Case Study—Equivalent-Circulating-Density Management Strategy Solves Lost-Circulation Issues on Complex Salt Diapirs/Paleocene Reservoir

Summary 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 pre-existing 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, high-strength (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.