Factors Affecting Hydraulic Fracture Initiation in High In-Situ Stress Conditions: A Wellbore Stress Modelling Approach
Authors: Akgun F. , Yang Z. , Crosby D.G. , Rahman S.S. 
Published: December 2007

Premature screen-outs, and low in-place proppant concentrations occur frequently during hydraulic fracture treatments carried out in Central Australia. Two and three dimensional numerical modelling studies have been carried out, investigating factors affecting hydraulic fracture initiation, and near-wellbore fracture tortuosity, in highly stressed conditions. The 2D modelling suggests that drilling induced shear fractures, if oriented close to the maximum horizontal in-situ stress direction and inflated during treatment, may promote the initiation of multiple hydraulic fractures. Near-wellbore tortuosity and screen-outs are more likely in such situations. The results of this study also suggest that the elongated borehole geometry due to breakouts does not significantly alter the impact of preexisting fractures (either natural or induced) on hydraulic fracture initiation. 3D stress modelling indicated that fracture initiation may occur from perforations oriented even at large angles with respect to the maximum in-situ stress direction. Both numerical modelling and analytical analysis suggest that starter fractures initiate at the base, rather than the tip of perforations and that the initiation of horizontal starter fractures from perforations is independent of fluid pressure. For properly oriented perforations, horizontal starter fractures are unlikely to initiate because the strong reverse faulting regime required is rare at most reservoir depths. In strike-slip stress regimes, such as that experienced in Central Australia, however, the initiation of horizontal starter fractures is possible if perforations are misaligned with the minimum horizontal in-situ stress. This significantly increases the likelihood of near-wellbore tortuosity and the possibility of near-wellbore screen-outs. These studies highlight the benefits of aligning perforations in the maximum horizontal stress direction in eliminating reduced near-wellbore tortuosity.

Introduction

Hydraulic fracture treatments of tight formations in Central Australia often experience abnormally high treating pressures and fail to achieve adequate fracture conductivities due to low proppant concentration. The length of these fractures are commonly shorter than anticipated as a result of premature screen-out. A recent study has found that the hydrocarbon bearing formations in Central Australia have relatively high horizontal in-situ stresses and that the stress regime in the region is probably strike-slip faulting. In addition, FMS log images of selected wellbores in the region show widespread and consistent borehole breakouts in sandstones or coals, and tensile fractures in shales. Intuitively, these high horizontal stresses and associated induced shear fractures which cause borehole breakouts (see Fig. 1) may, in a manner similar to that of natural fractures, cause near-wellbore hydraulic fracture tortuosity However, the significant difference between these two types of fractures is that induced shear fractures occur in planes parallel with the minimum horizontal stress () (Fig. 1), whereas natural fractures are randomly oriented. It is therefore unclear what role such stress induced shear fractures play in the complication of hydraulic fracture initiation.

The significant majority of high treating pressures and premature screen-outs in Central Australia, as experienced elsewhere, is ultimately near-wellbore fracture tortuosity which is manifested as multiple fractures or fracture reorientation. Regardless of form, the origin of near-wellbore tortuosity can be traced back to hydraulic fracture initiation, which is in turn controlled by the stress distribution around the wellbore during the breakdown stage. Therefore, a study of hydraulic fracture initiation in the presence of pre-existing fractures (either natural or induced shear fractures) under the influence of near-wellbore stresses, may help engineers establish a link between high in-situ stresses and an increased risk of premature screen-out.

There exists much literature regarding experimental studies of hydraulic fracture initiation.

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