In the drilling of deep horizons, the mud-weight window between pore-pressure gradient and fracture gradient often narrows because of rock properties and underground-stress state. Nonproductive-time events such as lost circulation, wellbore instability, kicks, underground crossflow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Bridging pre-existing natural fractures or drilling-induced fractures with lost-circulation materials (LCM) is often performed to increase fracture gradient and widen the mud-weight window. This technique, referred to as wellbore strengthening, includes the stress cage, fracture-closure stress, and fracture-propagation-resistance methods. Although these methods are often used, several aspects of these approaches are still not thoroughly understood.To reduce the risk of lost circulation while drilling in formations with narrow mud-weight windows, such as pressure-depleted reservoirs and deepwater formations, a good understanding of the mechanisms of wellbore strengthening in different downhole scenarios helps engineers to optimize the design of drilling fluids and operational procedures.This paper discusses the mechanism of wellbore strengthening, with a focus on hoop stresses at and near the wellbore, in elastic and poroelastic models, by use of the finite-element method to evaluate wellbore and near-wellbore stresses during fracture creation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior, such as horizontal-stress anisotropy, LCM-bridging location, initial pore pressure, and fluid leakoff, are investigated. A better understanding of the several interacting events local to the wellbore and near-wellbore regions can result in improved operational practices related to lost-circulation prevention and mitigation. IntroductionThe purpose of wellbore strengthening during the drilling of oil/ gas wells is to widen the mud-weight window. To maintain downhole pressure within the mud-weight window, drilling fluids are designed to provide wellbore-hydrodynamic pressure low enough to prevent downhole lost circulation but high enough to prevent borehole instability or kicking. While drilling in deep horizons or depleted reservoirs, the window between pore-pressure gradient and fracture gradient often narrows because of rock behavior and the near-wellbore-stress state. Nonproductive-time (NPT) events such as lost circulation, wellbore instability, kicks, underground crossflow, and pipe sticking are more likely. Such problems greatly increase drilling costs.To reduce NPT, drilling engineers often use wellbore strengthening to widen the drilling-mud-weight window and increase the fracture gradient in the formations being drilled, thereby preventing lost circulation and its related problems. Although NPT reduction is desirable, it may at times not be the most-important consideration. Sometimes lost-circulation problems determine whether the well can be completed without requiring additional casing strings and associated change...

In drilling deep horizons, the mud weight window between pore pressure gradient and fracture gradient often narrows due to rock properties and underground stress state. Non-productive time (NPT) events such as lost circulation, wellbore instability, kicks, underground cross-flow, and pipe sticking are more likely. Such problems greatly increase drilling costs. Plugging preexisting natural fractures or drilling-induced fractures with lost circulation materials (LCM) is often used to increase fracture gradient and widen the mud weight window. This technique, called 'wellbore strengthening', includes several strengthening methods, but there are several factors affecting these procedures are not thoroughly understood.To reduce the risk of loss circulation while drilling in formations with narrow mud weight windows such as pressure depleted reservoirs and deep-water formations, a good understanding of the mechanisms of wellbore strengthening in different downhole scenarios helps engineers to optimize the design of drilling fluids and operation procedures. This paper discusses the mechanism of wellbore strengthening in elastic and pore-elastic models, utilizing the finite element method to evaluate wellbore and near-wellbore stresses during fracture initiation and propagation, and after plugging fractures with LCM. Factors affecting fracture behavior such as formation stress anisotropy, LCM bridging location, initial pore pressure, and fluid leak-off are investigated. A better understanding of the several interacting events local to the wellbore and nearwellbore regions can result in improved operational practices related to lost circulation events.