Tool Measures Pressure Behind Subsea-Well Casing

At OTC 2011, Emerson Process Management launched the Roxar Downhole Wireless PT Sensor System - Annulus B, a wireless tool for measuring pressure behind wall casing, to improve offshore safety. The new instrument will measure online and in real time previously inaccessible pressure and temperature information from behind the casing in subsea production wells, providing operators with an important new tool for well integrity monitoring. The annulus of an oil well is the space between two concentric objects, such as between the wellbore and casing or between casing and tubing, where fluid can flow.

In a completed well, there are normally at least two annuli. The A annulus is the space between the production tubing and the smallest casing string, with the B annulus located between different casing strings. Cement seals behind the wellbore casing provide a barrier against the high pressures encountered deeper in the well. However, poor or deteriorating cement sealing or casing collapse, in which the casing will heat up and thermally expand due to the production flow, can lead to a loss in casing integrity, allowing oil or gas to migrate vertically towards the surface along the outside of the casing.

This can result in potentially hazardous situations, especially during workover operations, where uncontrolled gas may escape at the surface. In the worst-case scenario, a shallow gas blow out might occur due to the failed barriers in the casing systems. Through online pressure monitoring, the Roxar Downhole Wireless PT Sensor System - Annulus B can provide positive confirmation of the pressure barrier's integrity. The tool also negates the potentially excessive and expensive over-dimensioning of casings that can sometimes take place to compensate for worst-case scenarios.

In addition, it aims to save on costs previously incurred in shutting in wells, due to their lack of ability to verify barrier integrity. The Roxar Downhole Wireless PT Sensor 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 comprises an Integrated Downhole Network (IDN) system, to carry signals from the wellbore to the customer monitoring system; a Downhole Network Controller Card (DHNC), placed in the subsea structure and connected to a 1/4in electrical cable coupled to a tubing hanger penetrator; and a series of up to 32 sensors distributed throughout the completion string. Other key components of the system include a wireless reader, a wireless PT Transponder and antennas to monitor activity in the B Annulus, and a transponder and reader carrier. The system is qualified to last for a minimum of 20 years at temperatures of up to 302F and pressures up to 10,000psi.

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