基于流固耦合离散元模型的水力压裂过程微观机理研究

Micro mechanism investigation of hydraulic fracturing process based a fluid-solid coupling discrete element model

Abstract: Hydraulic fracturing is a critical technology employed in the exploitation of shale gas and the enhancement of geothermal resources. It is important but challenge to investigate the hydraulic fracturing process at the micro scale. Based on the discrete element method, a fluid–solid coupling approach is presented to study the hydraulic fracturing process in a granite block featuring a single fracture. The results indicated that a negative relationship between the initial fracturing pressure and the prefabricated fracture angle. The characteristics and relationship of displacement, stress, pore pressure, micro-crack and energy variation throughout the fracturing process are investigated, with which the hydraulic fracturing process can be divided into three phases: micro-crack development, seepage and macro fracturing. During the numerical simulations, a series of micro-cracks are generated and expanded, and a low pore pressure zone is formed around the tips of the propagating fracture. Such low pore pressure zone promotes the fluid seepage and change the fluid state around the fracture. Based on the received vibration signal, the source and strength of the signal are analyzed. This study presents a novel fluid–solid coupling model for an accurate description of hydraulic fracturing process at micro pore scale.

Keywords: Hydraulic fracturing; Numerical simulation; Discrete element method; Fluid–solid coupling; MatDEM

Fig.6 (a) Schematic diagram of a hydraulic fracturing experiment, where the angle between the prefabricated fracture and the horizontal direction is

θ; (b) Hydraulic fracturing model with a prefabricated fracture, colors represent element radii;

Fig.7 Stress–strain curves and failure modes of uniaxial compression tests. (a) Experiment; (b) Simulation;

Fig. 8 (a) Fracture propagation with different prefabricated fracture angles; (b)Schematic diagram of penetrating fracture and prefabricated fracture,the black lines represent the prefabricated fracture, and the red dashed lines represent the hydraulic fracture generated by fracturing; (c) Displacement fields of simulation models with different prefabricated fracture angles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 10 States of the model at different times: (a) Displacement; (b) Stress in the horizontal direction (positive for tensile stress and negative for compressive stress); (c) Pore pressure; (d) Micro-crack distribution.

Fig. 12 (a) DIC tensile strain diagram under 30◦ initial fissure; (b) DIC shear strain diagram under 30◦ initial fissure; (c) Development of tensile micro-cracks during the simulation (the short black line represents the breakage of the spring between the elements at this location). (d) Development of shear micro-cracks during the simulation.

Fig. 14 The simulation results in the seepage phase: (a) The displacement of elements; (b) The connections between elements. The green line segments represent complete connections between elements; (c) Stress state in the horizontal direction. Positive numbers represent tensile stresses and negative numbers represent compressive stresses; (d) The pore pressure distribution. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Zhu, Y., Liu, C., Zhang, H., Zhao, C., Wang, B., Mao, M., & Geng, H. (2024). Micro mechanism investigation of hydraulic fracturing process based a fluid-solid coupling discrete element model. Computers and Geotechnics, 174, 106640. https://doi.org/10.1016/j.compgeo.2024.106640