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Application of optical-fiber temperature logging, an example in a sedimentary environment

by A. Förster,* J. Schrötter,* D. F. Merriam,+ and D. D. Blackwell,#
*GeoForschungsZentrum Potsdam,Telegrafenberg, 14473 Potsdam, Germany; + Kansas Geological Survey, The University of Kansas, Lawrence, Kansas 66047 USA; # Department of Geological Sciences, Southern Methodist University, Dallas, Texas 75275 USA


Continuous temperature-depth logs, especially when recorded in boreholes under thermal equilibrium conditions, provide detailed information of the subsurface thermal structure, which for example, is necessary for reliable heat-flow density determinations and rock thermal properties. In conjunction with independent thermal-conductivity determinations, thermal logging data also allow the separation of heat conduction effects from thermal convection effects by fluid flow driven by various pressure differences such as pore fluid pressure. The Earth's thermal field is related intimately to geothermal resources and hydrocarbon resources. Therefore, the characterization of temperature in the subsurface and its relationship to lithology is of critical importance.

Through the years, a broad range of thermal logging tools has been developed and refined to record subsurface temperatures in boreholes (Blackwell and Spafford, 1987). For the application of continuous, high-resolution temperature logging for scientific purposes, two major types of logging tools are used currently: (1) conventional "electric-line" systems with real-time surface readout; and (2) temperature sensing "slick-line" or "memory tool" computer systems connected by a solid cable, with no real-time data readout (Wisian and others, 1996). Each of the systems have advantages and disadvantages. For example in both of these types of systems a slow logging rate on the order of meters/minute is required to ensure that the equilibrium temperature is measured by the probe in the borehole. The need to transfer data to the surface via cable can be a source of error because insulation failure, variations in the cable resistance, or electromagnetic induction may affect the temperature readout (Beck and others, 1994). These types of error can be avoided using "memory tools", however in this situation the logging results are available only after the logging run, and therefore there is no quality control during the logging. Another disadvantage of the memory tools is that the temperature and depth are recorded separately and thus have to be related afterwards. For details on the use of conventional temperature recording equipment, the reader is referred also to Haenel and others (1988), Blackwell and Steele (1989), Jessop (1990), and Pfister and Rybach (1995).

Since 1981 there have been several efforts to introduce and apply a new temperature monitoring method using optical fibers (Gerges and others, 1988). The innovative technique of Distributed optical fiber Temperature Sensing (DTS) (Farries and Rogers, 1984; Dakin and others, 1985; Rogers, 1988; Hartog and Gamble, 1991; Boiarski, 1993) is based on the Optical Time Domain Reflection (OTDR) concept (Suzuki and others, 1984, 1986; Stone and others, 1985; Wanser and others, 1993. This technique uses the optical fiber as the sensing element with the intensity of the Raman back-scattered light of a laser pulse as a temperature dependent parameter. The method allows the measurement of temperature instantaneously along a fiber with an exact allocation of temperature to distance because of the known velocity of light propagation in the fiber. In terms of borehole logging, the DTS principle differs completely from the conventional temperature logging techniques and so has a broad range of potential applications that can not be realized easily with conventional methods. For example the technique has a great potential for monitoring dynamic systems because the temperature measurements are made simultaneously at all depths in the borehole.

The first application of the DTS technique for geoscience purposes was in 1992 (Hurtig and others, 1993). An optical-fiber was lowered in a borehole and a temperature log was obtained under steadystate borehole conditions. Another well-log application was in boreholes of the Grimsel Rock Laboratory of NAGRA (Switzerland) that focused on measuring the impact of injected warm and cold fluids on the temperature profiles (Hurtig and others, 1994).

The object of this note is to report the results of the application of the Distributed optical-fiber Temperature Sensing technique (DTS) in boreholes in a sedimentary environment to evaluate the accuracy and precision of the DTS method in the field by direct comparison to results obtained using a conventional logging technique. This study shows in particular to what degree the DTS technique can resolve the thermal structure in a well by measurements in a thin-bedded sedimentary environment with rapid vertical variations in thermal gradient and by a comparison of these results to conventional temperature logs. Previous borehole applications of the DTS method lacked such a borehole validation.

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Updated Sept. 30, 1996
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