Experimental and numerical investigation of time-delayed pressurization for the manipulation of breakdown pressure and acoustic emissions during hydraulic stimulation

For the mitigation of micro-seismicity during hydraulic stimulation, we introduce the concept of time-delayed pressurization (TDP) which involves arresting the constant-rate process of borehole pressurization for a given delay period, and then resuming it until breakdown occurs. The additional fluid permeation into a porous media by the TDP scheme allows for the manipulation of pore fluid pressure buildup near a borehole, thereby resulting in the mechanical and seismic behaviors at failure in a controlled manner. Laboratory fracturing experiments with acoustic emission monitoring were conducted for several delay periods using mortar specimens to evaluate the breakdown pressure and acoustic emission (AE) in the TDP fracturing process. In addition, the morphological characteristics of induced fractures were visually investigated by 3D X-ray CT analysis on the fractured specimens. Numerical simulations using a phase-field approach were also performed to further examine the evolution of pore pressure during the fracturing process. The results show the considerable volume of permeated fluid and the elevated pore pressure during the delay period are the main factors responsible for the decreased breakdown pressure and AE.