Producing hydrocarbons in a deepwater environment is quite challenging and the goal is always to sustain production for as long as possible. Any shutdown, whether intended or unintended, can prove costly. Such was the case on Oyo-8 when the down-hole safety valve failed to open after a planned production curtailment. The several attempts to open the valve were unsuccessful due to suspected loss of hydraulic communication to the operating mechanism or the malfunction of the mechanism. A well intervention was therefore deemed necessary to re-enter the well. Well intervention in a deepwater environment is particularly expensive, risky and complex, whether it is rig based or rigless. A vessel based riserless light well intervention (RLWI) operation was determined as the optimal solution for Oyo-8. This entails a through-tubing operation to install a retrofit sub-surface controlled safety valve (velocity valve) assembly across the tubing-retrievable safety valve (TRSV) to permanently hold open its flapper and serve as a sub-surface controlled safety valve. The notable advantages of a vessel based operation were the reduced personnel and reduced number of third-party service contractor interfaces to manage. The rig-up is significantly simpler than the requirements for rig-based completion/de-completion operations. The fact that it is riserless also eliminates the debris management issues usually associated with the marine risers of mobile drilling units. The key components of RLWI are the subsea well-control package, subsea wireline lubricator, subsea pressure-control head, a compensated winch mounted on a module-handling tower, compensated heavy-duty over-side crane, slickline unit, and two work-class ROVs. The RLWI operation was completed within the budgeted time with less than 3% nonproductive time. The recovery of the horizontal tree crown plugs was uneventful, as the ROV high-pressure jetting tool was effective in flushing out debris from the top of the upper plug before installing the well control package and carrying out the slickline runs. The velocity valve assembly was installed hanging off the end of a thru-tubing packer set in blank tubing. A carefully planned combined operation with the FPSO ensured that control of Oyo-8 horizontal tree was maintained during well intervention from the production topside. The intervention operation achieved the set objectives of restoring Oyo-8 to full production within allotted time and budget without any QHSE incidents. The RLWI operation was the first of its kind in Nigerian waters.

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.

No abstract

Shell Petroleum Development Company of Nigeria Limited (SPDC) recently embarked on a campaign to abandon some wells in line with a corporate drive to enhance the safety of the wells in her portfolio. It is also to ensure that SPDC as a responsible operator aims to safeguard people and environment as required by law. The campaign is part of a larger campaign that had the objective of enhancing safety integrity in SPDC. It will also afford the opportunity to carry out relatively inexpensive repair and recompletions. A total of six wells were abandoned in the land and swamp terrain within a year period utilising the Hydraulic Workover Units (HWU) for Land anda combination of the HWU and Self Elevating Workover Platform (SEWOP) in the Swamp. The 6 wells were amongst the most critical wells for abandonment based on SPDC defined criteria (e.g wells in Navigable water ways or in built up neighborhoods). In one of the wells on land, human habitation had since grown close to its proximity and the well experienced high annular pressures, hence the need for the abandonment. The others are in the waterways in the swamp with other technical problems. Most of the wells were drilled over 30 years ago and they have either outlived their useful life or there is plan to recover the remaining reserves in other offset wells. The abandonment involved full subsurface and surface abandonment with the entire wellhead and casing structures removed from below the mudline or cellar (for land). The use of the HWU / SEWOP arrangement brought significant cost savings (25 - 50 %) when compared to the cost of using conventional workover rig. This paper reviews the work that was carried out in the abandonment and highlights the challenges as well as strategies that were applied towards the successful abandonment operations. Introduction Although, Well abandonment (decommissioning) is part of a Field development Plan, this activity has often being delayed because it is a non-oil generating activity. Moreso, it is most often ranked low when competing for the limited funds with other oil generating activities. However, SPDC has recognised the need to accommodate ‘limited’ well abandonment activities every year to ensure that as a responsible operator it aims to safeguard people and the environment as required by law. The well abandonment campaign is part of SPDC corporate drive to enhance the safety of some critical wells in its portfolio and to ensure that all wells are operated within the acceptable safe envelope.