theoretical calculations for the 103up02 workface of Yanzhou Coal Mining Group showed that artificial fracture networks reduced the first breakage distance of the sandstone layer from 250 m to 123 m. ③ For rock burst elimination mechanism by ground-based artificial fracture networks, numerous fractures created structural weak surfaces in the strata, reducing the elastic energy accumulation in hard roofs and weakening seismic intensity caused by roof ruptures, thereby mitigating rock bursts in workfaces and roadways. For rock burst elimination mechanism by underground artificial directional fractures, directional cracks reduced or eliminated elastic energy near roadways, decreasing the static load increment on the roof-cutting side and effectively controlling rock bursts within roadways.

The prevention and control of coupled disasters caused by strong mining pressure and high gas is currently the main challenge during coal seam deep mining in the southeastern mining areas of Shanxi Province. This paper takes the 1310 working face of Hudi Coal Mine as the engineering background, analyzing its on-site strong mining pressure event and triggering factors. A reasonable hydraulic fracturing scheme (including layer selection, drilling parameter design, etc.) is proposed based on theoretical analysis of the principles and advantages of hydraulic fracturing technology. Then, the physical analog modeling (PAM) method was used to study the movement law and fracture development of the overlying strata during coal seam mining after hydraulic fracturing. The weakening effect of mining pressure was analyzed through the evolution law of roof stress. The deformation of the surrounding rock in the roadway, coal drilling cuttings, support working resistance, and roof fracture development of the in situ measurement results show that hydraulic fracturing has a good effect on weakening mining pressure. It has achieved safe and efficient mining of coal seams while providing a reference for coal mines with similar conditions.

… model including coal reservoir and roof and floor. Through single-parameter simulation analysis, the … Perforation section length is an important parameter in fracturing operation. With the …

Indirect fracturing in the roof of broken soft coal seams has been demonstrated to be a feasible technology. In this work, the No. 5 coal seam in the Hancheng block was taken as the research object. Based on the findings of true triaxial hydraulic fracturing experiments and field pilot under this technology and the cohesive element method, a 3D numerical model of indirect fracturing in the roof of broken soft coal seams was established, the fracture morphology propagation and evolution law under different conditions was investigated, and analysis of main controlling factors of fracture parameters was conducted with the combination weight method, which was based on grey incidence, analytic hierarchy process and entropy weight method. The results show that “士”-shaped fractures, T-shaped fractures, cross fractures, H-shaped fractures, and “干”-shaped fractures dominated by horizontal fractures were formed. Different parameter combinations can form different fracture morphologies. When the coal seam permeability is lower and the minimum horizontal principal stress difference between layers and fracturing fluid injection rate are both larger, it tends to form “士”-shaped fractures. When the coal seam permeability and minimum horizontal principal stress between layers and perforation position are moderate, cross fractures are easily generated. Different fracture parameters have different main controlling factors. Engineering factors of perforation location, fracturing fluid injection rate and viscosity are the dominant factors of hydraulic fracture shape parameters. This study can provide a reference for the design of indirect fracturing in the roof of broken soft coal seams.

… the fracture propagation mechanism by using cohesive element in ABAQUS version 2020. The main coal seam thickness of the Luling coal mine’s coal-… : the roof (5 m in height), the coal-…

The fracturing crossing coal seam roof is a technology that fulfills the fracturing of a coal seam through the vertical propagation of fractures. Geological conditions are the key factors determining the effect of this kind of fracturing, but there is hardly any research on this aspect. To determine the favorable geological conditions for through-roof fracturing, based on a 3D fracture propagation model, and considering the interlayer vertical fracture toughness and leak-off heterogeneity, a mathematical model of fracturing through a horizontal well in a coal seam roof was established, and the calculation method of fractures crossing layer propagation was determined. In this method, the effect of fracture communication with the coal seam is evaluated by taking the area and the area ratio of fractures in the coal seam as the objective functions. The effects of parameters such as in situ stress combination profile, coal seam fracture toughness, and fluid loss coefficient on fracturing results were evaluated. The reasonable distance from the horizontal well to the coal seam’s top surface was determined in this work. The study results show that: (i) the fracturing effect is better when the coal seam is lower in in situ stress; (ii) the distance between the horizontal well and the top surface of the coal seam is recommended to be less than 4 m to obtain the ideal fracturing effect; and (iii) the combination of the in situ stress profile is the key factor, and the fracture toughness and fluid loss coefficient of the coal seam, fluid viscosity, and the number of perforations in one cluster are the secondary factors affecting the fracturing effect.

During fully mechanized caving in coal seams with medium-to-high strength hard roofs, gob-side roadways often experience significant dynamic pressure, severely compromising safety and mining efficiency. Hydraulic fracturing is widely used for hard roof pre-fracturing, but in high-level roofs with low vertical stress, it often produces horizontal fractures, limiting pressure relief effectiveness. To address the challenge of limited fracture height, a novel approach of radial borehole-assisted hydraulic fracturing is proposed. This method employs radial boreholes to overcome the constraints imposed by low vertical stress and to enhance vertical fracture propagation. True triaxial experiments combined with 3D reconstruction were conducted on sandstone samples collected from the field to characterize fractures induced by radial boreholes. Compared with conventional fracturing, analyses incorporating fracture surface parameters revealed vertical propagation patterns influenced by borehole diameter. The results indicate that radial boreholes can induce fracture initiation and guide vertical propagation, thereby significantly enhancing the fracture height. Increasing the diameter of radial boreholes can significantly enhance the induced stress field around the borehole, thereby improving the vertical propagation capacity of fractures. These findings offer valuable insights for longitudinal stimulation performance and pressure relief strategies in high-level coal seam roofs.

… ; both effects expand the dry zone of the crack and alter the propagation pattern. This … fracturing system for hard roof weakening, offering practical implications for optimizing fracturing …

… According to extensive engineering practice, the staged fracturing of horizontal wells in coal seam roofs is … Fracture propagation law and sensitive factors analysis of layer-penetrating …

Vertical fracture propagation mechanism is important to understand the effect of hydraulic fracturing on the roof of outburst coal seams. In this paper, the differences in physical parameters and in situ stress between an outburst coal seam and its roof strata were compared, and influencing factors of roof-fracturing fractures connecting coal seams were vertically analyzed. The impact of different fracturing strata and injection rates on fracture propagation was studied by numerical models. Results show that the horizontal principal stress of an outburst coal seam is less than that of roof strata, and the fracture length of roof fracturing is larger than that of an outburst coal seam. Roof-fracturing fracture of an outburst coal seam has the material conditions to communicate downward with the coal seam. The downward propagation height of roof-fracturing fracture is positively correlated with the minimum horizontal stress difference between an outburst coal seam and its roof strata. Soft coal fracturing cannot form a long fracture dominated by tensile failure, so the coal seam can be communicated by transforming the roof strata of soft coal to form vertical fractures. The injection rate affects the failure location, fracture width, and fracture-propagation path of the numerical model. Construction parameters should be reasonably designed in accordance with the physical parameters of coal and rock, construction displacement, and in situ stress. These research results can provide a theoretical basis and data support for the design, and optimization of construction parameters of roof fracturing in outburst coal seams.

Hydraulic fracturing can increase the fracture of coal seams, improve the permeability in the coal seam, and reduce the risk of coal and gas outburst. Most of the existing experimental specimens are homogeneous, and the influence of the roof and floor on hydraulic fracture expansion is not considered. Therefore, the hydraulic fracturing test of the simulated combination of the coal seam and the roof and floor under different stress conditions was carried out using the self-developed true triaxial coal mine dynamic disaster large-scale simulation test rig. The results show that (1) under the condition of triaxial unequal pressure, the hydraulic fractures are vertical in the coal seam, and the extension direction of hydraulic fractures in the coal seam will be deflected, with the increase of the ratio of the horizontal maximum principal stress to the horizontal minimum principal stress. The angle between the extension direction of the hydraulic fracture and the horizontal maximum principal stress decreases. (2) Under the condition of triaxial equal confining pressure, the extension of hydraulic fractures in the coal seam are random, and the hydraulic fracture will expand along the dominant fracture surface and form a unilateral expansion fracture when a crack is formed. (3) When the pressure in one direction is unloaded under the condition of the triaxial unequal pressure, the hydraulic fractures in the coal seam will reorientate, and the cracks will expand in the direction of the decreased confining pressure, forming almost mutually perpendicular turning cracks.

A hydraulic fracture extension model was established, an extension criterion and an extension mode of hydraulic fracture were analyzed, and the theoretical prediction was compared with the practical results, a good agreement was observed. Furthermore, the direction of hydraulic fracture extension was also discussed, the results showed that the hydraulic fracture propagates from the cut to the bedding plane, forming a complex mixture of longitudinal and transverse fractures fracture network. The hydraulic fracture extension direction is influenced by its extension critical pressure and its extension pressure in the rock formation. Practice shows that hydraulic fracturing can effectively weaken the strength and integrity of the roof, so that the roof in the mining area can collapse in layers and stages. The present theoretical analysis can be used for reducing or eliminating the hazard of the hard roof to the working faces.

To extract coalbed methane (CBM) from tectonically deformed coal seams, a horizontal well was drilled in the roof of coal seam. Staged hydraulic fracturing was then conducted to connect the horizontal wellbore and underlying coal seam. A finite element model that coupled seepage, stress, and damage theories was built to investigate the influence of coal-roof interface on vertical propagation of hydraulic fracture. Results showed that the coefficient of friction and crossing stress ratio were the two primary factors controlling the fracture penetration. A higher interfacial shear strength is beneficial to fracture penetration. The crossing stress ratio required for fracture penetration decreases as the interface friction coefficient increases. The numerical simulation results agree well with the field pilot test and can provide theoretical support for effective CBM development in similar coal seams. [Received: October 28, 2020; Accepted: March 10, 2021]

Multistaged fracturing in the roof of outburst coal seam is an efficient and creative technology for coalbed methane (CBM) drainage, which can effectively improve the permeability of coal seam. To reveal its mechanism of permeability enhancement, the effect of coal-rock interface on multistaged fracturing in the roof of outburst coal seam was simulated and discussed in this paper. Firstly, the lithological difference between outburst coal seam and roof was compared, and the concept and significance of multistaged fracturing in the roof of outburst coal seam were explained. Then, the mechanical conditions of multiple fractures in the roof traversing coal-rock interface were analyzed. The effects of mechanical parameters on multiple fractures were numerically simulated. The results indicated that fracturing borehole in adjacent rocks of outburst coal seam is much easier to drill and maintain gas drainage. Considering gas drainage efficiency and avoiding being blocked by coal fines, multistaged fracturing borehole is generally drilled in the stable rock stratum of roof. Whether the multiple fractures in the roof can traverse coal-rock interface is related to mechanical parameters of coal and rock, friction factor of coal-rock interface, angle between horizontal profile and coal-rock interface, cementing strength of coal-rock interface, minimum horizontal stress, and other factors. Higher fracturing fluid pressure contributes to propagating from the reservoir with low elastic modulus to the one with high elastic modulus for hydraulic fracture. Hydraulic fracture is more likely to propagate in the rock stratum with high brittleness index. The research results can improve multistaged fracturing theory and provide technological support for field test.

Horizontal wells within the roof are an effective method to develop gas in broken and soft coal seams, and layer-penetrating fracturing is a key engineering method for the stimulating of horizontal wells within the roof of a coal seam. To understand the propagation law of fracture in the composite roof of coal seams, this study conducted research using numerical simulation and physical similarity simulation methods. Furthermore, engineering experiments were carried out at the Panxie coal mine in the Huainan Mining Area and the Luling coal mine in Huaibei Mining Area, to further validate this technology. The numerical simulation results indicated that fracture within the coal seam roof can propagate from the roof to the target coal seam, effectively fracturing the coal seam. Due to the coal seam’s plasticity being greater than that of the roof mudstone, the coal seam forms a broader fracture than the roof. With the increase in pseudo roof mudstone thickness and being under constant fracturing displacement, the energy consumed by the pseudo roof mudstone during fracturing causes a decrease in pore pressure when fracture extends to the coal seam, resulting in a reduction in fracture width. Therefore, the pseudo roof mudstone is an adverse factor for the expansion of hydraulic fracturing. Physical similarity simulation results demonstrated that when horizontal boreholes were arranged within the siltstone of the coal seam roof, were under reasonable vertical distance and high flow rate fracturing via fluid injection conditions, and if the coal seam had a thin pseudo roof mudstone, the fracture could propagate through the direct roof-pseudo roof interface and the pseudo roof-coal seam interface, extending to the lower coal seam. The fracture form was curved and had irregular vertical fractures, indicating that hydraulic fracturing can achieve production enhancement and the transformation of soft and hard coal seams. However, when the coal seam had a thick pseudo roof mudstone, the mudstone posed strong resistance to hydraulic fracturing, making it difficult for the fracture to propagate to the lower coal seam. Therefore, the pseudo roof mudstone plays a detrimental role in hydraulic fracturing and the production enhancement of coal seams. The engineering verification conducted at Panxie coal mine and Luling coal mine showed that by utilizing a construction drainage rate of 7.5 cubic meters per minute at Panxie coal mine, the maximum fracture length reached 218.3 m, with a maximum fracture height of 36.8 m. The maximum daily gas production of a single well reached 1450 cubic meters per day, with a total gas extraction volume of 43.62 × 104 cubic meters across 671 days. At Luling coal mine, utilizing a construction drainage rate of 10 cubic meters per minute, the maximum fracture length reached 169.1 m, with a maximum fracture height of 20.5 m. The maximum daily gas production of a single well reached 10,775 cubic meters per day, with a total gas extraction volume of 590 × 104 cubic meters for 1090 days. This indicated that the fracture within the roof of coal seams can penetrate the composite roof of coal seams and extend to the interior of the coal seams, achieving the purpose of transforming fractured and low-permeability coal seams and providing an effective mode of gas extraction.

Abstract As coalbed methane (CBM) reservoirs have extremely low permeability, hydraulic fracturing is a common stimulation process for enhancing the CBM production. An in-depth understanding about the propagation mechanism of hydraulic fractures in coal is important for designing a hydraulic fracturing process and thus for improving the CBM production. This study performed a set of true tri-axial fracturing experiments on six block samples (300 mm × 300 mm × 300 mm, including raw coal and artificial roof/floor) with consideration of in situ conditions, aiming at simulating the propagation of hydraulic fractures in the CBM reservoir in the Changzhi field, southern Qinshui basin. Four groups of experiments were organized to evaluate the influences from the pre-existing natural fracture, in situ stresses and injection flow rates on the hydraulic fracture propagation. Meanwhile, five series of numerical simulations were constructed to model the relationship between in situ horizontal stresses and hydraulic fracture propagation. The results show that the hydraulic fracture propagates only along the pre-existing natural fracture direction under a small approaching angle, while it propagates along both the directions of the pre-existing natural fracture and the maximum horizontal principal stress (σH) under a large approaching angle. Whether pre-existing natural fractures exist or not can result in a distinct influence on hydraulic fracture propagation. Hydraulic fractures straightly propagate along the σH direction under a high value of horizontal stresses difference coefficient (Kh), while they tend to deviate from the σH direction under a low Kh value. The influence of Kh is greater than that of the horizontal stresses difference (Δσ) in determining fracture propagation to extend along the σH direction in the coal seam. Large approaching angle, high in situ stresses and a high injection flow rate are three major factors to cause the roof/floor broken by hydraulic fluids.

… of coal resources in … roof fracture with longwall coal mining. A novel model generation method in the universal distinct element code was proposed for the investigation of a propagating …

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.