Hydraulic Fracture Propagation in the Presence of Stress Variation

Abstract

ABSTRACT This paper presents results of a study regarding the effects of horizontal stress gradient as a barrier limiting vertical growth of an induced hydraulic fracture. Dry blocks (6 × 12 × 18 in.) of various rock materials were subjected to stresses by a polyaxial cell capable of delivering up to 3000 psi loads. Actual stress distribution inside the block and along fracturing planes was determined by a 3-D finite element model using elastic elements. Effects of stress magnitudes and stress profiles were investigated and fracturing pressure and fracture width were monitored at the center of the borehole during experiments. After splitting the block along the fractured plane, the final geometry was observed. The study has shown that stress variation between different levels (zones) of the block can greatly modify the induced fracture shape. However, the magnitudes of stress difference between zones to contain a fracture were found to be rock type dependent. Tests on blocks of softer material (such as hydrostone) needed lower stress contrast to contain a fracture than if a stiffer rock (such as Carthage marble) was used. In tests where fractures were initiated at higher stress zones adjacent to lower stress zones, fractures extended into the lower zones. The ratio between fracture areas in both zones was dependent on the difference in stresses between the two zones. At a constant stress contrast between zones, the growth of fracture height into the higher stress zone was found to be a nonlinear function of the injection rates. Experiments have also indicated that the stress contrast needed for containment depends on the height of the middle zone (simulated pay zone height). The ISIP measured during the tests were 11% higher than those predicted by the 3-D finite element model. Calculation of the fracture toughness values (K1c) of the rock from the experimental data were significantly higher than those obtained by standard fracture toughness testing.