This study investigated the mechanisms influencing hydraulic fracture propagation under the influence of cleat complex geometries. The study established a 3D-DEM (three-dimensional discrete element method) model for complex fracture propagation, utilizing the discrete element method and incorporating complex cleat geometries. The model simulates the propagation patterns of hydraulic fractures within coal seams. The findings indicate the following: (1) The fracture width within coal seam cleats undergoes significant variations. A quantification method for these variations and a novel concept of cleat-induced fracture deflection angle are proposed. As the cleat angle increases from 0° to 45°, the cleat-induced fracture deflection angle also increases, reaching 17.1°, demonstrating that cleats have a directional inducing effect on hydraulic fracture propagation; (2) injection hole pressure decreases during fracture capture by coal cleats, whereas pressure escalation occurs during penetration through these cleats; (3) a smaller angle between the face cleat orientation and the direction of maximum principal stress results in longer fracture lengths and narrower fracture widths; and (4) higher injection rates augment the fracture width, facilitating the entry of proppants.

In this paper, a two dimensional-discrete element method complex fracture extension model was developed to simulate the fracture extension behavior in reservoirs during multi-cluster perforation fracturing of horizontal wells by particulate flow and considering temporary plugging conditions. The effect of particle inhomogeneity on the extension of multi-cluster fractures, the change of fracture extending characteristics before and after temporary plugging, and the effect of geological and construction parameters on the extension of multi-cluster fractures under the consideration of temporary plugging conditions are investigated in this model. The results show that: (1) the middle cluster is more difficult to extend than the outermost cluster due to interstitial stress interference; the increased nonuniformity of particles aggravates interstitial stress interference and favors shear fracture extension. (2) Fractures expand substantially when the fracturing fluid reaches 3/4 of the total fluid volume; after temporary plugging, it can promote the uniform extension of fractures and increase the total length of fractures, but the fractures formed in the middle are easy to be captured by the fractures at the two ends and expand along it, and it is not easy to form complex fractures; the timing of plugging is based on the percentage of perforation clusters with fractures already formed, with fracture extension being preferred. (3) The horizontal principal stress difference mainly affects the direction of fracture extension; the number of clusters mainly affects the uniformity of fracture extension; and the fluid flow mainly affects the length of fracture extension.

With the continuous advancement of coal resource extraction and coalbed methane (CBM) development, revealing the propagation laws of hydraulic fractures in cleated thin-layer coal rocks has become particularly important. This study establishes a three-dimensional complex fracture propagation model considering the cleat system based on the Block Discrete Element Method, systematically investigating the influences of geological, engineering, and perforation factors on hydraulic fracture propagation in coal seams. The results show that: (1) the thickness of coal seams affects the expansion pattern of hydraulic fracture networks by regulating stress distribution, showing a positive correlation with hydraulic fracture length and width, and a negative correlation with fracture height. (2) Cleat spacing significantly affects the fracture network in cleated thin-layer coal rocks; smaller spacing leads to more frequent interactions between fractures and cleat surfaces, resulting in a more complex network. (3) Cleat angle exerts a notable guiding effect on the propagation direction of fractures in thin coal seams; the maximum fracture deflection angle and higher fracture complexity index occur at a cleat angle of 45°. (4) A high injection rate (16–20 m3/min) effectively increases fracture width and stimulated reservoir volume in cleated thin-layer coal rocks, while reducing the risk of fracture penetration into the roof and floor, thus improving CBM recovery efficiency. The findings provide a theoretical basis and technical support for efficient CBM development.

This study investigates nonuniform hydraulic fracture propagation in vertically stacked thin coal seams with small interlayer spacing. A discrete element-based numerical model incorporating dynamic interlayer flow allocation was developed to simulate competitive fracture behavior. Key findings include: (1) As interlayer stress difference increases (from 1.5 to 4 MPa), fractures shift from interlayer penetration to coal seam propagation, with main seam fracture length growth dropping sharply (from 18.37–20.73% to 2.59–6.07%) and nonuniform propagation coefficient rising (from 0.06 to 0.33). When the leakoff coefficient increases (from 0.0005 to 0.0030 m/min^0.5), the thinnest seam’s fracture length reduces by 36.70% while its flow proportion increases (from 12 to 33%). (2) Flow allocation trends: higher stress difference favors thick seams; elevated leakoff benefits thin seams; increased thickness proportion advantages thick seams (11#flow: from 38 to 60%). (3) Due to significant physical property differences between coal seams and roof/floor rocks (coal elastic modulus: 5–12 GPa vs roof/floor elastic modulus: 25 GPa), fractures primarily extend along coal seams with strong roof/floor barrier effects (coal flow >90%). (4) Increased coal seam thickness proportion enhances the competitive advantage of thick seams, with the nonuniform propagation coefficient remaining at 0.28 and thin seam propagation being suppressed. This work provides a theoretical basis for optimizing multithin-layer coal seam fracturing. This work provides a theoretical basis for optimizing multithin-layer coal seam fracturing.

Shale gas, as an unconventional resource, requires hydraulic fracturing to create complex fracture networks due to its low porosity and permeability. However, the presence of interlayers significantly affects fracture propagation, leading to highly complex fracture morphologies. This study focuses on the interbedded shale of the WJP Formation in southern China. A three-dimensional block discrete element method (BDEM) was employed to establish a hydraulic fracture propagation model, systematically investigating the effects of geological parameters (stress difference, interlayer thickness), engineering parameters pumping rate, fluid volume, viscosity), and perforation parameters (cluster number, cluster spacing, perforation location) on fracture network morphology. The results indicate that: (1) Among geological parameters, interlayer thickness is the key factor inhibiting vertical fracture propagation. Due to the influence of interlayers, an increase in stress difference promotes fracture length but suppresses fracture height and stimulated reservoir volume (SRV); (2) For engineering parameters, there exists a “threshold effect” for pumping rate and fluid volume, with 16 m³/min and 2000 m³ identified as the critical thresholds for interlayer breakthrough. Low viscosity (1 mPa·s) is conducive to forming complex fracture networks, while high viscosity extends fracture length but reduces SRV; (3) Regarding perforation parameters, the optimal stimulation effect is achieved with 6–7 clusters, a cluster spacing of 10 m, and perforation locations in the center of the main shale layer (19.85–21.6 m); (4) By introducing grey relational analysis, the degree of correlation between various influencing factors and the response to interlayer breakthrough is systematically evaluated based on the breakthrough conditions under different factors. Thin interlayers or low stress differences can reduce the critical pumping rate, whereas thick interlayers (≥3 m) become the primary constraint, making breakthrough difficult even at high pumping rates. Reliable interlayer breakthrough requires the simultaneous satisfaction of Δσ ≤ 16 MPa, h < 1 m, and Q ≥ 16 m³/min. The reliability of the model was verified by comparing numerical simulation results with field microseismic data. This study reveals the extension laws of complex fracture networks in interbedded shale, providing a theoretical basis for fracturing design and development optimization.