Robust and Cost Effective Distributed Coaxial Cable Temperature Sensor Verified as Real-Time Permanent Downhole Monitoring for Groundwater Safety in Geological CO₂ Storage

Downhole monitoring plays a crucial part in a geological carbon dioxide (CO2) sequestration project, especially in providing early warnings of failure. However, the current downhole monitoring technologies are either low in spatial resolution and time-consuming, or expensive and have system longevity issues. To address a robust and cost effective real-time downhole monitoring system, distributed coaxial cable temperature sensor is proposed using the Fabry-Perot interferometer technology. The coaxial cable sensor (CCS) is made in house and tested with a high pressure high temperature (HPHT) testing apparatus with water to study the sensor accuracy, sensitivity, long-term stability and crosstalk effect in simulated downhole conditions. The laboratory test results indicate that the sensor can work under simulated downhole conditions of pressure up to 1000psia and temperature up to 110 °C. At 1 ATM, the sensor has an accuracy more than 1%. At 1000 psia, the sensor sensitivity tends to be stable after repeated heating and cooling treatment. The hysteresis phenomenon is observed, but it is reduced after several treatment cycles. The pressure crosstalk effect is observed during the test. The temperature and pressure range of the distributed CCS allows a long-term in-situ monitoring for a surface casing well section up to 2500 feet, which would prove great value in detecting wellbore leakage that will contaminate the ground water