Temperature sensing with DAS: A case study from a deepwater subsea producer

Abstract Time-domain phase-based distributed acoustic sensing (DAS) provides high-resolution measurements along the fiber length. Sensitive to changes in temperature and strain, a DAS system deployed in a subsea well is a well-documented way of obtaining a high-resolution seismic image. The industry's goal has been to create additional value for DAS beyond seismic imaging, such as extracting temperature responses from DAS measurements. This is particularly valuable as temperature changes can indicate fluid movement, out-of-zone injections, and wellbore integrity issues. Distributed temperature sensing that would normally be used to obtain temperature profiles faces challenges in deepwater subsea environments due to large step-out distances and poor signal-to-noise ratio, while temperature gauges offer only point measurements. DAS, however, can monitor temperature changes along the entire fiber length. The main challenge is that DAS measurements combine temperature and mechanical strain changes, leading to interpretation ambiguity. We investigate this ambiguity using continuous DAS data from a subsea producer recorded during an active seismic campaign. Exploratory analysis of long continuous DAS recordings and co-located temperature gauge measurements across different frequency bands leads us to focus on the low- and ultra-low-frequency range. We find that the DAS response in this study is primarily temperature-driven at 0–0.5 mHz, with a robust linear correlation (R2 > 0.95). Thus, DAS may reliably serve as a temperature proxy at these frequencies. Additionally, with temperature gauges as control points and a reliable initial reference depth profile, ultra-low-frequency DAS may be used for absolute temperature field estimation.