Scientists at Heriot-Watt University have published cautionary research which concludes that the carbon dioxide (CO2) storage potential of the proposed Captain Sandstone site in the Moray Firth has been significantly overstated.
The paper, published in Interpretation, shows how the uplift, tilt and resulting fracturing of the UK’s subsurface approximately 55 million years ago (Early Cenozoic) has dramatic implications for the country’s fledgling Carbon Capture and Storage (CCS) industry.
Professor John Underhill, the University’s Chief Scientist, said: “Many geoscientists, including myself, believe geologic storage of CO2 offers a significant opportunity to arrest greenhouse gas emissions into the atmosphere and reduce our carbon footprint.
“Previous CO2 storage studies have primarily focused on the use of subsurface reservoirs in depleted oil and gas fields or regionally extensive saline aquifers. Given the perceived scale of the challenge and the amount of CO2 that needs to be sequestered to stabilise or reverse emission levels, the geological focus has largely been on regional saline aquifers because of their lateral continuity, gross rock volume, and large storage capacity.
“However, it is essential that the right site is chosen to prove the potential of this technique and demonstrate that CO2 can be safely stored and will not leak to the surface. Poor site selection and gas leakage will undermine the credibility of geological storage.
“For the past seven years, the Captain Sandstone saline aquifer, which lies buried beneath the Moray Firth, has been widely hailed as having the potential to store between 15-100 years of CO2 output from Scotland’s power industry.
“Our research concludes that this is the wrong exemplar to choose because the tilt of Britain leads to it rising to subcrop the seabed with few barriers to arrest gas escape and has caused fault reactivation. If leakage occurs, which the geology suggests it will, then the case for CO2 storage will be weakened and potentially undermined.
“We should be focusing attention on sites where CO2 has been found already as that proves that the trap and seal works on geological time scales. Although these sites are the exploration disappointments of the oil and gas companies, they present a clear opportunity for carbon storage.”
The interpretation of the seismic data highlighted that the 120 Million year old Lower Cretaceous Captain Sandstone Member is a continuous, interconnected reservoir that rises to subcrop in the western areas of the basin as a result of Early Cenozoic uplift and tilt. The aquifer therefore forms an open system with few barriers or sizable closures to arrest or entrap light fluids and gases en route to the western subcrop.
Our findings lend further weight to the importance of greater investment in geoscience research and education and of undertaking robust and forensic geologic screening of any prospective storage sites prior to injection.
The data also indicates that the saline aquifer is cut by several west-southwest/east-northeast-striking reactivated normal faults. Although migration along the faults permitted hydrocarbons to get into structurally elevated traps, such as the Captain Field itself, some faults also breach the seal of the Captain Sandstone Member aquifer, rise to the seabed and increase the risk of seabed leakage. Despite its large storage capacity, the dip, subcrop and fault reactivation affecting it suggests that it lacks integrity as a potential storage site.
The research was funded by the Scottish Overseas Research Scholarship Award Scheme (SORSAS) and co-authored by PhD student Gustavo Guariguata-Rojas.