Pulsating hydraulic fracturing (PHF) is useful in the development of methane from geologically complex coal seams in China because it can relieve pressure in a great range and increase the permeability of coal seams. Current research studies on PHF mainly focus on coal rock damage via experimental methods; however, to the best of our knowledge, none of these studies consider stress disturbances, which significantly impact the stimulation effect. In this paper, a numerical model is proposed to investigate the features of stress disturbances under loads of PHF in coal seams.
Based on field data, a stress-response numerical model applies a high-order, staggered-grid, finite-difference scheme to investigate the stress disturbance features of coal seams. This model combines the linear slip deformation model and the preloading method to describe the cleats and confining pressure, respectively. Moreover, the implementation of an infinite formation makes use of the perfectly matched layer technique. The model has been verified for the static and dynamic theoretical calculation cases, which can be regarded as sufficiently precise for analyzing the stress disturbance.
The numerical simulation results show several unique stress behaviors with PHF. First, the stress disturbance is characterized by an extensively altered scope and regional stress concentration, and the effective stress disturbance zone can be much larger compared with conventional fracturing. Second, the PHF disturbance effects can be improved by increasing the source frequency and amplitude. Third, the strong compressibility (i.e., the high elastic Young's modulus and low Poisson's ratio) of coal rock is beneficial for stress propagation and disturbances in improving the fracturing effect. Fourth, the crack density (0−0.2) has little impact on the PHF stress disturbance effect because of the diffraction of the stress wave.
The proposed model is able to describe the static and dynamic stress response in an infinite fractured formation with confining pressure. The model clarifies the propagation and distribution of stress, and it can help us understand the fracture mechanism and suitable coal seam conditions for PHF from the perspective of the formation stress disturbance. Based on this model, the results can improve the optimization of the technical parameters and the investigation of the stimulation mechanics of PHF in coal seams.