| Multi-stage hydraulic fracturing (HF) is an essential technology for completion of horizontal wells in unconventional hydrocarbon reservoirs. In engineering design for multi-stage HF treatments of horizontal well stimulation, it is ideal to promote simultaneous growth of all fractures in each stage in order to reduce the number of non-producing perforation clusters. While increased attention has been given to studies of multiple HF growth, subcritical crack growth is not typically considered as a factor affecting the HF process. However, laboratory experiments have shown that subcritical crack growth plays a pivotal role in the initiation of hydraulic fracture(s).;Inspired by laboratory observations, this research is aimed at the study of the behavior of single/multiple hydraulic fracture(s) under subcritical growth conditions. This work consists of three main parts. First, a numerical model accounting for the subcritical crack growth is developed for simulating the initiation and propagation of single hydraulic fracture. It is seen that subcritical crack growth leads to significant changes in both the evolution of crack length and the wellbore peak pressure. Then, this model is extended to the case of simultaneous growth of multiple hydraulic fractures. A parametric study is carried out to investigate the competition between the effect of stress shadowing and subcritical crack growth. Finally, laboratory HF experiments are performed to explore the occurrence of time-dependent HF initiation in various rocks.;By showing the existence of time-dependent HF initiation and explaining that its underlying mechanism is due to the stable growth of the hydraulic fracture under subcritical conditions, this research leads to new insights for promoting more evenly growth of multiple hydraulic fractures in multi-stage HF treatments. Most importantly, this work shows that reducing the subcritical index shortens the time delay associated with hydraulic fracture initiation at wellbore pressures that are insufficient to induce instantaneous initiation. The experiments show that choice of fluid can impact the effective subcritical index, thereby leading to the practically-relevant outcome that fluid(s) can be chosen in order to promote initiation and growth of multiple hydraulic fractures and/or single hydraulic fractures under conditions where the required wellbore pressure for instantaneous initiation cannot be reached. |
