A downhole CO2 sensor can continuously collect real-time data about CO2 movement and concentration changes at subsurface conditions. These data are very valuable for better understanding of subsurface uncertainties and quality-controlling theoretical studies such as reaction, transport, and mechanics in oil and gas formations. This paper describes the development of a downhole CO2 sensor tested under high pressure and reservoir conditions to monitor aqueous CO2 concentration change. The CO2 sensor developed is a Severinghaus-type sensor, which includes a metal-oxide electrode, a gas-permeable membrane, a porous steel cup, and a bicarbonate-based internal electrolyte solution. The CO2 sensor thus prepared 0.7 in. in diameter and 1.5 in long. A linear correlation was observed between a change in sensor output potential and dissolved CO2 in water under 1,000 psi pressure. CO2/brine coreflooding tests were performed to simulate the CO2 storage process and the sensor was deployed to monitor CO2 movement. The results indicated that the CO2 sensor could monitor CO2 movement in-situ in CO2 storage processes.
Geologic sequestration of CO2 involves putting CO2 into long-term storage in geologic zones at subsurface conditions . Such sites as deep saline aquifers and unmined coal seams onshore, and depleted oil or gas formations both onshore and offshore have been recommended for further serious consideration . Thus far, in various regions of the world (Pacific Ocean, Gulf of Mexico, North Sea, Chinese East sea, and the Atlantic Ocean), a large part of research studies and pilot projects have looked at the feasibility of geological sequestration of CO2 . The first commercial project occurred in Norway in 1996, in which CO2 was captured from natural gas streams and around 1 million tons of CO2 per year were into the Utsira formation  and to provide insight into CO2 migration . All these pilot studies, located in Kansas, Virginia, West Virginia and Canada, are either under consideration or have been initiated.