Experimental Study of the Simultaneous Initiation of Multiple Hydraulic Fractures Driven by Static Fatigue and Pressure Shock

Abstract

One of the most important challenges in horizontal well fracturing is to create multiple tightly-spaced hydraulic fractures (HFs) within each fracturing stage in tight reservoirs, especially with the strong heterogeneity. This study explores the influences of static fatigue and pressure shock on the simultaneous initiation of multi-HF from an open-hole horizontal wellbore in a formation with the variability in the rock strength. Laboratory-scale fracturing experiments were conducted on cuboid-shaped specimens (8 cm × 8 cm × 10 cm) of varying brittleness under the tri-axial confining stress. The rock strength variability in a specimen was illustrated as the variation in the width or length of the preset natural fractures (NFs) within the open-hole section (OHS). Inside the wellbore, three pressurization regimes, corresponding to the normal, static fatigue and pressure shock conditions, were obtained by the constant-rate injection, constant-pressure injection, and high-pressure build-up and rapid release, respectively. The results demonstrate that under the pressure shock conditions, bottom hole pressure (BHP) can be forced to exceed the normal breakdown pressure in a conventional fracturing, and generate a relatively high breakdown pressure. It has the potential to improve the simultaneous initiation of multiple transverse HFs from NFs of different strengths and that of longitudinal HF within a single fracturing stage. A lower limit BHP is required to overcome the variations in strength across different potential initiation points. Meanwhile, the low-BHP, which is lower than the normal breakdown pressure, is maintained to induce the static fatigue failure of the surrounding rock. Under static fatigue conditions, a single HF is generally initiated from the weak NF. The time to breakdown decreases evidently with the increase in the BHP and rock brittleness. In low-brittle specimens, the time-dependent initiation of HF may be difficult in the OHS without connecting with the NFs. In practice, simultaneous initiation of multi-HF may be enhanced by the combined effects of static fatigue and pressure shock.