Robust and Cost Effective Distributed Coaxial Cable Sensors Verified As Real-Time Permanent Downhole Monitoring for Groundwater Safety in Geological CO2 Storage
Authors: Li Yurong , Nygaard Runar , Zhu Wenge , Xiao Hai 
Published: November 2015

A program for monitoring of Carbon Dioxide (CO2) distribution is required once injection begins in order to manage the injection process, delineate and identify leakage risk or actual leakage that may endanger underground source of drinking water, verify and provide input into reservoir models, and provide early warnings of failure. However, the current 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, a distributed coaxial cable temperature/strain system is proposed using the Fabry-Perot interferometer technology. The coaxial cable sensors were made in house by creating two reflectors on a flexible coaxial cable with a separation of 4 inch. A high pressure high temperature (HPHT) testing apparatus was fabricated to simulate downhole conditions. The sensors were tested with the HPHT testing apparatus with water to study the accuracy, sensitivity, long-term stability and pressure/temperature crosstalk of the sensor in simulated downhole conditions. A central composite experiment design and regression analysis were conducted for sensor calibration. The laboratory test results indicate that the pre-stressed coaxial cable sensors have a stable performance under simulated downhole conditions of pressure up to 1000psia and temperature up to 110 Celsius. The sensors show a relatively constant sensitivity after 5 heating and cooling treatment in the HPHT testing apparatus with each treatment lasting 48 hours. The hysteresis phenomenon was observed on the temperature sensor, which might be due to the residual stress on the coaxial cable, but it was reduced after each treatment cycle. The maximum measurement deviation of temperature is within ±1 Celsius over a 50 hour period. The strain sensor has the ability to measure strain up to 1%. The pressure crosstalk exists on the sensor, which was identified as an over measurement of temperature when the pressure was reduced. Further tests were conducted under varied pressure conditions and the result indicates that the pressure influence has different properties under different temperatures. To calibrate the sensor, a central composite experiment design and regression analysis were conducted and a regression model was developed. In addition, the test result shows that the sensor response time is instantaneous. The temperature and pressure range of the distributed coaxial cable sensors allow 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. As a novel downhole sensing technology, the low cost and robustness of the distributed coaxial cable sensors will not only lower the downhole monitoring cost, but will also enhance the monitoring system stability and longevity, which will provide continuous monitoring during each stage of well operation and throughout the lifetime of the well to provide input to reservoir characterization, wellbore stability analysis, fracture operation evaluation and production appraisal.