Mature fields often include wells or platforms that require a plug and abandonment (P&A) job and decommissioning. Two high-pressure gas wells were identified in a field offshore Brunei that required a coiled tubing unit (CTU) intervention to secure them. The job was further complicated by high reservoir pressure in a tight formation, a small platform area, and no existing surface facility.
Because the flowline and pipeline equipment on the platform are not in service, a method was needed to take large volumes of returns. To overcome this challenge, an integrated operation was designed and planned that uses a complete catenary CTU and a pumping and flowback package with a mobile storage support vessel. The returns system will enable liquid (brine and spacer) and gas returns with pressures of up to 5,000 psi. The mobile storage support vessel enables the returns to be stored before disposal at an onshore facility.
Before killing the wells and pumping cement across the perforation, multiple tubing plugs must be removed. High tubing pressure indicated some communication to the reservoir; consequently, the plug removal strategy with coiled tubing (CT) must include risk mitigation to ensure that the plugs can be removed safely.
After all plugs were removed, the wells were successfully killed by using a 17 kPa/m (14.45 ppg) calcium chloride/calcium bromide (CaCl2/CaBr2) blend brine. The next operation was to spot cement across the perforation to a depth that is below where the tubing will be cut during the P&A process. The spacers and cement design must be tailored to avoid compatibility issues with the CaCl2/CaBr2 and the presence of carbon dioxide (CO2). The cement placement with CT is critical to prevent flash reaction with the kill fluid and to minimize the volume of contaminated cement. The cement was placed successfully and tested in accordance with the Brunei Shell Petroleum requirement.
The wells were safely secured and ready for the planned P&A process. These operations marked the first successful operation on high-pressure, tight gas wells with no surface facilities on the platform within a sensitive environment. The use of a mobile storage support vessel enabled the wells to be killed successfully. This approach demonstrates that a similar method and similar planning can be used to safely and economically perform future interventions.