Experimental Validation of Fiber Bragg Grating (FBG) Measurement on Early Time Downhole Cement Stress

ABSTRACT: Accurately characterizing the cement stress at the early stages can affect the estimation of wellbore integrity. Laboratory characterizations on the early-time cement stress suffer limitations as the conventional measurements, i.e., using strain gauges, can hardly be applicable to the hostile underground environments. Instead, fiber optical sensing maybe an alternative measurement technology where the fiber can sustain the underground hot and corrosive environments. In this study, we attempt to validate the capacity of fiber bragg grating (FBG) for real-time cement characterizations. We design a prototype FBG cement-casing experimental assembly - mimicking the cement-casing in actual industrial practices. The validation test sets were using water as the media inside the casing. As such, strains caused by pressurizing the water can be theoretically predicted. We validated that the accuracy of FBG measurement achieves an expected error of approximately 2%. We further demonstrated the ability of FBG by applying it for actual measurement on the cement curing inside the casing. The early-stage transition and the development of anisotropic stresses have been successfully identified, combining the FBG and other measurements. Although these identifications have avoided using the output from strain gauges, FBG measurement is clearly affected by temperature changes. This study demonstrates the fiber optical sensing technology maybe a useful alternative for real-time monitoring in the underground hostile environment.