Mechanism of fracture initiation and propagation using a tri-axial hydraulic fracturing test system in naturally fractured reservoirs

A series of laboratory hydraulic fracture experiments was conducted on synthetic rock samples to investigate the mechanism of hydraulic fracture initiation and propagation of a pressurised wellbore in non-fractured and fractured reservoirs. To achieve this goal, the influence of pre-existing fracture (both on and far from the wellbore wall) and horizontal differential stress was investigated on the fracture initiation and propagation in different stress regimes. In reservoirs with no fracture due to the high stress concentration in surrounding rock of the well, the fracture initiation and propagation pressure are increased. In fractured reservoirs, the presence of pre-existing fracture on the wellbore wall reduced the effect of the original stress concentration around the wellbore, which led to a drastic decrease in the fracture initiation and propagation pressure. In the far-wellbore region by increasing dip and strike of the pre-fracture, the fracture propagation pressure was decreased when it intersected the pre-fracture and arrest behaviour of the fracture was also decreased. In-situ horizontal differential stress, ( [GRAPHICS] ) played an important role on the fracture propagation behaviour and the fracture initiation and propagation pressure both in non-fractured and fractured reservoirs. Unlike low differential stress, at high differential stress due to decreasing the interaction between hydraulic and pre-existing fractures, the dominant behaviour of fracture propagation was changed from arrest to crossing mode. In addition, at high differential stress because of decreasing the stress state concentration around the wellbore, initiation and propagation pressure of the hydraulic fracture were decreased.