Early-Time Cement Stress Characterization using Conventional and Fiber-Optic Bragg Gratings

Summary Accurate characterization of early-time cement stress and behavior is essential for assessing wellbore integrity. Fiber Bragg grating (FBG) technology offers a robust solution for deployment in hostile downhole environments, where conventional strain gauge measurements are often infeasible. For this study, we designed a measurement strategy and validated the accuracy of FBG fibers for early-time downhole cement assessment under simulated laboratory conditions. A steel pipe and water/cement assembly were used to mimic the downhole cement curing process under controlled stress conditions. Using water as the testing material, we validated the accuracy of FBG technology and quantified its uncertainties, achieving an expected error of approximately 2%, which is even smaller than the expected error of 5% for conventional strain gauge measurements in the current tests. Both FBG and strain gauge measurements exhibited drift, although they manifested differently. While the FBG measurement drift diverged with loading history, the averaged FBG measurements remained accurate, which offers valuable insight into long-term deployment viability and suggests how to design robust field systems. In the cement tests, we observed the typical exponential decrease in early-time radial stress during cement setting. With a 20-MPa hydrostatic pressure, cement initial stress decreased to 6.5–8.3 MPa after 2,500–4,900 minutes. Additionally, the cement tests highlighted the influence of temperature on both strain gauge and FBG measurements. Results from this study demonstrate the advantages and accuracy of FBG technology for downhole wellbore integrity assessment and cement stress characterization, which could support the earlier detection of cement integrity issues and help reduce long-term intervention costs.