Greenhouse gas emissions from fossil-fuelled power plants and large industrial point sources such as cement plants or steel works contribute significantly to climate change. In order to limit global warming to 2°C above pre-industrial levels – a threshold that in the opinion of climate scientists would reduce the risks and impact of climate change – these emissions have to be drastically reduced as soon as possible. This is where a suite of technologies known as carbon capture and storage (CCS) may be of crucial importance. The idea of CCS is simple: capture the greenhouse gas carbon dioxide (CO2) where it is produced, for example at coal-fired power plants, before it reaches the atmosphere. Then transport the captured CO2 to storage sites where it can be safely held for long periods of time without being released into the atmosphere. However, CCS comes with challenges: the capture process has to be as cost effective and energy efficient as possible, transportation must be safe and fit for purpose, and storage sites have to be secure and, if possible, located close to the CO2 source. In the last few years, more and more large-scale capture facilities have become operational – for example, at the coal-fired Boundary Dam power station in Saskatchewan in Canada, where one million tonnes of CO2 are captured each year – and more are planned or are under construction. Transportation of CO2 via pipelines has been proven to be secure as pipelines for CO2 transportation have been around for decades, with the US having a pipeline network of more than 2000km alone. This shows that technological and engineering obstacles associated with capture and transport of CO2 can be overcome and that widespread rollout of capture facilities is possible in the near future. Scientists have proposed several ways to store the captured CO2, including storing it at the bottom of oceans, in volcanic rocks, in abandoned mine shafts and in porous sedimentary rocks located deep underground. The latter method is favourable for many researchers, as sedimentary rocks are widespread all over the world and they are well understood – mainly because they host most of the known oil and gas fields worldwide.

Carbon capture and storage sites

The key aspect for storing CO2 is to ensure that it is separated from the atmosphere for at least ten thousand years. At the Sleipner CO2 storage project in the North Sea, off the coast of Norway, CO2 has been injected into deep sedimentary rocks for close to twenty years, making it one of the oldest storage sites in the world. While Sleipner shows no indications for being an insecure storage site from which CO2 may be released to the atmosphere, how can we be certain that CO2 injected into the subsurface at Sleipner or other storage sites stays there for the thousands of years required? Leaks of CO2 from storage sites would not only contribute to greenhouse gas emissions to the atmosphere, but pose a health risk and undermine public confidence in CCS technology. In order to better understand what makes carbon storage sites safe and what may lead to migration of CO2 from the underground storage reservoirs to the surface, myself and fellow researchers from the Universities of Edinburgh and Strathclyde have studied naturally occurring underground reservoirs of carbon dioxide. What better way to understand what happens at storage sites on the long-term basis than looking at natural sites where CO2 has been stored for millions of years? Carbon dioxide can naturally accumulate in underground rock formations and remain trapped in excess of a million years and in some cases for tens of millions of years. The CO2 within these natural reservoirs was formed as a result of geological changes, volcanic activity, or from decayed plants and animals. The CO2 is held within the pore spaces of porous and permeable rock layers such as sandstones and impermeable rock layers above the reservoir stops any upwards migration. These natural CO2 stores are ideal analogues for the long-term behaviour of CO2 in engineered storage sites and many of them have been studied by researchers before. However, until now a study that compares naturally occurring CO2 stores on a global scale in order to identify key criteria for storing the greenhouse gas effectively has been missing. We studied 76 naturally occurring CO2 stores, of which the vast majority (66 reservoirs) has held CO2 securely in the subsurface for geological timescales. Six of the studied reservoirs show clear evidence of CO2 moving from the underground reservoir to the surface while the evidence for either migration or the successful retention of CO2 is inconclusive. Our dataset includes the geological characteristics of all reservoirs, such as the location and depth of the reservoirs, the type of rocks which form the carbon store, the type of rocks which form an impermeable seal above the store and thus prohibit vertical migration of the CO2, and the temperature and pressure of the reservoirs. Comparing and contrasting the secure and insecure reservoirs enabled the identification of geological conditions best suited for long-term CO2 storage.

CO2 retention in underground reservoirs

Successful retention of CO2 in underground reservoirs is governed by the complex relationships between reservoir depth, reservoir temperature and pressure, and the state and density of stored CO2. Both temperature and pressure rise with increasing depth while CO2 state and density is controlled by temperature and pressure. At subsurface conditions, CO2 will be either gaseous or in a supercritical state. Generally gaseous CO2 will be much lighter than the water found in rock pores while supercritical CO2 can be lighter to close to the same weight. This has implications as the lighter the CO2 the higher the buoyancy pressure that is exerted on the impermeable rock layer above the porous rock layer that acts as carbon store. If the pressure is high enough the sealing rock will fail and CO2 is released from the store to the rock layers above and eventually to the surface. At naturally occurring CO2 stores secure retention is more likely at reservoir depths deeper than 1200 metres, ‘normal’ temperature gradients and pressure close to hydrostatic pressure. This ensures that CO2 is stored in supercritical phase and has a relatively high density. It is beneficial if a reservoir has multiple, thick impermeable rocks to cap reservoirs as then a failure of the retention mechanisms is less likely. The highest risk of movement of CO2 from the underground reservoir to the surface occurs at faults, tectonic features which vertically cut through the rock layers, discontinuing the sealing rocks and creating high permeability pathways. All insecure naturally occurring CO2 stores failed to withhold in the underground because of upwards movement along faults. However, the nature of faults is very important as nearly half of the secure carbon stores also feature faults without impairing the safety of the storage site. The results of the study on natural CO2 stores show that the current screening methods for engineered storage sites would successfully identify only suitable reservoirs. Nonetheless, we have proposed some stricter screening rules to ensure that storage site selection is secure in the long term and we think that this will influence the selection and design of future CO2 storage sites. To read the full paper, published in the International Journal of Greenhouse Gas Control, click here. Johannes Miocic, PhD Student School of GeoSciences The University of Edinburgh Grant Institute James Hutton Road King's Buildings Edinburgh EH9 3FE e: w: