Wellbore Strengthening (WBS) is an enabling technology to simplify casing designs, prevent costly lost circulation events, and complete wells with a narrow drilling margin. Given its effectiveness to help drill today's challenging wells economically, WBS has become a routine part of the drilling fluid design and the well construction process in general. However, the drilling industry still lacks a general model to optimally exploit WBS for various formation types and fluid systems.

In this paper, we have conducted a systematic investigation of the major parameters which affect WBS treatment in permeable formations. Large-scale fracturing experiments were conducted on two rock types with significantly different permeability and stiffness: Berea sandstone (stiff - moderately permeable) and Castlegate sandstone (weak - highly permeable). Several fluid injection cycles were carried out on each rock sample to characterize the Fracture Initiation Pressure (FIP) and Fracture Propagation Pressure (FPP). Further, we investigated the effect of Lost Circulation Material (LCM) - which are used for WBS purposes - on the FIP and the FPP. Also, we studied the effects of the concentration and the Particle Size Distribution (PSD) of LCM blends, the confining pressure, the stiffness and the permeability of rock samples.

The experimental results confirmed that the FIP is mainly a function of rock tensile strength and stress concentration around the borehole, and not affected by the presence of LCM. The FPP, however, can be significantly enhanced by adding LCM to drilling fluids. Further, FPP enhancement is largely independent of the rock stiffness. We also found that a minimum concentration of LCM is necessary for effective WBS. However, no significant additional enhancement in the strengthening benefits was observed for concentrations above a certain upper threshold. In addition, our results showed that a higher LCM concentration is required for effective WBS in formations with a lower permeability (i.e. tighter formations). PSD dominates the strengthening benefits of an LCM blend and, for both tested rock types, LCM blends with a broad bimodal PSD provide superior strengthening benefits compared to those with a unimodal PSD.

In this paper, we introduce design guidelines to maximize the attainable strengthening benefits of LCM blends. The findings of this paper are relevant to improve the construction of (ultra-) deep-water wells with a narrow drilling margin through systematic optimization of WBS treatments and minimization of non-productive time.

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