This paper presents the results from a long term development project to produce an instrument that measures pressure behind the casing in subsea production wells: online and in real-time.
Such an instrument will be an important application in protecting well integrity, where effective cement seals behind the wellbore casing provide a barrier against the high pressures encountered deeper in the well. Poor or deteriorating cement sealing and/or loss of casing integrity can allow oil or gas to migrate vertically towards the surface along the outside of the casing which can lead to a number of unwanted and potentially hazardous conditions. To this end, an instrument that detects any variations in pressure behind the casing will provide early warning of these conditions and allow intervention or other remedial actions to be planned and implemented in a timely manner. The instrument also has a health & safety application in verifying the integrity of the B annulus.
Two challenges to be overcome were the need to avoid any penetrations in the casing string, thus maintaining its full pressure integrity, and the requirement to provide power to instrumentation behind this casing without using batteries. The paper will describe the development process, technology choices and laboratory testing of the instrument. It will also describe the applications and limitations of the new system, which has been developed as part of a joint industry project with Statoil (who co-author the paper) in competition with other instrument vendors.
The paper will be highly significant for oil and gas operators planning subsea production or injection wells and for government regulating agencies in i) improving well integrity; ii) meeting safety and environmental protection; and iii) in the development of a new technology instrument not previously available on the market.
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. The presence of an annulus gives the ability to circulate fluid in the well.
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.
With conventional land wells and dry wellhead offshore wells, operators normally have easy access to the B annulus via valves at the wellhead, ensuring that pressure can be regularly checked and if necessary adjusted. This is not the case in subsea wells, however, where there is often 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).
The result is that the continuous monitoring of the pressure in the annulus of the casing may not be performed following completion - casing which provides an important barrier against the high pressures encountered deeper in the well.