Numerical Simulation of Hydraulic Fracture Propagation in Coalbed with Cleats

ABSTRACT Cleats play a crucial role in the propagation of fractures during coalbed hydraulic fracturing. However, current numerical methods still suffer from significant drawbacks in terms of computational efficiency and fracture trajectory prediction. In this study, we used the displacement discontinuity method (DDM) to calculate the stress in the coal body and corrected fluid leakage using the Brinkman equation. We also derived a mechanical criterion for cleat failure under different conditions and proposed a new model for fracture propagation in coalbeds with orthogonal cleats. Our numerical results indicate that simple plane fractures are more likely to form when the horizontal stress difference in the coalbed is greater. When the angle between the cleats and the horizontal principal stress is close to 45°, hydraulic fractures are more likely to activate the cleats. The friction coefficient of the cleats only affects the net pressure within a specific range in the critical crack. As the cohesion of the cleats decreases, the minimum net pressure required for shear failure decreases significantly, making shear failure more likely. We also introduced a reconstruction coefficient, η, to evaluate the effectiveness of coalbed fracturing stimulation. When the cleats are activated, the reconstruction coefficient η increases, indicating that the flow width per unit fracture length improves and is more conducive to production. INTRODUCTION Against the backdrop of carbon emission reduction, coalbed methane (CBM) is becoming an increasingly significant clean energy source for future use, playing a crucial role in energy supply and economic development (Wang et al., 2021; Zhang et al., 2018; Li et al., 2020; Fan 2021). Hydraulic fracturing is a well-established technology for CBM production and has proven to be highly effective over decades of application (Ai et al., 2021; Li et al., 2017). The morphology of fracture propagation in coalbeds is complex, with numerous factors affecting the process. Along with coal's low strength, low elastic modulus, and high Poisson's ratio, the presence of cleats also significantly influences fracture propagation. (Li et al., 2019a; Li et al., 2019b; Su et al., 2005; Xu 2020). Cleats within the coalbed develop in an orthogonal network, and their interaction with hydraulic fractures (HF) can alter the HF propagation direction, adding complexity to the fracture system and posing challenges for predicting the trajectory of HF within coalbeds. Despite the significance of cleats on fracture propagation in coalbeds, there is still a lack of theoretical calculation methods capable of accurately simulating and predicting the morphology of fracture propagation.