Temperature Logging by the Distributed Temperature Sensing Technique During Injection Tests

Abstract

The Distributed Temperature Sensing (DTS) technique using an optical fiber sensor is a relatively new method in temperature logging. Using the Optical Time Domain Refractometry (OTDR) technique and temperature dependency of the Raman backscattering light, it is possible to measure the temperature along the entire length of the optical fiber at an interval of ca. 1m at a time. Because it is not necessary to move point sensor during measurement, the DTS technique enables us to make simultaneous monitoring of the temperature profile of the well at a time interval of a few to several minutes. This feature of the DTS temperature logging system is suitable for detecting temporal change of the temperature profile of a geothermal well such as during injection and production tests. Recently, we introduced a new optical fiber sensor combined with a capillary tube, and are able to measure the temperature profile and pressure at the end of the sensor simultaneously. The major results of field experiments during injection tests at the Sumikawa geothermal area, northeast Japan, and the Sugawara geothermal area, southweat Japan, are as follows. 1. Locations of fractures can be detected clearly by the temporal change of the temperature. Fractures are more clearly detected during injection than recovery.. The DTS logging can show the fracture locations more clearly and easily than conventional temperature logging systems. 2. The water level can be traced by the DTS logging. The pressure profile of a borehole can be calculated from the water level and the temperature profile data. 3. Comparison between the calculated pressure value from temperature profile and the measured pressure value suggests that it may be possible to evaluate the amount or rate of inflow into the borehole from the reservoir.. To ensure this comparison is valid, precise calibration of the single-ended fiber sensor is required.