A variety of Wellbore Strengthening (WBS) methods have been introduced and successfully applied in field drilling practice over the last decade, with origins going back to the late 1980's when the DEA13 investigation was carried out. In addition, a beneficial casing "smear" effect has been observed in recent years during drilling-with-casing or casing-while-drilling (CWD) operations, preventing lost circulation even when drilling highly depleted formations at high annular circulating pressures. Casing smear mimics WBS effects observed during regular drilling operations in its ability to effectively extend fracture gradients and prevent lost circulation, which is of great benefit to drilling difficult wells with tight drilling margins such as (ultra-)deepwater wells. In fact, when used appropriately, WBS and casing smear may offer well construction benefits that are on par with those of managed pressure drilling (MPD) and dual gradient drilling (DGD), at reduced equipment usage, complexity and cost.

To explain WBS and casing smear phenomena, a variety of different mechanisms have been proposed. These broadly fall into three categories: (1) Wellbore Stress Augmentation (WSA) - augmenting the hoop stress or closure stress around the wellbore by propping open induced fractures either at the wellbore face or somewhere into the fracture; (2) Fracture Propagation Resistance (FPR) - effective solids plugging of any fracture tip, thereby interfering with effective fracture growth; (3) Wellbore Face Sealing (WFS) - forming a uniform seal around the wellbore that prevents fluid leak-off and interferes with fracturing. Here, we offer a re-examination of previously published data (such as open-hole leak-off tests) and theoretical results, and present our own independent results and assessments to show that WBS and casing smear have a common, unified origin. It will be shown that the only mechanism that consistently explains all the data obtained to date is, in fact, the FPR mechanism. Moreover, guidelines are presented to exploit FPR optimally for the benefit of complex well construction.

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