UKCCSRC Knowledge Exchange Conference 2024 – Parallel 1b: CO2 Storage (ECR Meeting Fund)

Azibayam Amabogha (University of Glasgow) and Robyn Mottram (UCL) share their takeaways from “Parallel 1b: CO2 Storage” at the UKCCSRC Knowledge Exchange Conference 2024.

The first talk of the session was presented by David Healy, from the University of Leeds, and was titled ‘Pump it up? Quantifying the stress paths of depleted oil & gas reservoirs to prepare for CO₂ injection’. David Healy is a professor at Geosolutions Leeds within the Earth Sciences department. They work on Hamilton (East Irish Sea) and Viking A (North Sea) carbon storage sites, both of which are depleted oil reservoirs.

In the UK, proposed storage sites for CO₂ are in depleted oil reservoirs, which have previously housed oil and gas. Thus, they possess the storage capacity and properties required for long-term CO₂ storage. However, David pointed out that this may be a more complex challenge. He brought our attention to uncertainties surrounding the geomechanical/structural integrity of these reservoirs. David presented the uncertainties to us with the aid of Mohr circles: diagrams that show changes in stress within rocks.

If stresses within a rock increase to reach the failure envelope, fracturing occurs. Theoretically, adding fluid moves the Mohr circle towards the failure envelope, and removing fluid moves the Mohr circle away from the failure envelope. This is an important consideration for CO₂ injection into depleted oil reservoirs. David explained that when hydrocarbons are removed from the reservoirs, there is a significant change in effective stress and a decrease in minimum horizontal stress as pore pressure decreases. This can be represented by a Mohr circle growing bigger and moving away from the failure envelope over the life cycle of oil extraction.

Uncertainties arise when determining what happens if pore pressure is subsequently increased because of CO₂ injection. To reduce uncertainty, what happened to the reservoir rocks during hydrocarbon removal must be identified.  There are three possibilities: compaction, poroelasticity (isotropic) or normal faulting. Each scenario produces a different Mohr circle, but it is not currently possible to identify which scenario has occurred – this forms the crucial point of the research conducted by David’s group, which is investigating how this can be identified.

David highlighted that due to constant extraction of hydrocarbons and reduced pore pressure, the safe window for drilling in the southern North Sea and Irish Sea (where Viking A and Hamilton CCS sites are based) has narrowed. This trend can also be expected in other oil and gas reservoirs worldwide. Therefore, it is essential to understand these stress change processes better to ensure safe CO₂ injection.

In summary, the depletion of hydrocarbon reservoirs has likely changed the stress regimen, and injecting CO₂ will change this stress again, but there are uncertainties surrounding these changes.

The second talk was presented by Abbas Rajaei, from the University of Bradford. Abbas is researching Petrochemical Engineering and has been working in CCS for two years. Abbas’ talk was titled ‘CO₂ Sequestration in the Steel and Cement Industry Using Indirect Mineral Carbonation (IMC) Process’.

The steel and cement industries are considered hard to abate industries, with 1 tonne of steel producing 1.91 tonnes of CO_2, and 1 tonne of cement producing 0.8 tonnes of CO₂.  Abbas’ research aims to help reduce the carbon intensity of steel and cement production by locking the produced CO₂ in CaCO₃.

Abbas is conducting an experimental study of IMC using cement and steel wastes. The process is conducted at ambient pressure and temperature, so is not an energy intensive process. There are four main stages:

1. Extraction – Three acids (HCl, HNO₃ and H₂SO₄) are being tested for suitability in removing minerals from steel and cement wastes to form a mineral-enriched leachate.
2. pH swing – The leachate is titrated with NaOH to increase pH and remove impurities such as Fe, Al and Mg. The remaining solution is high in Ca.
3. CO2 capture – CO₂ is injected into the NaOH to produce Na₂CO₃. CO₂ absorption is calculated.
4. Carbonation – Na₂CO₃ is added to the Ca-rich solution, resulting in the formation of CaCO₃ and locking up the CO₂.

At present, this method allows for 39.5% of CO₂ emitted from steel production to be locked up in CaCO₃ after 40 days. Abbas and his team are investigating how to optimise this process.

Similar to the previous speaker, the session’s final speaker, Zongqian Ren, was also working on enhancing carbon sequestration via carbonation, albeit by taking a direct approach.  Zongqian is a second-year PhD researcher at the Geoenvironmental Research Centre, University of Cardiff. His talk was on “Enhanced Carbon Sequestration via Carbonation of Biochar-Steel Slag Composite”.

Zongqian proposed using biochar as a catalyst to enhance the carbon sequestration capabilities of steel slag. Steel slag is a waste material generated during the manufacture of steel. Its indiscriminate disposal can result in a slew of environmental problems. The speaker pointed out, however, that despite its reputation as a waste material, steel slag can be a good candidate for carbon dioxide sequestration due to the presence of several calcium mineral phases. An example would be the reaction between calcium silicate and CO₂ to form a stable solid product like calcium carbonate (CaCO₃). This aspect of the speaker’s talk emphasises the importance of a circular economy where waste materials are repurposed into valuable resources while simultaneously working to reduce further environmental problems.

Zongqian discussed that despite its vast potential, the low carbonation degree of crystalline steel slag limits its efficacy for carbon sequestration. Thus, to manage this, his research aims to use biochar—a byproduct from the pyrolysis of biomass—as a catalyst to enhance the carbonation degree of steel slag. According to Zongqian, the benefit of this, beyond the presence of stable carbons, is that the biochar due to its high porosity can provide additional diffusion channels, allowing for gas transport to reactive sites within the steel slag during the carbonation reaction process.

The process of achieving the enhancement discussed in the talk involves three iterative steps: mixing, carbonation and oven drying.

After the experiment, Zongqian reported that adding biochar significantly improved the CO₂ sequestration capabilities of steel slag, with approximately 20 wt.% biochar yielding a 57% increase in CO₂ adsorption. Furthermore, the speaker discussed that the benefit of this mixture extends beyond the scope of carbon sequestration alone as they found that the binding effect of the two materials enhanced phosphorus removal and was effective in pH regulation as well as reducing conductivity compared to steel slag alone. This potential points towards effectively using the bonded material for wastewater treatment. Other use cases discussed by the speaker include soil remediation and cementitious materials (lightweight aggregates).

The parallel session on CO₂ storage highlighted some challenges and innovative solutions being researched in the carbon storage domain. It encompassed the complexities of stress regimes in depleted oil reservoirs discussed by David Healy, and the reuse of industrial wastes in the talks given by Abbas Rajaei and Zongqian Ren. Ultimately, the session underscored the importance of interdisciplinary approaches in managing the multifaceted problems associated with CO₂ sequestration.