Analysis of distributed temperature and acoustic data to assess cementing of a surface casing in a geothermal well

Permanently installed fiber optic cables allow for distributed sensing of subsurface infrastructures. Preliminary studies show that distributed temperature sensing (DTS) and distributed acoustic sensing (DAS) data may be utilized to assess well integrity. The completion of a geothermal well in Munich included the installation of a permanent fiber optic cable in the annulus behind the surface casing. The cable was attached through clamps to the outside of the casing while running in hole, reaching a depth of roughly 700 m. This allowed the acquisition of DTS- and DAS-data during cement placement, wait on cement (WOC), and further operations.
Within this study, the fiber optic data is analyzed to assess cementing operation, cement curing, and cement sheath integrity. The temperature data provides for each meter a continuous measure of the released heat during hydration, whereas the acoustic data allows for an understanding of the wave field along the entire measurement. This wavefield and its changes are examined in the time and frequency domains to identify features, which are correlated with technical operations and physical properties of the curing cement. The goal is to extract distinct features, which are conclusive through various other forms of assessment like logging. Going beyond, distributed temperature and acoustic data may prove a viable solution to monitor the evolution of any well’s integrity over its lifetime.