The ability to “see” downhole in subsea production wells to monitor and measure pressure and temperature could potentially prevent a catastrophic event, as well deter sub-optimal gas lift design.

For operators, the ability to have an eye in the well annulus – where previously it was not possible – to provide positive confirmation of the pressure barrier’s integrity online is something that in today’s ultra-sensitive industry environment can be a welcome addition to the portfolio. It is also something that government regulatory agencies are likely to pay particular attention to with increased global focus to improve well integrity and meet tighter safety and environmental protection requirements.

This solution is the result of a long-term development project to produce an instrument that measures online and in real time previously inaccessible pressure and temperature information behind the casing in subsea production wells and injection wells. The Roxar Downhole Wireless PT Sensor System – Annulus B – was launched at this year’s Offshore Technology Conference in Houston.

Emerson Process Management’s new Roxar downhole sensor system, developed over several years as part of a joint industry project with Statoil, could represent a significant step forward in protecting well integrity.

A better solution

Conventional land wells and offshore wells with a dry wellhead have valves that provide operators with easy access to the B annulus to check the pressure and make any necessary adjustments. Not only is this useful for gas lift purposes, it is a simple way to ensure no gas is leaking into the annulus from the formation or because of poor cementing.

This is of course not an option on subsea wells, where there often is no access to the B annulus following the landing of the casing hanger and sealing and cementing of the casing. This is due to specifications for subsea wells with a requirement not to breach a primary well barrier (i.e ISO 13628-4 and API17D).

This can result in potentially hazardous situations, especially during workover operations, where uncontrolled gas can escape at the surface. In a worst-case scenario, failed barriers in the casing systems could result in a shallow gas blowout with significant safety implications.

Instruments capable of regularly monitoring online and detecting any variations in pressure behind the casing can provide early warning of these conditions and allow intervention and other remedial actions to be planned and implemented in a timely manner. The instrument also has an HSE application in verifying the integrity of the B annulus. Subsequent completions can benefit from a more accurate model prediction.

The Roxar Downhole Wireless PT Sensor System – Annulus B – will be able to directly measure pressure and temperature behind the casing in subsea production wells. (Image courtesy of Emerson Process Management)

The tool also will negate the sometimes excessive over-dimensioning of casings that takes place to compensate for worst-case scenarios.

Additionally, the sensor system can provide operators with significant cost savings previously incurred in shutting in wells due to a limited ability to verify barrier integrity.

Protecting subsea well integrity

The new wireless PT Sensor System and its permanent monitoring capabilities will give added certainty to the well integrity monitoring process as well as valuable input during well trouble-shooting.

Terje Baustad of Emerson Process Management described the system as representing “a real step change in protecting subsea well integrity, as it tracks pressure in an area which was previously off limits for all operators.”

The new wireless instrument attaches to the same cable as the reservoir monitoring gauges and can detect variations in pressure behind the casing string. It will provide early warning of these conditions and allow intervention or other remedial actions to be planned and implemented in a timely manner, he explained.

“For oil and gas operators planning subsea production or injection wells and looking for improved well integrity monitoring, and for government regulatory agencies overseeing safety and environmental protection, the Roxar Downhole Wireless PT Sensor System represents a milestone moment,” Baustad said.

The system monitors the B annulus pressure and temperature without any degradation to the original barrier element consisting of the A casing system and can be retrofitted to the monitoring system design of current subsea systems.

The system consists of an integrated downhole network that carries signals from the wellbore to the monitoring system with a downhole network controller card placed in the subsea structure and connected to a ¼-in. electrical cable coupled to a tubing hanger penetrator and up to 32 sensors distributed through the completion string.

Other key components of the system include a wireless reader, a wireless PT transponder and antennae to monitor activity in the B annulus, and a transponder and reader carrier. The system is accurate to +/- 2.5 psi - +/- 0.18°F.

It also is qualified to last for a minimum of 20 years at temperatures up to 302°F (150°C) and pressures up to 10,000 psi.

Based on an electronic wireless system, where the signals and power are transmitted using induction technology applying electromagnetism, the system ensures a long life. This lifespan is far longer than battery-based solutions, which also have temperature restrictions. Because of the passive nature of the meter module, the unit can be turned off to prolong its operational life.

The power is transferred from a section in the production tubing. All that is required during installation is a part on the casing that is run like a regular casing joint. The completion – with the receiver in the tubing – is run like any other regular completion.

B annulus project – work scope and conclusions

The scope of the project was to address well integrity issues and provide an instrument that could monitor the B annulus pressure and temperature without degradation to the original barrier element consisting of the A casing system. The proposed instruments also had to be able to greatly increase the operator’s ability to verify the integrity of the barriers throughout the well life.

Due to the position of the device as part of the casing string, several other issues had to be addressed:

  • The components and the system had to be qualified to last for a minimum of 20 years at temperatures to 302°F (150°C) and pressures to 10,000 psi;
  • The system could not make the well barriers less reliable;
  • It had to be possible to retrofit the monitoring system to the present designs of subsea systems;
  • The system needed to be able to monitor pressure and temperature in both the A and B annuli;
  • It had to be possible to read the sensors at any time; and
  • It had to be possible to install or retrofit the system without making any changes to the existing control systems.

Phase 1 was a feasibility study in 2005, followed by a development agreement in 2007.

A steering committee was also set up to sponsor the process, which included representatives from Roxar, Statoil, and other sponsors and a representative from Det Norske Veritas, who acted as observer. Regular meetings took place throughout the design and qualification process, and all members are currently involved in seeking/selecting suitable pilot wells for installation following a rigorous qualification process.

Final qualification testing for the new tool will take place this year, and further longevity testing will continue into 2012.