Experimental study on the propagation rules of hydraulic fracturing cracks in deep rock-coal-rock composites under true triaxial conditions

Deep low-permeability coal reservoirs are characterized by high in situ stress, low permeability, and strong heterogeneity, and their production enhancement depends on the complexity of the fracture network created by hydraulic fracturing. Owing to the complex lithology of the strata and significant differences in coal-rock strength, the propagation of fractures across layers and their morphological evolution are controlled by geological factors (such as in situ stress) and operational factors (such as fracturing fluid volume). Revealing the cross-layer mechanisms of fractures and the patterns of fracture propagation in composite strata has become a core objective for the efficient development of coalbed methane in deep coal reservoirs.

This study considers the low-permeability coal reservoir in the deep inclined bottom layer of Gemudi, Liupanshui, as an example. Targeting the “three high” geomechanical environment of deep coal reservoirs, a “rock-coal-rock” composite similarity physical model simulating the real coal seam roof and floor was constructed. By conducting true triaxial physical fracturing experiments, the effects of different horizontal stress differences and fracturing fluid volumes on fracture propagation were systematically analyzed. Additionally, computed tomography (CT) was used to three-dimensionally (3D) reconstruct the spatial distribution and morphology of fractures. Furthermore, a 3D fluid-solid coupling numerical model was established in Abaqus by embedding cohesive elements that quantitatively simulated the dynamic evolution of fracture length, width, and height, achieving effective validation of the physical experimental results.

Physical experiments show that fractures are regular and singular under high differential horizontal stress but tend to branch more easily under low differential horizontal stress; CT 3D reconstruction models demonstrate that the main fractures maintain good continuity in space, and branch fractures are only connected to the main fractures in local areas, without forming a complex fracture network. Under high-pressure fracturing fluid discharge, pump pressure fluctuates, and fracture closure pressure is delayed; CT 3D reconstruction shows a denser fracture network, with significantly improved connectivity between the main and branch fractures, forming a multi-level network structure of main fractures–branch fractures–microfractures. Under low-pressure fracturing fluid discharge, fractures mainly extend as single primary fractures; pump pressure is stable, the morphology is clear, and the extension range is large, allowing for higher energy accumulation and significantly increasing fracture pressure. Numerical simulations show that as the minimum horizontal stress differential increases, fracture length increases, whereas fracture width and height decrease. When the minimum horizontal stress differential increases to 1 MPa, it can effectively control fracture height to prevent fracture cross-layering. The larger the fracturing fluid discharge, the greater the fracture length and width; however, when the discharge exceeds 0.8 m³·min^-1, the increase is only 24.3%.

The fracture patterns of hydraulic fracturing in deep coal reservoirs are dominated by in situ stress. By controlling the horizontal stress difference, directional height drainage and interlayer penetration can be effectively achieved. Although increasing the fracturing fluid volume is an effective means to construct complex, multistage fracture networks, it is necessary to balance pumping costs with fracture gains. Considering both fracture propagation efficiency and engineering economics, it is recommended to use a medium volume of 0.8 m³·min^-1. These research findings can guide hydraulic fracturing operations in deep coal reservoirs.