Applications of Distributed Fiber Optic Strain Sensing for Real-Time Wellbore Integrity Monitoring

The integrity of casing and cement is of utmost importance in order to increase the lifecycle and to improve safe operations of geothermal wells. This contribution focuses on the potential of real-time downhole monitoring techniques along fiber optic cables which are permanently installed behind casing. Distributed fiber optic temperature and strain sensing technology are used to measure thermal as well as load signatures during the completion of a low-enthalpy well for geothermal energy storage (Gt BChb1/2015, ATES Fasanenstrasse, Berlin, Germany). Gravel and cement pumping was monitored with distributed strain sensing. The pumping of gravel leads to a density change in the annulus which results in a measureable strain reading on the fiber optic cable. A simultaneous measurement with a gamma-gamma density log shows that the strain data from the fiber indicates the position of the gravel head in the annulus. In addition, a delayed consolidation of the gravel packing was monitored with the fiber. During cement pumping, it was observed that fluid shear stresses generate a measureable strain on the cable. The magnitude of these forces can be used to estimate rheological parameters such as fluid density and viscosity of the pumped medium. An experimental study was conducted to validate the field observations. Using distributed strain sensing, we can extract relevant downhole information (such as fluid/material changes) in real-time without interfering with the operational schedule of a well.