Effect of Pressure Increasing Rate on Rock-Mechanical Response Modes During Energized Fracturing of Water-Injectors

Energized fracturing in water injectors has become a new approach for the efficient development of low-permeability reservoirs. To investigate rock damage and the mechanical-response and rupture mechanisms, core-scale physical simulation experiments were conducted, changes in the near-wellbore stress field were analyzed, criteria for calculating fracture-initiation and rupture pressures were established, and the principal controls on rock-mechanical response modes at low pressure increasing rates were identified. The results indicate that, compared with the traditional fracturing process, energized fracturing can significantly increase the near-wellbore pore pressure, causing a drastic change in the third stress field derived from pore pressure, which in turn changes the rock-mechanical response mode from the tensile rupture mode in traditional fracturing to the evolution of progressive damage: By controlling the injection rate and viscosity, as the bottomhole pressure increasing rate increases, the rock sequentially exhibits three mechanical response modes: pore elastic-plastic permeability enhancement, channel plastic permeability enhancement, and fracture-type rupture flow diversion. At lower pressure increasing rates, the rock near wellbore is subjected to the combined effects of mesoscale expansion and rupture induced by pore-fluid pressure and macroscale failure governed by effective stress; both initiation and rupture pressures decrease as the bottomhole-pressure increasing rate decreases. The location of the initiation point deviates from the wellbore, and its distance from the wellbore increases as the pressure increasing rate decreases. The fracture-mechanics-based classification criterion successfully captures the relationships among initiation pressure, rupture pressure, initiation location, and pressure-increasing rate.These findings can support process design for large-scale energized-fracturing treatments.