An experimental investigation into the influence of pre-existing natural fracture on the behavior and length of propagating hydraulic fracture

Abstract

Propagation of hydraulic fracture (HF) in the presence of natural fracture (NF) fundamentally differs with fracture propagation in reservoirs without natural fracture. Natural fracture can significantly affect the behavior and length of propagating hydraulic fracture. In this study, a series of true triaxial hydraulic fracturing tests on large size synthetic rock specimens with a single pre-existing natural fracture was conducted in the laboratory scale in order to investigate the propagation behavior and length of hydraulic fracture in naturally fractured reservoirs. The parameters of laboratory experiments were scaled based on the scaling analysis to simulate hydraulic fracturing process at the field conditions. For the purpose of controlling the hydraulic fracture propagation behavior, different values for approaching angle, θ (30°–90°) and horizontal differential stress, Δσ (1.5–10 MPa) were considered. Four modes of interaction between hydraulic fracture and pre-existing natural fracture were found including (1) net-crossing, (2) net-opening, (3) crossing-opening and (4) opening-crossing. Modes of crossing-opening and opening-crossing were introduced as two new behaviors of hydraulic fracture propagation. These interaction modes play a significant role on the length of growing hydraulic fracture. It was observed that by increasing the approaching angle and differential stress, the hydraulic fracture propagation behavior change from opening (net-opening and opening-crossing) to crossing (crossing-opening and net-crossing) and, consequently, the length of hydraulic fracture increases from a minimum of 9.10 cm to a maximum of 14.30 cm. The fracture initiation (break-down) pressure decreases with the increase of differential stress from 1.5 MPa to 10 MPa. Besides, increasing the differential stress and approaching angle results in a decrease in the pressure fluctuations of hydraulic fracture propagation.