Well integrity can be summarised as the application of equipment and procedures to prevent the uncontrolled release of well fluids throughout the life of the well. This is achieved by including multiple well barriers into the well design, each of which must function normally to maintain the desired redundancy to ensure on-going well integrity.

When well parameters point to the failure of a barrier it is necessary to identify the source of the failure in order to plan remediation, or to demonstrate the continued integrity of the barrier for normal operations. Distributed fibre optic sensing offers a unique way to quickly and efficiently detect the fluid movement which may indicate barrier failure.

Where an optical fibre cable is pre-existing within the well the iDASintelligent Distributed Acoustic Sensor (and the ULTIMA DTS, Distributed Temperature Sensor) may be retrofitted to interrogate the optical path without the need to intervene in the well. Since the entire fibre path is surveyed simultaneously, well integrity issues at undetermined depths can be detected and localised almost immediately.

An alternative approach is to convey the optical fibre cable into the well as part of an intervention, then with the cable across the entire survey interval distributed acoustic and temperature sensing can be used to detect and localise anomalies which may indicate leak events or which may suggest fluid movements outside the tubing or casing.
Conventional point sensors, such as noise based leak detection tools, need to be placed adjacent to the anomaly which can result in lengthy operations to attempt to find the leak by moving the tool up or down in the well. In the event of an intermittent leak attempting to have a point sensor at the right depth at the right time can be even more difficult. Distributed sensing overcomes this limitation by enabling continuous monitoring of the entire survey area simultaneously; ensuring that any detectable anomaly will be captured and with minimal operating time.

Silixa provides field software to integrated temperature and acoustic data in real time and to perform basic acoustic data processing that can highlight leak events whether they are isolated discrete events or continuous low activity leaks. This is achieved by monitoring acoustic data in both the time and frequency domains in real time.

Where it is desirable to permanently monitor well integrity, such as in the disposal of CO2 where well barrier and caprock integrity may be of equal importance, the iDAS and ULTIMA DTS offer an unsurpassed method of achieving this.

Silixa’s iDAS and ULTIMA DTS operate interchangeably on either singlemode or multimode fibre so where an opportunity exists to utilise pre-existing installations there is no limit to the measurement opportunities.

Key benefits

  • Intervention-less well integrity monitoring using existing fibres
  • Rapid leak detection by scanning whole well
  • Real-time software enables rapid conclusions and optimises operating time

Applications

  • Tubing to annulus communication detection
  • Identify and confirm leaking completion components such as valves and plugs
  • Detect fluid movement behind casing and track to origin
  • Listen continuously for signs of cap rock failure during injection or disposal of CO2

With an optical fibre cable placed in an abandoned well, the iDAS was able to listen continuously over a period of several hours and subsequently capture a single discrete event signalling the periodic release of accumulated gas pressure behind casing. Unlike point sensor leak detection, the distributed nature of the iDAS measurement ensures that all such events are captured.