Downhole Temperatures from Optical Fiber
Send a pulse of light through a strand of optical fiber running inside a wellbore, and the well’s temperature profile reflects back. This is both the promise and reality of distributed temperature sensing in the oil field today. The many uses for this data are taking fiber-optic technology into the forefront of production monitoring and diagnostics.
George Brown Southampton, England
Oilfield ReviewWinter 2008/2009: 20, no. 4. Copyright © 2009 Schlumberger.
For help in preparation of this article, thanks to Matt Garber, Rosharon, Texas, USA; and Dominic Haughton, Technical Editing Services, Chester, England.
ACTive, THERMA and WellWatcher BriteBlue are marks of Schlumberger.
1. Leonardon EG: “The Economic Utility of Thermometric Measurements in Drill Holes in Connection with Drilling and Cementing Problems,” Geophysics1, no. 1 (January 1936): 115–126.
2. For a discussion of permanent downhole fiber-optic sensors: Al-Asimi M, Butler G, Brown G, Hartog A, Clancy T, Cosad C, Fitzgerald J, Navarro J, Gabb A, Ingham J, Kimminau S, Smith J and Stephenson K: “Advances in Well and Reservoir Surveillance,” Oilfield Review14, no. 4 (Winter 2002/2003): 14–35.
Temperature plays an important role in many downhole processes, and thermal measurements have long been used to monitor the performance of producing wells. Indeed, since the 1930s, engineers have used wellbore-temperature data for calculating flow contributions, evaluating water-injection profiles, diagnosing the effective - ness of fracture jobs, finding cement tops behind casing and identifying crossflow between zones.1 For many years, the popularity of this very basic measurement was largely overshadowed by other, more exotic measurements obtained through sophisticated suites of logging tools. However, the development of fiber-optic technology has helped spur a resurgence of interest in temperature measurements.
Though first used in the oil patch as a medium for transmitting data and commands, optical fiber has evolved into an intrinsic down hole sensor. During the 1980s, researchers in fiber optics developed a means for measuring temperature along the length of the optical fiber, and this technology was integrated into certain types of oil and gas completions by the early 1990s.2
Requiring no moving parts or downhole electronics, distributed tem per ature sensing (DTS) relies on a laser beam and a continuous strand of optical fiber to collect spatially distributed temperature data.
Rather than recording a single temperature snapshot during occasional and infrequent wireline logging runs, a fiber-optic DTS system can obtain extremely sensitive wellbore-temperature measure ments at regular time intervals along each meter [3.3 ft] of a well. This uniform sampling enables the DTS system to pinpoint the time and position of temperature changes as they occur, improving understanding of the processes that are taking place inside the wellbore. This article describes the workings of a distributed temperature sensing system and discusses how knowledge of a key principle governing the thermal behavior of oil or gas in wellbores helps engineers deduce what is happening downhole.
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