Material Model Test Study on Multi-Layer Superimposed Coalbed Methane Production Layer Fracturing

The lithology of multilayer superposed coal-measure reservoirs is highly interbedded, and the mechanical contrast between adjacent layers is significant, resulting in strong uncertainty in the initiation and propagation behavior of hydraulic fractures. To address the problem that the fracture-propagation mechanism under multi-lithology assemblages remains insufficiently understood, typical layered composite specimens were constructed, and large-scale true triaxial hydraulic fracturing physical simulation tests were performed to systematically investigate the effects of coal seam thickness, interlayer thickness, injection rate, and fracturing-fluid viscosity on fracturing pressure, fracture propagation path, and propagation capacity. The results show that when the coal seam thickness does not exceed 90 mm, cross-layer connectivity at the fracture breakthrough interface is more likely to occur. Interlayer thickness directly controls fracture-height growth. When the mudstone interlayer thickness is 40 mm, the fracture still retains the ability to propagate across layers, whereas this ability decreases significantly as the interlayer becomes thicker. When the injection rate is increased from 20 mL min−1 to 30 mL min−1, the overall pump-pressure platform rises, accompanied by a simultaneous increase in fracture extension scale and connectivity. As the fracturing-fluid viscosity increases from 3 mPa·s to 24 mPa·s, both the fracturing pressure and platform pressure increase significantly, and the fracture morphology gradually changes from dispersed propagation to more concentrated extension. The results further indicate that structural constraint factors (coal seam thickness and interlayer thickness) and dynamic driving factors (injection rate and fracturing-fluid viscosity) jointly control the spatial structure and pressure-response characteristics of fractures. Among these factors, interlayer thickness determines the conditions for cross-layer fracture propagation, injection rate and fluid viscosity control the ability to maintain net pressure within the fracture, and coal seam thickness constitutes an important geometric constraint. These findings provide an experimental basis for fracturing-parameter optimization and cross-layer stimulation design in multilayer superposed reservoirs.