A long-term suspended subsea exploration well within a producing gas reservoir needed to be decommissioned after 21 years. During a pre-decommissioning diving campaign, bubbles confirmed as reservoir gas were observed to be percolating from the well bore through a hard silt / cement debris plug inside the wellhead. A pressure study established that the reservoir may have re-charged to 2,200 psi. An alternative pressure controlled well re-entry method was required to safely re-enter, tie-back the well to surface with 16-in. high pressure riser, install BOP while preventing gas from reaching the rig floor from seabed. Two existing cement plugs would then be drilled out under controlled conditions due to the potential for high-pressure gas beneath the plugs. Casing integrity evaluation and cement bond logging would be carried out to establish the path of gas ingress into the wellbore. Remedial work would be conducted, and permanent abandonment barriers installed in the well. Casings and wellheads would then be recovered from a depth below the seabed. A customized managed pressure drilling (MPD) system was designed using a rotating control device (RCD) and modified drilling chokes. A pioneering plan was developed to meet the specific well re-entry requirements of the percolating suspended well to account for the potential for virgin reservoir pressure at seabed and the wellhead silt plug preventing deployment of BOP test tools. A hazard and operability study (HAZOP) was conducted with key personnel, which supported development of well-specific operating procedures and decision matrices. Successful deployment included MPD system calibration, well behavior fingerprinting, and training of rig personnel at the well site. The combination of experienced personnel, innovative MPD equipment, specific procedures, team interactions and risk analyses were key to safely completing this well re-entry and decommissioning scope. The strategy enabled drilling out of two cement plugs with potential high-pressure gas trapped beneath them. Both cement plugs, 356ft and 669ft long, were drilled without any well-control or plugged-choke events. Throughout the process, the well was monitored using MPD equipment, which included an RCD on top of rig's BOP, modular drilling chokes and multiple pressure gauges and sensors installed at critical points. Additionally, temporary modifications were made to the rig and new lines of communication between the rig crew and the MPD team were established to ensure all pressures were correctly interpreted and the decision matrix was correctly applied. An effective close partnership developed between the equipment service provider, well operator and drilling contractor was a key enabler to deliver this very challenging novel implementation of MPD technology within eight weeks. The MPD approach was estimated to have saved 9 days of rig time, when compared to alternative coiled tubing-based solutions. This paper describes the first MPD-assisted well re-entry for well decommissioning in the UK North Sea sector. The novel application of existing technology can help operators to cost effectively re-enter and decommission troublesome legacy wells without harm to people, environment or assets. This new approach resulted in the safe unconventional re-entry and decommissioning of a potentially live gas well.
Whilst drilling the 12 ½" and 8 ½" sections on a HPHT field in the North Sea, fluid losses were encountered which required the pumping of high concentration Loss Circulation Material (LCM) as well as multiple ‘through the bit’ cement squeezes. The reservoir had been under production for many years and was now at a point where it had significantly depleted and the formation was very delicate, making it easily subjected to fracturing. Losses were expected to be an upcoming challenge as the mud weight window was so narrow and due to the HPHT rating of the offshore reservoir and the fractured nature of the geology, this posed as a serious risk that required pre-planned mitigation. The ability to pump ultra-high LCM concentrations was discussed in pre-planning as being one of the key mitigation strategies to stemming losses. However, it was thought that pumping LCM would block a typical MWD pulser due to the narrow geometries of the flow paths within. Successful onshore testing was conducted prior to drilling operations to prove that the MWD pulser selected had the ability to permit the flow of ultra-high concentrations of LCM. A closed loop system was created onshore and LCM with concentrations up to 420kg/m3 (147lbs/bbl) was pumped through the MWD tool without blocking it up and with full functionality throughout. This gave the operator the confidence to pump LCM through the MWD tools, if required in the field. The use of an MPD (Managed Pressure Drilling) system was also required to keep the equivalent circulating density within the tight mud weight window by maintaining a constant bottom-hole pressure through the application of Surface Back-Pressure (SBP). This was critical as the well, when static, was underbalanced. The incorporation of MPD also gave the operator the ability to ‘squeeze’ LCM/cement into the well and formation by applying surface back pressure during ‘through the bit’ LCM/cement jobs. During field operations, the operator did go on to pump a 320 kg/m3 (112 lbs/bbl) LCM pill through the MWD tool without plugging during a loss scenario. Pumping LCM through the MWD tool reinforced the formation and enlarged the mud weight window from <0.01sg to 0.03sg allowing operations to continue. In addition, muliple "through the bit" MPD LCM/cement squeezes were conducted on two wells in the same field with no negative effect on any of the MWD tools. The partnership between the MWD and MPD systems formed a key part of the losses management. The MWD system could identify exactly what downhole pressure losses were seen at, which was then transmitted by the pulser to the surface. The MPD system would then use an algorithm based calculation to apply the exact required SBP to maintain a constant bottomhole pressure, that was within the mud window. This provided the operator a safe and efficient solution to continue to the target depth in a very challenging narrow margin well.
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