Well Integrity Surveillance with High Sensitivity Temperature Distributed Sensing

Abstract

Increasingly well integrity monitoring is being done with fiber optic temperature and acoustic measurements. Historically temperature and acoustics have required two separate types of systems. Advances in fiber optic sensing have resulted in temperature measurements that can be conducted using an Interrogator Unit (IU) that is traditionally used for distributed acoustic sensing (DAS) acoustics analysis. The Rayleigh based Distributed Temperature (RDTS) measurements are sensitive to a 1/1000th of a degree and can be used to assess subtle integrity issues like leaks or problems in the various annuli of wells. It is this great sensitivity to small temperature fluctuations that is the focus of this paper. These DAS measurements have higher sensitivity than conventional DTS measurements and can illuminate previously unexplored wellbore dynamics. Various measurements with fiber optic cables were conducted with DAS IU's in different engineering and downhole settings. The DAS measurements in wellbore conditions were processed for temperature focusing in the low frequency content of the signals. The measurements were initially validated against other sensing tools like DTS (Raman) and PT gauges to ensure that the temperature changes measured by the DAS unit are representative of real thermal regimes. Measurements were taken from a well setting that included permanent fiber optic cable cemented behind casing. However, these same tools are deployed in wells where the fiber is installed attached to the production tubing hence reducing deployment costs. The DAS data processing was processed for temperature fluctuations by focusing on low frequency components where fluid regimes reside. Fluid movement within the wellbore was identified by small temperature changes recorded by the IU. These fluid dynamic processes were subtle and quiet enough that a traditional acoustic analysis was not able to identify anomalous zones. The resulting measurements were able to determine fluid velocities illuminated by temperature changes. We show how high sensitivity RDTS signals capture small temperature features and various fluid dynamic responses within the well not possible with other tools. The responses from the system clearly showcase quiet zones above which clear entry points of fluids can be identified as the origin of leaks.