From Completion to Production Without Intervention in a Subsea Well
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As part of an improvement program focused on increasing standardization and efficiency on subsea well operations on the Norwegian continental shelf, the operator aimed to standardize future subsea wells using vertical trees (VXT) instead of horizontal trees. This would enable batch completion of several wells with a rig, followed by XT installation with an installation, maintenance, and repair (IMR) vessel, eliminating the need for a rig or lightweight intervention (LWI) vessel. The complete paper describes the development and implementation of a glass-plug solution that closed the technical gaps that had previously inhibited fully intervention-free operation for completion installation.
Background and Field Information
Trestakk is an oil and gas field in the Norwegian Sea in Block 6406/3. The field lies in 300 m of water approximately 27 km southeast of Åsgard A. Trestakk was discovered in 1986 and the plan for development and operation was approved in 2017. Equinor operates the field with 59.1% ownership interest, with Vår Energi owning the remaining 40.9%.
The field development covers a subsea template with four well slots and one satellite well. A total of five wells will be drilled—three for production and two for gas injection. Production from the Trestakk subsea field on Haltenbanken in the Norwegian Sea began 16 July 2019.
Trestakk is tied back to the Åsgard A floating production vessel. The Petroleum Safety Authority Norway and the Norwegian Petroleum Directorate approved the application for extending the lifetime of the installation to 2031 as a result of the additional recoverable volumes from Trestakk, for which field production is expected to last for 12 years.
The goal for Trestakk was to achieve first recoverable oil at surface as soon as possible. The method selected was deployment of the completion string and VXT from a rig. The proposed solution was a barrier valve to be integrated as part of the completion string. As an operational improvement, the Trestakk team decided to design the wells with the intention of excluding wireline. Common methods for suspension and initiation of subsea wells during blowout-preventer (BOP) removal and VXT installation include installing shallow-set bridge plugs in the tubing or a tubing hanger plug. The plug is then removed using a riserless LWI vessel or rig. This method is often associated with high cost and enhanced operational risk.
A technology group was assembled to overcome the obstacles inhibiting a fully intervention-free operation for completion installation. A joint operator/service company technology team collaborated to develop a glass-plug solution that met all necessary requirements while still minimizing associated risk. Because the IMR vessel used to install the VXT is not rigged for wireline operations, the goal was to achieve a wireline-intervention-free subsea completion. New or improved technology was sought for temporary well suspension, from removal of the BOP to installation and testing of the VXT. A glass plug with fluid bypass installed as part of the completion string was proposed.
Glass-plug technology enables three positions—open, closed, and then open again. The glass plug is first installed with the bypass ports in open position, allowing for standard completion activities to be conducted. For example, lighter fluid can be pumped into the tubing before setting a packer, allowing for bullheading in a well-control situation. The bypass is shifted to the closed position after a predefined number of tubing pressure cycles have acted on the glass-plug system. Once the bypass is closed, a well barrier is established. The glass-plug barrier is shattered only after the tubing above the glass has been subjected to a predefined number of pressure cycles while applying a specific overbalance. After the barrier element has shattered the valve, it activates its final open state, resulting in full-bore inner diameter through the valve. To initiate production, the glass plug can be opened either from a rig or from the Åsgard A platform. The solution was developed and qualified according to ISO 14310 V0Q1 / ISO 28781 V1Q1 and was installed in less than a year.
Technical Challenges and Design
The technical challenge was to develop a valve with a throughbore initial open state, an intermediate closed state, and a final open state that could be integrated as part of the completion string above the production packer. To achieve a robust design, a new technology was developed based on a field-proven inter-remote shatter valve (IRSV). The IRSV is a glass plug qualified to ISO 14310 V0Q1 and ISO 28781 V1Q1. It is installed in closed position as part of the completion string or below an intervention packer. The tool acts either as barrier, packer setting device, or a combination of both. The IRSV is opened by subjecting it to a predetermined number of tubing pressure cycles. The required cycling pressures are determined based on well-specific parameters such as pressure below, pressure above, and available wellhead pressure when opening the plug.
The solution developed for Trestakk is an inter-remote barrier valve (IRBV), a shallow glass valve deployed as part of the completion string in the open position, allowing for the completion activities to be conducted and the possibility of bullheading in the event of a well-control situation. Before BOP removal, the IRBV is activated to the closed position, leaving an ISO 14310 V0 barrier in the well (Fig. 1). When the VXT has been installed and tested, the barrier is removed by tubing pressure cycles, shattering the glass valve (Fig. 2).
The IRBV valve housing is supplied with crossovers and tubing pup joints at both ends, forming an independent tubing joint for running between regular tubing joints. All internal tubing body threads are metal-to-metal seal premium connections. The valve is installed with the bypass ports in open position, allowing the completion activities to be performed.
The bypass can be closed, and the barrier inside the valve housing is removable. Both functions are activated by cycling the pressure inside the tubing. When the barrier has been removed, the IRBV housing remains as an integral part of the tubing string.
Case Study, Trestakk A-4H
The successfully developed and qualified IRBV was installed on the first Trestakk well, A-4H. The purpose of the IRBV in this application was to act as a shallow secondary barrier when removing the BOP and installing and testing the VXT with a rig.
The glass shattered at bleed down at 1,015-psi overbalance, as expected. When implementing the shallow glass plug with bypass in line with a deeper secondary barrier, the first oil on surface was achieved at an optimized schedule. This optimization was achieved even while using the rig to deploy and install the VXT. Therefore, in larger fields, when the wells are predrilled and completed in advance of being hooked up to production facilities, the most-cost-efficient method would be to batch-complete the wells and install the VXTs at a later stage with an IMR vessel.
By incorporating the valve into the completion string and ensuring that well design and operation are planned accordingly, wireline intervention can be avoided entirely. The optimized completion design was one of the cost-reducing contributors to Trestakk field development. By implementing the new technology, the operator could carry out the VXT subsea completion from a rig without wireline intervention and without needing to wait for an LWI or IMR to start production.
The tool can be used in various applications, including as a shallow and deep barrier for both conventional and subsea completion. It can act as a shallow barrier when removing the BOP until the XMT is installed and tested. It can also function as a deep barrier, allowing for communication down the tubing until it is closed (i.e., to avoid running the upper completion closed-ended or allowing for communication when running standalone screens).
By using the glass plug, the drilling rig can batch-complete subsea wells and at a later stage batch-install the VXT with an IMR vessel. The plug can also be installed as part of a conventional completion string when a shallow barrier is required, eliminating the need for intervention.
The IRBV was successfully developed within 12 months in close collaboration with the Trestakk team and the operator’s technical headquarters. Since then it has been successfully installed and operated on three wells on Trestakk, and a new size has been developed. The estimated time saved by using a shallow glass plug with a bypass is 5 to 6 days, resulting in a total cost savings per well of approximately $1 to $2 million.
From Completion to Production Without Intervention in a Subsea Well
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