3D Distributed Fibre Optic Sensing Characteristics of Well Leaks: A Dynamically De-Spooling Fibre Leak Library

Abstract Dynamically de-spooling fibre optic is discharged from a single use probe as it descends into the well through tubing or casing, increasing the accessibility of Distributed Fibre Optic Sensing (DFOS) leak detection surveys and data. Common well leak paths and their 3D Distributed Acoustic Sensing (DAS) and Distributed Temperature Sensing (DTS) characteristics are presented, providing a leak reference library to assist well Engineers extract maximum utility from DFOS well integrity surveys. Data acquired using dynamically unspooled fibre optic, which was used to plan integrity remediation or well decommissioning, illustrates leaks. 3D DAS and DTS leak characteristics, displayed as time, well depth and acoustic energy or temperature are visually represented with an accompanying leak description. Surface pressure data, cement bond logs, gas sampling and DTS are used to corroborate DAS leak characteristics to provide a leak diagnosis. Large continuous leaks and formation activity are observed as a depth zone of elevated acoustic energy. Contrastingly, small leaks resulting in as little as 20PSI sustained casing pressure, are intermittent at the leak location and are distinct in their graphical representation. Examples include gas from a plug intermittently leaking into wellbore liquid, where acoustic energy dissipates 1000m upwards in 45s. High-definition DAS data from a behind casing formation to wellbore leak and a tubing leak display as chevron type visual representations, with leak locations at the apex. The related energy can dissipate over 100m in a fraction of a second. Tubing or casing leaks adjacent to disposable fibre are graphically well defined with a qualitative assessment on relative leak severity possible. The axial direction of DAS energy dissipation in conjunction with known fluids, tubing pressure and annulus pressure is used to determine leak direction. Behind pipe DTS leak characteristics include distinct visual representations where liquid from out of zone injection or zonal cross flow is moving behind cemented production casing. Also captured in DTS data, a small gas leak through a casing plug shows Joules-Thomson cooling. Wireline well integrity diagnostic logs are common and widely understood whereas 3D DFOS well integrity surveys, that are increasingly accessible through dynamically unspooled fibre, require increased industry familiarity. The paper presents common DFOS leak characteristics, corroborated by widely understood diagnostic tools, increasing well Engineer confidence in DFOS well integrity survey interpretation.