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This paper describes how a downhole CO 2 chemical sensor was prepared for use in determining the concentration of CO 2 in water, particularly in harsh, high-pressure environments such as those found in geological formations considered for the geologic storage of CO 2. The sensor consisted of a pair of Ir/IrO x and Ag/AgCl electrodes, a gas-permeable membrane, a bicarbonate-based internal electrolyte solution, and porous stainless steel as supporting material. The sensor was tested in the solution with different concentrations of CO 2 and displayed very good CO 2 sensing performance under pressures of 1000, 2000, and 3000 psi. A linear relationship between the sensor response potential and the logarithm of the CO 2 concentration was obtained for the sensor under different pressures. Reproducibility of the sensor was examined and the results indicated that the sensor displayed excellent reproducibility. CO 2 /brine core flooding tests were carried out to evaluate the performance of the CO 2 sensor in simulate CO 2 storage process. The results indicated that the sensor could detect CO 2 movement in the tests. Further studies showed that the sensor could be reused by brine flooding after CO 2 /brine flushed the core. The results of the core flooding tests demonstrated that the sensor had potential application for CO 2 monitoring in carbon storage.

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