Closing an Information Gap in Geothermal Well Construction: Continuous Distributed Fiber-Optic Sensing for Improved Displacement Efficiency

Geothermal reservoirs require reliable well-completion techniques to reach well integrity. Well, integrity means there are no flow paths behind the casing at all. While constructing the well, the drilling mud must be fully displaced by uncontaminated cement, measured in displacement efficiency. Conventional real-time measurements show average pumping parameters to control the cement job's success. However, studies show issues with well integrity worldwide. We aim to improve well integrity by closing an information gap within the construction phase using continuous distributed fiber-optic sensing with dense spatial sampling. This study investigated the primary cementing of an 874 m surface casing in Munich, Germany. A fiber optic cable deployed behind the casing enabled the measurement of distributed dynamic strain rate (DDSS or DAS) and distributed temperature (DTS) during cement placement. We used field data from the cementing service, developed a fluid displacement model, and compared the results with those of the fiber optics. While we can trace only the rise of a cold front in the temperature data, the combined interpretation of DAS data shows features that allow comprehensive insights into the subsurface displacement process. We observed two rising velocities at a constant pumping rate. We were able to correlate them to the rise of different fluid interfaces. We conclude that the freshwater spacer does not displace the drilling mud but the first arrival of cement. Once the breakouts are filled, the succeeding cement seeks the path of least resistance without displacing cement in the breakouts again. Our findings suggest the possibility of tracking the rise of different fluids and the stability of their interfaces in real-time with distributed dynamic strain sensing. Having sensors along the borehole to track the displacement efficiency enables on-site reactions to ensure the cement jobs' success.