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.
Conventional techniques for Plug and Abandonment (P&A) operations usually involve multiple trips downhole to mill the casing and underream the section. This paper describes new tools and methodologies developed to reduce rig time in Plug and Abandonment operations, specifically section milling and underreaming.The service provider has been working with an operator in its on-going Brent Field abandonment campaign, during which over 140 wells will be abandoned. The scope of work requires section milling a window in the 9-5/8" casing, and then underreaming the open hole to a half inch diameter over the original wellbore size. This is to allow placement of an abandonment cement plug against the original formation. The complete operation requires displacing the wellbore to milling fluid, section milling and then underreaming, all of which have traditionally been performed in separate runs.In response to these operational/economic challenges, a P&A system was developed using an innovative Ball Activated Drilling Valve (BADV) and a unique High Ratio Underreamer (HRU). This combination of tools made it possible to execute all operations in a single trip. The system allows the operator to mill and underream a window in a single trip; this will allow placing a rock-to-rock abandonment barrier plug in the required position.While tripping in hole, the BADV is open to the annulus which allows the string to self-fill, so no extra time is required top filling the pipe. Once the Bottom-hole Assembly (BHA) is at the section milling depth, the wellbore is displaced to milling fluid through the open BADV. Once displacement is complete and fluid rheology is suitable to ensure good hole cleaning of metal swarf during milling, a ball is dropped to shut the valve. Cut-out and section milling are then performed to achieve the required window length, once this has been competed the HRU tool is activated by drop ball and the open-hole section underreamed.A field test plan was presented and discussed with the offshore supervisors during pre-job briefing and they were introduced to the tool's functionality in the workshop. The first run with the combined section mill, HRU and BADV assembly took place on Brent and saved over 24 hours rig time. The system was then successfully trialed on seven challenging 9-5/8Љ casing milling jobs which resulted in total savings of over six days rig time. In addition, there was a reduction in the safety exposure to the crew by handling only a single BHA.Over the course of the campaign, the P&A team made several changes to the abandonment process enabling the operator to eliminate dedicated cleanout trips between section milling and underreaming runs. This streamlining process successfully combined operations that previously required four trips to complete down to one. Additional design changes/modifications were made during field testing and performance continually monitored to ensure the field test objectives were met and improvements to the tool/system reliability were being made. Product Engineers and Fie...
The initial proposal for abandoning a subsea field in the North Sea was to carry out the pre-abandonment work from the Light Well Intervention Vessel (LWIV) followed by the complete Plug and Abandon (P&A) operation conducted by a Mobile Offshore Drilling Unit (MODU). During the detailed design of the workscope, the four wells in the field were identified as through-tubing abandonment (TTA) candidates. To maximise the use of the LWIV and optimise the MODU sequence these TTA operations were planned to be executed from the LWIV. The TTA cement plug is installed by circulating cement down the production tubing and taking returns from the annulus to place the cement plug above the production packer and deep-set plug. An agitator tool is installed in the production tubing to improve the cement coverage in the annulus. As the LWIV does not have a permanently installed cementing system, a temporary system was fitted on the vessel which proved to be challenging given the deck limitations of the installation. Two of the four TTA candidate wells included a downhole gauge cable in the completion design across the zone that was required to be cemented. To ensure the long-term integrity of the TTA cement plug with the gauge cable, an expanding self-healing cement was utilised.
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