| Title | DISTRIBUTED TEMPERATURE SENSING IN FRACTURED LOW-TEMPERATURE RESERVOIRS — LESSONS LEARNED IN NEW ZEALAND AND CENTRAL EUROPE |
|---|---|
| Authors | L. Janku-Capova, P. Dedecek, T. Fischer, R. Sutherland and J. Townend |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | DTS, fibre-optic cable, geothermal gradient, heat flux, permeable fractures |
| Abstract | DFDP-2B is a ~900 m-deep borehole drilled in 2014 in the hanging-wall of the Alpine Fault, in the South Island of New Zealand. PVGT-LT1 is a 2 km-deep hole drilled in 2007 in the Bohemian Massif, in the town of Litoměřice, Czech Republic. Both boreholes intersect foliated metamorphic rocks and are equipped with a fibre-optic cable that has been used for temperature monitoring and locating permeable fracture zones. By comparing data from the two boreholes we identify common problems and develop best practices for downhole applications of Distributed Temperature Sensing (DTS). DTS measurements are sensitive not only to intrinsic properties of the optical fibre and the measuring unit, but also to ambient temperatures surrounding the measuring unit. A drift may occur in the recorded data if the temperature of the logging unit is not uniform, and here we describe different corrections (linear, polynomial) to this drift. The resulting data are also sensitive to filtering and time-averaging. Results from the two boreholes suggest that setting a longer sampling interval in the DTS unit generates less noise, although longer durations of measurement (1 hour or more) and averaging can effectively reduce the noise. Our results highlight how different filtering techniques are appropriate depending on the scale of interest. For example, a 5 m moving window is useful for identifying individual fractured zones whereas a 100 m moving window enables zones of conductive heat transfer to be distinguished. We show that DTS can provide reliable continuous downhole temperature measurements in fractured metamorphic rocks with accuracy ±0.5°C and <5 m spatial resolution, provided that the temperature of the measuring unit is kept stable or appropriate corrections made to correct for drift. This is sufficient for monitoring dynamic temperature changes in boreholes and more convenient than conventional logging for acquiring data on an on-going basis. |