Resolving CO2 trapping and tracking CO2 migration (Flexible Funding 2018)

Stuart Gilfillan (PI) and Emma Martin-Roberts (PDRA), at the University of Edinburgh, were awarded funding in our Flexible Funding 2018 call to investigate “Resolving CO2 trapping and tracking CO2 migration in the Carbon Management Canada Field Research Station, Canada, using inherent tracing tools.”

Improvements in CO2 monitoring are needed to provide regulator and public assurance of secure CO2 storage, both during and following injection operations. The monitoring tools should allow the fate of the injected CO2 to be quantified and provide reassurance that CO2 is not leaking from the storage site.

The ability to identify buoyant and potentially mobile structurally retained CO2, and residual or solubility trapped CO2, is essential to understand how to engineer a secure storage site. However, this is difficult to measure and often poses the single greatest uncertainty in a CCUS project. To date, few studies have attempted to provide a full assessment of residually and solubility stored CO2 from within the storage site at reservoir scale. Furthermore, leakage of CO2 from a breached storage site may intercept shallow aquifers and escape to the atmosphere, which can be hazardous to both ecosystems and human health. Whilst artificial chemical tracers have proved to be successful at tracking CO2 injection at the pilot scale, these tracer compounds are expensive and their co-injection can be cost prohibitive at commercial scale storage sites, where millions of tonnes of CO2 will be injected over decades.

The aim of this project was to determine the fate of CO2 after its injection into the subsurface. This was achieved geochemically, using a combination of inherent noble gas and stable isotopes to quantify the amount of CO2 dissolved. This work also set out to establish the effectiveness of inherent tracers in identifying CO2 migration from a storage site.

We worked with one of the world’s leading research organisations focused on CCS – the Containment and Monitoring Institute of Carbon Management Canada – who operate a dedicated research facility focused on CO2 storage, their Field Research Station (FRS) located near the town of Brooks in Alberta. At the start of this project, the site had secured $9M of Federal and Provincial Government funding providing a unique opportunity to develop, refine and calibrate monitoring systems and technologies. The project also had the support of Natural Resources Canada, who undertake bi-monthly sampling of the shallow groundwaters above the FRS, and this programme of work complimented their ongoing monitoring operation.

As with any research that requires experimental work, the sampling campaign was impacted by COVID-19, meaning travel to Canada could not take place in the planned timeframe. However, work remained on previous samples that were obtained from the site for earlier projects (Figure 1), and we were able to apply the core aims of this research to the already collected, but unanalysed data. In collaboration with partners at the Scottish Universities Environmental Research Centre (SUERC), the remaining samples underwent analysis for noble gases (He, Ne, Ar, Kr, Xe) and major gas compositions including d13C and dD.

Copper tubes for gas samples

Figure 1: Copper tubes used to collect gas samples for stable isotope and noble gas analyses

The results of the work presented the first multi-well gas and groundwater characterisation of the geochemical baseline at the FRS. We found that all our sampled gases exhibited extremely low CO2 concentrations, implying that bulk gas monitoring may be an effective first step to identify CO2 migration. However, we also discovered pervasive biogenic methane (CH4) in both groundwater and gases within the stratigraphy, which contain numerous coal seams, making any upwardly migrating CO2 harder to monitor.

Results gained from the noble gas measurements implied the presence of a deep crustal flux (i.e., high values of radiogenic 4He) at the site. In contrast, the measured 4He concentrations in the shallowest groundwaters at the site were much lower, indicating 4He loss. We also found that the injected CO2 was low in helium, neon and argon concentrations, yet enriched in krypton and xenon (relative to argon). This key observation allowed us to assess the effectiveness of the specific isotopes of these gases as inherent noble gas fingerprints for geochemical tracing of the injected CO2 at the FRS. While specific to our research site, we were encouraged to learn that this modelling technique highlighted that, using certain noble gases, we were able to trace the signature of injected CO2 in background gas samples with high and confident resolution – an encouraging step for the future use of noble gases in providing assurance of CO2 storage security.

While the challenges of a global pandemic caused some delays and modification to the research, it also presented us with other unforeseen opportunities. It has allowed us to further analyse the already acquired data and apply this to previously published findings within the same sedimentary basin. We now have a basis for a follow-up study and related research paper focusing on the provenance of the economically important noble gases, such as helium, in the Alberta region.

Additionally, while waiting for key sample analysis we were also able to produce and publish a collaborative review paper highlighting the status and challenges facing carbon capture and storage over the last decade. The added time gave us an ideal opportunity to reassess efforts over the last decade and, importantly, to project and visualise the levels of dedicated geological storage required to meet future CO2 emission reduction targets.