TiO2 coated optical fibers for distributed real-time pH monitoring in wellbore conditions

Abstract

pH is a critical parameter for wellbore integrity and geochemical monitoring in wells for oil and gas production, CO₂ storage, H₂ subsurface storage, and geothermal systems. In situ real-time pH monitoring in subsurface wells is of significant value for wellbore integrity monitoring and predictive analysis of well component deterioration such as casing steel corrosion and cement carbonation. However, harsh environments in subsurface wells have limited many commonly used pH sensors. We have previously demonstrated optical fiber pH sensors coated with metal oxide-based sensing materials such as TiO₂, which offer stability at high pressures and temperatures. In this study, we demonstrated TiO₂ coated optical fibers for real-time distributed pH monitoring based on backscattered light interrogation. TiO₂ coated optical fibers were tested under ambient conditions and wellbore relevant conditions at elevated temperatures. TiO₂ coating was deposited on the optical fibers through a facile sol-gel method. TiO₂ coated optical fibers have shown promising pH sensing results under elevated temperatures and high pH conditions, making them suitable for wellbore cement monitoring.