Learnings from Successful Drilling in Heavily Depleted HPHT Reservoir with Up to 460 Bar Depletion

Gudrun is a high-pressure, high-temperature (HPHT) field on the Norwegian Continental Shelf which has been in production since 2014. The initial development called for predrilling of the producers prior to commencement of production through depletion drive. In 2020 a second drilling campaign was initiated where the goal was to drill several infill producers and two water injection wells. The issue of drilling in heavily depleted reservoirs was highlighted as a major risk since depletion in some of the layers was expected to be in excess of 450 bar. The operational window was small and uncertain, and several risks were anticipated. Differential depletion in this highly layered reservoir, with the potential for penetrating both heavily depleted layers and non-depleted layers, meant that drilling and completion operations required wellbore pressures in excess of the minimum stress in the heavily depleted layers. There was thus a significant risk for lost circulation and escalation to possible well kick/underground blowout events. To mitigate these risks several actions were taken including Managed pressure drilling (MPD), splitting reservoir drilling into several sections, drilling of near vertical reservoir intervals and the use of active Wellbore Strengthening (WBS)/ Lost Circulation Material (LCM) particles in the mud. The use of optimal background WBS particles was complicated in the first two wells due to risk of plugging of lower completions upon production and so compromises were required to the particle sizes that could be used. This paper summarizes the experience from the successful drilling of these infill wells. It confirms that the use of WBS particles is critical in providing a robust drilling window against losses when the Fracture Gradient (FG) is reliant on near wellbore processes and elevated hoop stress around the wellbore to support downhole pressures that exceed minimum stress deeper in the "body" of the depleted layers. The experience on Gudrun also suggests that the FG is sensitive to the temperature of the mud when drilling the stiff Gudrun layers. The influence of depletion on the minimum horizontal stress, as determined from this drilling campaign, is also discussed and this is related to rock mechanical tests performed on core plugs from the field.