Borehole failure mechanisms in naturally and hydraulically fractured formations investigated with a fast time-stepped linear superposition method (TLSM)

Abstract

The present study investigates the state of stress around a wellbore with nearby natural fractures using a new simulator based on a linear superposition method (LSM) of elastic displacements. 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. Rather than shear failure, tension failure may also occur at the wellbore. For example, wellbore stability is jeopardized by unintended circulation-loss when the mudload causes spontaneous hydraulic fracturing during drilling operations. LSM shows static solutions of the stress state, and a dynamic version of the method (TLSM) uses time-stepped superposition to model fracture propagation processes. TLSM solutions show how fluid pressure-loaded natural fractures near a newly drilled well may increase the risk of such lost circulation to occur. Nearby natural fractures, if loaded by an internal fluid pressure, will 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.