Drilling horizontal infill wells in the Pierce field in the UK Central North Sea is challenging due to the narrow drilling window caused by depletion in a highly fractured reservoir, which is further complicated by water injection and the proximity of other production wells. 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. During the next ten days over 5,000 barrels of non-aqueous fluid (NAF) were lost downhole. During the recovery from the losses event, 15 individual treatments with loss circulation materials (LCM) were made. More than 50MT of LCM was used without success, however, the well was stabilised enough to secure the drilled footage and complete the well. Two additional horizontal wells in the same field were successfully drilled using a low-equivalent circulating density (ECD)-management strategy combined with the development of a low-ECD drilling fluid system and the use of a high-fluid-loss, high-strength (HFHS) pill. The dual problems of barite sag and poor hole cleaning, that can be problematic when reducing ECD, were successfully managed. This low-ECD technique can be applied in other subsea depleted wells where narrow drilling windows exist. This paper describes how a detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism resulting in a revised ECD management strategy. An engineered low-ECD NAF with a HFHS pill combined with learnings on ECD awareness in the planning and execution phases mitigated the drilling risks on subsequent wells. This has implications when encountering very large fractures that are difficult to overcome, not only on future Central North Sea wells, but also has wider global significance for wells drilled in similar geological settings.

Drilling horizontal infill wells in the Pierce field in the UK Central North Sea (UKCNS) is challenging because of the narrow drilling window caused by depletion in a highly fractured reservoir, which is complicated further by water injection and the proximity of other production wells. Wellbore strengthening was attempted in the reservoir section of Pierce B5 (B5), although, when a preexisting fracture weakened further by depletion was encountered, losses occurred. During the next 10 days more than 5,000 bbl of nonaqueous fluid (NAF) was lost downhole. During the recovery from the losses event, 15 individual treatments with lost-circulation materials (LCMs) were made. More than 50 t of LCM was used without success; however, the well was stabilized enough to secure the drilled footage and to complete the well.Two additional horizontal wells in the same field were successfully drilled with a low-equivalent-circulating-density (ECD) management strategy combined with the development of a low-ECD drilling-fluid system and the use of a high-fluid-loss, highstrength (HFHS) pill. The dual problems of barite sag and poor hole cleaning, which can be problematic when reducing ECD through a reduction in fluid rheology, were successfully managed. This low-ECD technique can be applied in other subsea depleted wells in which a narrow drilling window exists.This paper describes how a detailed analysis of the losses event on Pierce B5 provided an improved understanding of the loss mechanism that resulted in a revised ECD-management strategy. An engineered low-ECD NAF with an HFHS pill, combined with learnings on ECD awareness in the planning and execution phases, mitigated the drilling risks on subsequent wells. This has implications when encountering very large fractures that are difficult to overcome, not only on future Central North Sea wells, but also globally for wells drilled in similar geological settings.