Santos Ltd (Santos) proposes to plug and abandon (P&A) the Tern-2 wellhead located within the Tern Field of retention lease license WA-27-R. All wells in the Tern field have been permanently plugged and abandoned with the exception of Tern-2, which is currently classified as Temporarily Abandoned. The permit area is in the Bonaparte Basin within Commonwealth waters, approximately 300 km WSW of Darwin, in water depths of approximately 83 m.
This EP will focus on installing and verifying additional well barriers to supplement the existing system of well barriers to support the removal of the wellhead and well infrastructure at the seabed. The P&A campaign is predominantly concerned with the removal of equipment to eliminate future hazards to the environment or other uses of the area as best as practicable. A carbon steel drilling template and remnants of a cement patio presumed buried below the mudline will not be retrieved and instead will remain in-situ in perpetuity. A contingency scenario to leave the wellhead in-situ has been assessed in this EP in the event that operational risks prevent full recovery of the wellhead. In the event it becomes necessary to leave the wellhead in-situ, the residual risk to other marine users and environment is considered very low.
Activities for the P&A of Tern-2 may include:
- Corrosion cap removal (including marine growth removal from wellhead infrastructure)
- Well integrity evaluation (i.e. wireline drift run, wireline cement bond logging of wellbore casing)
Permanent isolation of the reservoir (i.e. abandonment plug setting)
Recovery of wellhead (i.e. wellhead severance and recovery, wellhead leave in-situ contingency if full recovery is not feasible)
Vessel operations (LWIV and contingent support vessel)
ROV activities (well site surveys)
Helicopter activities (for emergency situations)
The P&A campaign will be conducted using a Light Well Intervention Vessel (LWIV). It is anticipated that a support vessel will not be required during P&A activities, however provision for a single support vessel is accounted for throughout the EP. A remotely operated vehicle (ROV) will also be used to carry out activities such as corrosion cap cleaning and an ‘as left’ ROV survey that will be conducted at the completion of the P&A activity. Helicopters and support vessel may be used to transfer crew and equipment to and from vessels/LWIV and assist in emergency situations as required.
The activity duration is expected to take up to approximately 10 days of continuous operations. However, 40 days of activity duration has been considered to allow for unforeseen delays. Once started, operations will be 24 hours per day, 7 days per week. The exact timing and duration of the P&A activities are subject to LWIV availability, regulatory and business approvals as well as metocean conditions (particularly cyclonic activity). To ensure conservatism, the EP is assessed for the activity occurring at any time of year for the approximate duration of the activity.
