Nonlinear fracturing characterization of hydraulic fracture:Utilizing full-waveform and multi-parameter analysis method of acoustic emission

Hydraulic fracturing is an effective approach in geo-energy extraction,such as enhancing the reservoir per-LMmeability of the flow mineral resource and controlling the surrounding rock in the mine.Due to the development of the fracture process zone(FPZ; microcrack zone) at the fracture tip and the hydraulic microcrack band(HMB) at both sides of the fracture surface,hydraulic fracture(HF) extension presents remarkable nonlinear fracturing characteristics.Therefore,characterizing the HF nonlinear fracturing is fundamental to deepening HF nonlinear fracturing theory and controlling HF propagation.Due to the physical mechanism that cracks generation releases elastic waves(i.e.,the acoustic emission(AE)),AE can be employed to characterize the nonlinear fracturing of hydraulic fracture.In this work,the AE full-waveform and multi-parameter analysis method was proposed,optimizing nonlinear fracturing characterization with a newly developed analysis program.The newly developed program involves high-quality waveform pick-up,waveform synchronization and vibration time calculation,and AE source positioning algorithm.Compared with the widely used commercial AE positioning program,the distribution of AE events obtained by the newly developed program is more consistent with the real fracture surface.Besides,the distribution dispersion of AE events is reduced.Then the spatial distribution of AE characteristic parameters can characterize the evolution of dissipated energy,damage,multi-size fracturing and tension-shear-collapsed fracture mechanism in FPZ and HMB,enriching the characterization parameters of the nonlinear fracture characteristics.To validate the AE full-waveform and multi-parameter analysis method,the true-triaxial fracturing experiment was conducted on the Sichuan white sandstone.AE was employed to precisely characterize the HF nonlinear fracturing.Several new findings were obtained.① The FPZ(31 mm long and 12 mm wide) and HMB are identified via the spatial distribution of AE energy.The strip-shaped FPZ on the HF cross-section presents non-isotropic development.FPZ is the HF dominant propagation path,and the real fracture surface is formed inside the FPZ and almost propagates unsteadily.The dissipated energy is symmetrically distributed along the width of FPZ and HMB,with a distribution pattern of high in the middle and low on both sides.The cumulative dissipated energy is linearly decreasing along the fracture propagation direction,with a linear correlation coefficient of 0.9.② The low AE wave velocity represents the high damage degree surrounding the seismic source.Along both the width and length of the FPZ and HMB,the AE wave velocity is low surrounding the fracture initiation point and high outside.This phenomenon indicates that damage degree in FPZ and HMB decreases from the fracture initiation point to the outside.③ The AE frequencies in FPZ and HMB presents high and low random distribution characteristics.These results indicate that both the FPZ development and the HMB extension obey coalescence and nucleation of microcracks at the same time.④ AE fracture mechanism reveals that micro-fractures are mainly tensile(51.7%-65.3%) during the initiation and propagation of plane hydraulic fractures.With the HF propagation,the proportion of tensile micro-fractures decreases from 65.3% to 51.7%.In contrast,the proportion of shear micro-fracture increases from 19.3% to 23.4%.We can infer that the compressive-shear effect will be enhanced surrounding the HF.Then the permeability surrounding the HF will be improved due to the increasing proportion of shear micro-fractures.