Recent Advances in Nonlinear Fracturing Characteristics of the Hydraulic Fracture in the Deep Reservoir

Abstract

During hydraulic fracturing in the deep reservoir, the rock surrounding the hydraulic fracture (HF) presents three remarkable nonlinear mechanical behaviors: the fracture process zone (FPZ; microcrack zone) developing at the HF tip, the microcrack band surrounding the HF surface, and the plastic zone growing due to the opening compression of the HF. The above three mechanical behaviors constitute the HF nonlinear fracturing characteristics. However, understanding the HF nonlinear fracturing characteristics is challenging and crucial. In this work, we present our recent published advances in HF nonlinear fracturing. (1) We developed a characterization method for rock FPZ by utilizing and integrating fiber Bragg grating (FBG), digital imaging correlation, and acoustic emission (AE). The integrated measurements showed that FPZ softening followed a linear response. The relationship between FPZ length and crack tip opening displacement was linear, and the relationship between dissipated energy and FPZ length was quadratic. (2) Detailed analysis of the AE energy during cyclic fracturing tests of sandstone revealed that the FPZ propagation rate of subcritical fractures was related to the increasing rate of dissipated energy accumulated from all activated microcracks. Thus, a general law (Zhang’s law) of dl/dN = (1/2CG)l −1(dG D/dN) was proposed and validated. (3) The HF microcrack band of high-temperature (120 °C) granite was identified using AE waveform analyses. The fracture energy was reduced by approximately 75% adjacent to the wellbore (approximately 40% of the fracture length) in the microcrack band. The effective width of the microcrack band was reduced by 40%–56.4% at 120 °C, indicating that high temperatures decreased the effective stimulated volume of HF. (4) We proposed a thermoplastic constitutive model for high-temperature rocks (such as shale) in deep reservoirs. The constitutive relation depends on hydrostatic pressure, stress deviator, and temperature. Parameters characterizing effects of temperature on thermoplasticity were also proposed. The above research advances can provide bases for HF in the deep reservoir.