Mesoscopic interpretation of hydraulic fracture initiation and breakdown pressure using discrete element method

Abstract

The relationship between initiation pressure and breakdown pressure, two vital parameters in hydraulic fracturing operations, requires further investigation, with the explicit mechanism underlying breakdown pressure yet to be revealed. This study presents an enhanced particle discrete element model with a regular-shaped borehole, wherein the pressure-updating equation of the pipe network flow model is improved to incorporate comprehensive fluid–solid coupling. Through simulation of the mesoscopic process of hydraulic fracturing initiation and propagation, the characteristic curve of borehole fluid pressure is obtained. By comparing the curves of borehole pressure and fluid flow into the fracture, the physical meaning of breakdown pressure is explained at the mesoscopic level. The results show that the breakdown pressure fundamentally arises from the unstable propagation of the fracture, with its numerical value corresponding to the borehole fluid pressure when the fluid flow into the fracture reaches the injection rate for the first time. Additionally, employing fluid–solid coupling modeling, the impacts of fluid viscosity, injection rate, and borehole diameter on breakdown pressure are investigated. Finally, the evolution of fracture morphology under different injection rates and elastic moduli is analyzed.