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When a well is hydraulically fractured, the propagation of the fracture away from the wellbore is dictated by the far field stresses in the reservoir. However, the fracture initiation from the wellbore depends strongly on the near wellbore stress state created by drilling the well. Misaligned fracture initiation and propagation planes can reduce the wellbore-to-reservoir connectivity causing operation failure and high post fracturing skin.Currently creating multiple fractures along a horizontal openhole requires mechanical isolation means such as openhole packers or sand plugs. They can be costly and time consuming. In addition, there is no control of fracture initiation within one isolated section. Undesirable competing fractures within the zone can occur to impact the fracture length. Significant improvement can be made if the factors controlling multiple fracture initiation without mechanical isolation can be understood.Experimental work in multiple fracture initiation has been rare, controlled multiple fracture initiation is non-existent. Therefore a series of laboratory experiments was performed in a true tri-axial stress frame to investigate how multiple fractures can be initiated in a controllable fashion. In the tests, notches at specific locations along the openhole wellbore were created. The impact of the notch depth on the orientation of the hydraulically induced fractures was studied.In addition to the experiments, continuum fracture mechanics modeling using finite element was also conducted to rationalize the experimental observations of fracturing initiation process in the rock.The results of block tests provided new insight in multiple fracture initiation. By monitoring the real time acoustic emission events, the sequence of fracture creation as wellbore pressure increased was visualized. The finite element modeling gives simple criteria to explain the observed orientation of initiated fracture as a function of notch depth. IntroductionThe productivity of horizontal wells may be enhanced by inducing multiple hydraulic fractures along the wellbore. The propagation of a fracture away from the wellbore eventually is determined by the far field stresses in the reservoir. Assuming isotropic homogeneous formation properties, a longitudinal fracture will propagate along the axis of the horizontal wellbore when the well is drilled into the maximum principal horizontal stress (σ H ). A transverse fracture will propagate if the well is drilled into the minimum principal horizontal stress (σ h ). However, the fracture initiation from the wellbore is more complex (Daneshy 2009). It depends not only on the reservoir stresses orientation, but also is influenced by the near wellbore local stress state due to the drilling of the well. The mechanical properties of the rock, the surface condition of the wellbore wall, the rate of pressurization, and the physical properties of the fracturing fluid also play important roles in fracture initiation direction. Fracture initiation, though does not necessarily alter the fa...

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In stimulating tight carbonate formations, the propagation of multiple transverse fractures is highly desirable to contact as much matrix as possible. The application of this method to openhole well environments is challenged by the dominating impact of hoop stresses in the near-wellbore vicinity rather than far-field stress in the producing layer. As a result, even if the open hole is drilled in the direction of minimal horizontal far-field stress, there is a high probability that hydraulic fractures initiate longitudinally and then turn to the preferred fracture plane, creating undesired tortuosity.One of the approaches towards controlling both the position and direction of fracture initiation is to cut notches in the wellbore wall at specified positions. As pressure increases during fracturing, those notches can locally eliminate the influence of the wellbore hoop stress and develop high tensile stress concentrations initiating transverse hydraulic fractures at lower pressures.A theoretical model is proposed herein that aims to predict the position, orientation, and pressure at which a fracture initiates. In the model, the 3D stress state around wellbore and notch(es) is analyzed using the brittle fracture criteria. In the numerical implementation, the stresses are efficiently resolved using the boundary element method. The model is used to interpret published laboratory data on fracture initiation including those from hydraulic fracturing block tests. It is shown that the conventional maximum tensile stress (MTS) criterion fails to reproduce the observed trends in initiation pressure and fracture orientation. The nonlocal modification of the MTS criterion based on the stress averaging technique (SAMTS), reveals a good match with initiation pressures in simplified tests. When applied to hydraulic fracturing block test data, SAMTS captures the observed fracture orientations while overestimating the absolute pressure values. The discussion of possible reasons for that overestimate and the way forward concludes the paper.