New technique measures leaks in underground CO 2 storage sites

University of Groningen PhD student Charlotte van Leeuwen has developed a technique for measuring carbon dioxide (CO₂) leaks from underground storage sites. This is important for monitoring whether underground storage does in fact prevent greenhouse gases from entering the atmosphere. Van Leeuwen’s PhD ceremony will be held on 27 November 2015 at the University of Groningen.

Charlotte van Leeuwen began her research in 2010, when the government was exploring the possibility of storing CO₂, a greenhouse gas, in empty gas fields. In the Netherlands there was considerable concern among people living near possible trial sites and the plan for underground storage was quickly dropped from the agenda. Van Leeuwen’s research continued, however.

Trial sites

‘There are various trial sites in other countries, including the US. It is important to know whether the CO₂ actually does stay underground,’ says Van Leeuwen. The injection of CO₂ is an expensive business, and it simply wouldn’t be worth it if a significant portion of the gas soon ended up back in the atmosphere.

More detectors

It is not easy to detect a CO₂ leak into the atmosphere because the escaped gas quickly blends with its surroundings and there is already a fairly large natural variation in atmospheric CO₂ concentrations. This makes a single CO₂ sensor useless. But if several sensors are used and one of them gives an abnormal reading, it could point to a leak. To test this, Van Leeuwen placed five fairly simple detectors around an artificial CO₂ leak in a 70 x 70 metre grid. The method worked, but the detectors responded differently to changes in temperature, requiring various corrections. A drawback of the method is that it does not distinguish between a CO₂ leak and other sources of CO₂, such as burning fossil fuels.

Carbon dioxide and oxygen

A technique that measures the atmospheric concentration of both carbon dioxide and oxygen (O₂) can make this distinction, however. ‘In natural processes the variation in the CO_2- and O₂ concentration is of equal, but opposite, magnitude,’ explains Van Leeuwen. ‘Plants, for example, absorb carbon dioxide during the day and release oxygen. And when wood or coal is burned, CO₂ is released and O₂ is consumed.’ A CO₂ leak from a storage facility, however, causes a rise in the concentration of carbon dioxide, without a corresponding drop in the oxygen concentration.

Order of magnitude

This is not a simple technique because it requires a highly accurate measurement of the oxygen concentration. The atmospheric variations of both gases are similar, but the background concentration of oxygen is many orders of magnitude higher than that of carbon dioxide (21% or 210,000 parts per million (ppm) of oxygen against 400-500 ppm of carbon dioxide). ‘In relative terms therefore, a difference of 10 ppm in the CO₂ concentration is much more than the same difference in the oxygenconcentration.’

Centre for Isotope Research

The University of Groningen’s Centre for Isotope Research (CIO) is one of the few institutes in the world to have such accurate measuring equipment in house. ‘But that’s done with equipment that is fixed in place in a lab. My job was to make equipment that could be transported fairly easily. It worked. But because the measurements are so complicated, and a lot of equipment is needed, including several gas cylinders, it remains quite a tricky operation.’

Detection limit

By measuring carbon dioxide and oxygen at the same time, Van Leeuwen was able to measure a CO₂ increase of 6 ppm above the natural variation. If the precision of the oxygen measurements is improved still further, the detection limit could be brought down to 3 ppm. This would mean detecting a leak of 1000 tons of CO₂ per annum (32 grams per second) at a distance of 500 metres.

Even if CO₂ is not stored underground, the tool Van Leeuwen has developed will be useful for research purposes. Because the system is mobile, it can easily be used in measurement programmes temporarily requiring oxygen measurements.

PhD

Charlotte van Leeuwen will be awarded her PhD on 27 November at the University of Groningen for research that she conducted at the Centre for Isotope Research, part of the Energy and Sustainability Research Institute Groningen (ESRIG). Her research was funded by CATO2. The title of her thesis is Highly precise atmospheric oxygen measurements as a tool to detect leaks of carbon dioxide from Carbon Capture and Storage sites . Her supervisor is Prof. H.A.J. (Harro) Meijer.