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

Abstract

Summary Annular pressure buildup due to fluid expansion can be mitigated by using a compressible fluid, typically nitrogen, as a cushion at the top of an annuli. 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 (GLVs), effectively making up a large gas cushion compared with the full length for the annulus. In light of 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 (DTS) and/or distributed acoustic sensing (DAS) 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 (FO) 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.