HSE & Sustainability

Annuli Liquid-Level Surveillance Using Distributed Fiber-Optic Sensing Data

Distributed fiber-optic sensing data from distributed temperature sensing and distributed acoustic sensing can be used to monitor annuli liquid levels in production wells, further allowing calculation of leak rates and inference of leak path.

Schematic section of a gas lift-assisted production well where the annuli are named and the liquid levels are indicated by triangles.
Source: Paper SPE 217989

Annular pressure buildup due to fluid expansion can be mitigated using a compressible fluid, typically nitrogen, as a cushion at the top of an annulus. The advantage of using a nitrogen cushion is that we do not have to manipulate annuli pressures as often during variations in production. A disadvantage is that it is more difficult to detect small leaks to or from an annulus. For gas lift-assisted production wells, Annulus A is used for the transportation of gas down to the gas lift valves, effectively making up a large gas cushion compared with the full length for the annulus. Considering this, monitoring annular pressures, and ensuring continuous control of fluid volumes are essential for effective well-barrier management.

We present relevant theory and show that we can track annuli liquid levels using distributed temperature sensing and/or distributed acoustic sensing data to detect leaks, estimate leak rates, and infer leak paths. We find that the main cause for observing liquid levels in these data is because the equilibrium temperature at the fiber is dependent on the fluid fill of the various annuli in addition to the temperature inside the tubing and outside of the well. Six data examples with variations in liquid level(s) are presented to demonstrate this. Furthermore, simple models for estimating changes in liquid levels are proposed and compared with liquid levels from distributed fiber-optic data. Being able to detect leaks to or from annuli makes it possible for the operator to apply mitigating action in a timely manner, prevent unwanted well integrity situations, and ensure production regularity.

This abstract is taken from paper SPE 217989 by Kjetil E. Haavik, Equinor ASA. The paper has been peer reviewed and is available as Open Access in SPE Journal on OnePetro.