In steam assisted gravity drainage (SAGD), temperature instrumentation is widely used to understand steam placement across openhole liners in producer and injector wells. The accuracy of temperature readings is crucial when monitoring wellbore integrity and optimizing production. While distributed temperature sensing (DTS) on fiber-optic cables has been widely used in temperature monitoring, challenges consistently exist regarding accuracy.
Several trials over a 2-year period were completed to quantify the accuracy of DTS systems. Each trial employed different operating conditions and instrumentation:
Case Study 1: Dual-laser DTS and thermocouples were permanently deployed in a newly drilled SAGD producer. The well was heated from geothermal to 240°C. Newly deployed thermocouples were compared against newly deployed fiber-optic cables.
Case Study 2: Dual-laser DTS temperature measurements were compared across a 1 dB point loss at a maximum optical budget of 13 dB.
Case Study 3: Dual-laser DTS vs. single-laser DTS vs. new thermocouples comparisons were made.
Case Study 4: Thermocouples and dual-laser DTS for a mid-life SAGD injector and producer were compared for accuracy after two years of operation.
Case Study 5: An independent temperature log containing dual-laser DTS was used to quantify the thermocouple temperature discrepancy on a SAGD producer in operation for 7 years.
The results of these case studies allowed Devon to determine several important learnings, as well as a troubleshooting methodology for evaluating DTS accuracy. When properly installed and calibrated, dual-laser systems are accurate up to ±2°C over the full range of SAGD operating temperatures for stable operating conditions. During the trial period, DTS was shown to maintain accuracy for 2 years.
When comparing the accuracy of temperature systems, it is recommended to compare against other known metrics during falloffs. New thermocouples are still considered the most accurate measurement; however, legacy thermocouples are susceptible to drift. Independent temperature logs have proven to be a valuable tool for assessing legacy instrumentation accuracy for thermocouples and DTS.