Finite Element Studies of Wellbore Strengthening

Abstract

Abstract 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 down-hole 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 near-wellbore regions can result in improved operational practices related to lost circulation events.