Post-calibration of DTS data and analysis of well completion process monitoring in CO2 geological storage demonstration site

Abstract

The DTS temperature profiles obtained during well completion of Koreas first CO2 monitoring well in Janggi, Pohang were analyzed to characterize each well completion process in terms of temperature anomalies and to evaluate the feasibility of DTS as a monitoring tool of well development process. Since DTS temperature, a measure of temperature effect on a scattering of light traveling in fiber, should be calibrated to real temperature and unavoidable depth mismatch during installation should also be corrected, several post calibration methods of depth and temperature are also investigated.

The unavoidable depth mismatch is corrected by applying conventional approach which subtracts the ratio of cable length error to TD (Total Depth) of well from each measurement depth, and confirmed its effectiveness by comparing the result with the location of transition point of steel and FRP (Fiber-Reinforce Plastic) and its effect on temperature change pattern during gas lift.

For temperature correction, several point-wise methods and conventional long-point pre-calibration method are investigated to suggest an effective post-calibration method. The conventional method by submerging a portion of cable longer than the length of resolution cell defined by pulse width or a gauge length of a measurement system in water showed preferable result than the other methods. However, point-wise methods which gave reasonable and comparable results with long-point method also showed that it could be a useful alternative when a long-point method-based pre- & post-calibration cannot be done.

The real-time DTS monitoring data revealed that each well completion processes can be characterized by their own distinctive temperature anomaly patterns. In gravel packing, the progression up the well of the emplaced sand was noticeable as an anomaly and creating a thermal bridge to the warmer formation. The DTS data clearly shows the exothermic reaction associated with curing of cement within the cementing boundary. During gas lift operations, the conventional highly oscillating pattern was observed in the DTS data, but in our case, the casing transition point was delineated. In summary, these imply that real-time DTS monitoring could not only be an effective tool of monitoring but also controlling, and optimization of well completion procedure.