Borehole Failure Mechanisms in Naturally Fractured Formations

The possibility that pressure-loaded natural fractures in naturally fractured formations may contribute to wellbore instability merits further investigation. The present study analyzes in detail the state of stress around a wellbore with pressurized natural fractures nearby. Synthetic examples are given of unanticipated well instability when pressure-loaded natural fractures occur in the vicinity of the wellbore at the time of first penetration of the naturally fractured formation. The locations of shear failure are controlled by the stress field distortion due to the fluid pressure in the natural fractures. Various shear failure criteria are used to quantify where and when the critical stress for shear is reached. Rather than shear failure, tension failure may also occur at the wellbore. For example, wellbore stability is jeopardized by unintended circulation loss when the mud load causes unexpected hydraulic fracturing during drilling operations. The method of solution is a recently developed linear superposition method (LSM) of elastic displacements. The LSM models show static solutions of the stress state, and a dynamic version of the method (TLSM) uses time-stepped superposition to model fracture propagation processes. Time-stepped solutions show how fluid pressure-loaded natural fractures near a newly drilled well may alter the fracture propagation paths. The study also includes examples of stress concentrations near boreholes prepared with slotted perforations commonly used during fracture treatment operations in both conventional and unconventional fields.