SPE/IADC-173073-MS Drilling Depleted Sands: Challenges Associated With Wellbore Strengthening Fluids Kuhan Chellappah, Arunesh Kumar, and Mark Aston, BP Copyright 2015, SPE/IADC Drilling Conference and Exhibition This paper was prepared for presentation at the SPE/IADC Drilling Conference and Exhibition held in London, United Kingdom, 17–19 March 2015. This paper was selected for presentation by an SPE/IADC program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers or the International Association of Drilling Contractors and are subject to correction by the author(s). The material does not necessarily reflect any position of the Society of Petroleum Engineers or the International Association of Drilling Contractors, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers or the International Association of Drilling Contractors is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE/IADC copyright. Abstract One way of overcoming the challenge of drilling depleted sands is to use so-called “wellbore strength- ening” (WBS) techniques, whereby engineered particles are added to drilling fluids to increase the fracture initiation or propagation resistance. Although the mechanisms are not comprehensively understood, most explanations suggest that an increasing proportion of large particles is required to achieve a greater extent of ‘strengthening’. These large particles can create practical challenges while drilling; such as particle attrition, settling, and erosion of equipment. Field examples are presented which highlight these issues. Experimental work to identify solutions to the practical challenges is presented. A laboratory attrition test developed in-house confirms walnut shell as a promising material to resist grind-down when large particles need to be retained in the system. Formulations prepared using controlled size distributions of this material are shown to give a good fracture sealing performance in the laboratory. The accurate maintenance of particle size will require regular solids monitoring at the rigsite to keep the fluid within design specifications. Some data from real-time particle sizing techniques are presented. Overall, the findings presented in this paper should lead the industry towards wellbore strengthening systems which are effective, more efficient and practical operationally. Introduction Hydrocarbon production from reservoirs leads to a decrease in formation pore pressure which in turn reduces the pressure a wellbore can withstand before fracturing. This reduction in ‘fracture gradient’ due to depletion can make it challenging to drill subsequent wells and access additional reserves. For instance, an increased number of casing strings may be required to prevent lost circulation, which can be costly and is not always feasible. An alternative approach is to apply so-called ‘wellbore strengthening’ (WBS) techniques which use drilling fluid additives to raise the fracture gradient. A widely used technique is to add engineered particulates to the fluid (1–4). This method has empirical roots, from field observations that the addition of particulates tends to reduce the frequency and severity of lost circulation events. Several mechanisms have been proposed to explain this phenomenon. The StressCage model (2,5,6) is one such proposed theory which has received popular industry support. With demand increasing for such techniques the practical limitations are becoming more apparent. The general