This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 166134, ’Case Study - ECD-Management Strategy Solves Lost-Circulation Issues in Complex Salt Diapirs/Paleocene Reservoir,’ by David Murray, Shell; Mark W. Sanders, SPE, and Kirsty Houston, SPE, M-I Swaco; and Hamish Hogg and Graeme Wylie, Shell, prepared for the 2013 SPE Annual Technical Conference and Exhibition, New Orleans, 30 September-2 October. The paper has not been peer reviewed. Drilling horizontal infill wells in the Pierce field in the UK central North Sea is challenging because of a narrow drilling window caused by depletion in a highly fractured reservoir. Wellbore strengthening was attempted in the reservoir section of Pierce B5 although, when a pre-existing fracture further weakened by depletion was encountered, losses occurred. A detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism, resulting in a revised equivalent-circulating-density (ECD) -management strategy. Introduction The Pierce field (Fig. 1) in the UK central North Sea is a brownfield development with 17 existing wells drilled around two complex salt diapirs. Oil production was depletion-driven for the first 7 years until pressure support was provided by three water injectors drilled around the southern salt diapir. Pierce B5 was drilled in the proximity of the southern diapir, and losses encountered in the reservoir section were severe enough that no further drilling progress could be made. Preexisting fractures further weakened by depletion were thought to be the cause of the losses. Unfortunately, from a well-planning perspective, these fractures cannot be predicted or avoided when drilling horizontal wells. During a three-well infill campaign (Wells B5, A11, and C1), considerable losses on the first well led the operator to re-evaluate how future wells would be drilled. The outcome of this was the development of a low-ECD-drilling- fluid system, a revised ECD-management procedure, and a much-improved loss-treatment strategy. This combined strategy was applied in the field and resulted in the successful drilling of the subsequent wells. Loss Event on Pierce B5 Because of the close proximity in the reservoir of the Pierce B5 well to a preexisting production well (Pierce B2), it was determined that the localized depletion seen at this location because of the drawdown effects of B2 would be on the order of 2,300 psi, which would result in a reduction in formation integrity. However, there was concern that the effects of depletion could be higher than that; 3,200 psi through open fractures was modeled as the worst case. Therefore, because of the potential for dynamic losses when encountering zones that have seen the lowest field depletion and to mitigate the risk of open fractures, the recommendation was made to use a fluid design incorporating wellbore-strengthening materials (WSMs).