Scientists at Heriot-Watt University in Edinburgh have published cautionary research which claims the carbon dioxide (CO2) storage potential of the proposed Captain Sandstone site in the Moray Firth in Scotland has been overstated.

The paper, featured in November’s Interpretation, published by the Society of Exploration Geophysics and American Association of Petroleum Geologists, said how the uplift, tilt and resulting fracturing of the UK’s subsurface approximately 55 million years ago, during the Early Cenozoic age, could have dramatic implications for the country’s fledgling Carbon Capture and Storage (CCS) industry.

Professor John Underhill, Chief Scientist at Heriot-Watt University, said many geoscientists, including himself, believe geologic storage of CO2 offers a significant opportunity to arrest greenhouse gas emissions into the atmosphere and reduce our carbon footprint.

He explained, “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.”

However, research at Heriot-Watt has suggested it 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.

Underhill continued, “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. 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.”

Interpretation of the seismic data suggested 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.

The data also suggested 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.

The research concluded that despite its large storage capacity, the dip, subcrop and fault reactivation affecting it suggests that it lacks integrity as a potential storage site.

Counter argument

However, UK research group Scottish Carbon Capture and Storage (SCCS) have argued the connected reservoir and the subcrop at the seabed have been identified as beneficial.

In a statement, SCCS said, “They enable pressure to dissipate, reducing the risk of the injection site becoming over-pressured and improving CO2 storage capacity and security. Calculations show that without these features the Captain Sandstone formation could store around 360 million tonnes of CO2, but that with the subcrop the capacity for secure storage increases to over 1,550 million tonnes. That is the equivalent of increasing capacity from about seven years of all of Scotland’s CO2 emissions to over 30 years of emissions storage. These findings were reported in a March 2011 public report ‘Progressing Scotland’s CO2 storage opportunities’.”

“Industry has undertaken robust geological screening to ensure the integrity of any CO2 storage in the Captain Sandstone formation. An extensive research study undertaken with industry to investigate storage in two or more sites in the Captain Sandstone identifies that this formation contains six oil and gas fields, as shown in the September 2015 public report ‘Optimising CO2 storage in geological formations; a case study offshore Scotland’. This fact demonstrates containment of oil and gas over millions of years. In addition to the geological features that lead to oil and gas trapping (called stratigraphic trapping), CO2 is additionally retained in the deep subsurface by a combination of factors. These include the CO2 dissolving into the pore water, making the water denser and so liable to migrate downdip away from the outcrop, and by leaving a trail of microscopic micro-bubbles isolated in pores (called residual trapping).”

“It is very useful and timely to draw attention to the potential of CO2 storage in the Captain Sandstone, since the UK is lagging behind our counterparts in Norway, and will need to use CO2 storage to meet greenhouse gas emissions targets by 2050 in a cost effective manner.”

“Learning gained from the appraisal, monitoring and operation of this and other Norwegian storage sites has been applied to the assessment of the UK Captain Sandstone, and due to its location, existing infrastructure, and especially the suitability of the geology, it is currently the most promising prospect for CO2 storage in UK Continental Shelf.”