Experimental Investigation on Hydraulic Fracture Propagation Behaviors of Coal-Measure Thin Interbedded Rocks

Clarifying the vertical propagation mechanism of hydraulic fractures (HFs) within thin interbedded rocks can provide the fundamental insights into cross-layer behaviors of HFs and valuable guidance for designing hydraulic fracturing pumping parameters in coal measure strata. Laboratory true-triaxial hydraulic fracturing experiments for horizontal wells are conducted using artificial thin interbedded samples consisting of the cement mortar encapsulating natural coal and mudstone interlayers. A three-dimensional (3D) fracture reconstruction method based on the fluorescent agent and laser scanning technique is proposed to quantitatively evaluate the stimulated reservoir area (SRA) and vertical fracture height (VFH) of HFs under different in-situ stresses, injection rates, fracturing fluid viscosities, lithological combinations and interfacial cementation strengths. Results show that the 3D geometry of HFs exhibits non-planar, asymmetric, and non-uniform propagation within thin interbedded rocks. The propagation patterns of HFs in thin interbedded rocks mainly include arresting, deflecting, penetrating and mixed pattern. The higher vertical stress difference coefficient (λ V ), injection rate and fracturing fluid viscosity facilitate the HF penetration through interlayers, thus increasing the SRA and VFH. A smaller horizontal stress difference coefficient (λ H ) induces the HF reorientation and significantly increases the SRA, although it has a minor impact on the vertical cross-layer propagation behaviors of HFs. Compared with the mudstone interlayer, the HF is prone to penetrate from the high-modulus sandstone layer into the low-modulus coal interlayer. The values of critical parameters (λ V , injection rate, and fracturing fluid viscosity) for cross-layer propagation of HFs increase in the presence of weakly cemented lithological interfaces within thin interbedded rocks. When HFs propagate across interlayers, the injection pressure curve typically exhibits noticeable fluctuations and an upward trend during the injection of low-viscosity fracturing fluid. The key findings of this paper offer valuable insights into the vertical propagation mechanism of HFs and provide guidance for designing hydraulic fracturing treatments in coal measure strata. Highlights The SRA and VFH of HFs within thin interbedded rocks are quantitatively evaluated using the 3D laser scanning technique under true-triaxial hydraulic fracturing experiments. The higher λ V , injection rate