Appendices

SUMMARY OF STORAGE MECHANISMS AND SECURITY

Four mechanisms exist for trapping CO₂ in the subsurface. These mechanisms occur simultaneously upon injection but occur at different rates (Appendix figure 1). The relative contribution of each trapping mechanism – physical, residual, dissolution, mineralisation – changes with time and with a CO₂ plume’s evolution. In the initial decades of a standard storage operation, physical trapping of free-phase CO₂ is the primary trapping mechanism. Trapping of CO₂ is strongly dependent on a site’s geology and local formation conditions (in-situ fluids, pressure, temperature). A portion of the CO₂ plume may always remain in its free phase, but physical trapping is permanent when the geologic setting is stable and the CO₂ plume is behaving in the reservoir as predicted.

Appendix figure 1: (Lower panel) The four trapping mechanisms operating in the subsurface to permanently store CO₂. (Upper panel) Relative contribution of the four trapping mechanisms to permanent CO₂ storage through time. Each mechanism operates simultaneously upon CO₂ injection, but they occur at different rates.

Source: IPCC (2005)

PHYSICAL TRAPPING

Physical trapping occurs when buoyant, free-phase CO₂ migrates into a body of rock that has been folded or faulted into a subsurface structure (or “trap”), which closes in three or four directions, and is contained below a low-permeability caprock (or “seal”) (see Appendix figure 2). Physical trapping is the same mechanism that traps hydrocarbons in the subsurface. Appendix figure 2 illustrates types of physical traps, including independent folded rock bodies and fault-dependent folds (which rely on closure against a fault for CO₂ containment). In certain geological settings, physical trapping of CO₂ occurs when a reservoir thins laterally and ultimately pinches-out. This is called a stratigraphic trap and is shown at “E” in Appendix figure 2.

Appendix figure 2: Schematic illustration of physical traps in the subsurface. Circles show “spill points” or fault dependency of structural closures. (A) Residual trapping can be the dominant trapping mechanism in gently dipping (that is, relatively flat-lying) rock bodies that do not exhibit structural closure. (B) A fault-independent folded rock body (anticline) can trap buoyant CO₂ down to its “spill point”, below which CO₂ will migrate out of the folded trap. (C) A fault-dependent (extensional fault) folded closure relies on the juxtaposition of sealing lithologies across the fault plane to prevent CO₂ migration out of the trap. (D) A fault-dependent (contractional fault) folded closure relies on the juxtaposition of sealing lithologies across the fault plane to prevent CO₂ migration out of the trap. (E) A stratigraphic trap relies on lateral changes in lithology (often lateral stratigraphic terminations or “pinch-outs”) to prevent CO₂ migration out of the trap.

RESIDUAL TRAPPING

As a CO₂ plume migrates through a reservoir, a portion of the plume will become trapped in the pore space and micro-scale reservoir heterogeneities by capillary forces (see Appendix figure 1). This process is called residual trapping and is controlled by the connectivity between pores, pore throat size, reservoir lithology, and pre-existing pore fluid chemistry. Pores in suitable reservoirs are typically <1 mm in size, well connected, and often make up 10–30 per cent of the bulk rock volume. Buoyancy forces of the CO₂ plume are generally strong enough to overcome capillary forces in rock pores; however, along the margins and tail of a migrating plume, capillary forces are strong enough to “snap-off” small amounts of CO₂ from the plume. These small amounts of CO₂ are held permanently in pores against the surface of mineral grains. As the CO₂ plume migrates away from the higher pressures at an injection well, residual trapping becomes increasingly important. Although residual trapping occurs at the micro-scale, the mass of CO₂ trapped by this mechanism becomes significant at the reservoir scale (tens of metres of thickness and over an area of hundreds of square kilometres). Residual trapping contributes significantly to permanent storage in the early decades of a storage project.

DISSOLUTION TRAPPING

Dissolution trapping is a simple mechanism that occurs when injected CO₂ comes into contact with a brine and the CO₂ is able to dissolve into the brine solution. CO₂ solubility is dependent on brine salinity and the temperature and pressure conditions of a reservoir. A CO₂-saturated brine solution is denser than unsaturated brine and will sink in a reservoir. Dissolution trapping is considered permanently trapped. Over time, the CO₂-saturated brine diffuses and disperses within the regional hydrogeological system of the basin. Dissolution trapping happens immediately on contact, but only becomes a significant contributor to storage at decadal to century timescales.

MINERAL TRAPPING

Mineral trapping occurs when injected CO₂ chemically reacts with the minerals in a reservoir rock to form solid stable product minerals – often carbonate minerals. Mineral trapping is a permanent form of storage. Reaction rates and the mineralogy of product minerals depend on reservoir pressure, temperature, and reservoir mineralogy. Reservoirs targeted for CO₂ storage often have favourable conditions for mineralisation. Mineral carbonation begins immediately upon injection, but is generally a minor component of a storage project until thousands of years have passed. At this timescale, in a conventional storage reservoir, the majority of CO₂ will have already been permanently stored by the three mechanisms discussed above. However, injection into some rock formations (such as basalts) that contain reactive iron and magnesium minerals can result in rapid mineralisation of the majority of the CO₂ in as quickly as two years (2).

CO2 STORAGE RESOURCE CATALOGUE

The CO₂ Storage Resource Catalogue is a comprehensive global database of storage resources classified according to their commercial readiness using the 2017 Society of Petroleum Engineers Storage Resources Management System (SRMS). The purpose of the catalogue is to accelerate the commercial-scale development of CCS projects, build confidence in storage resource estimates, provide a consistent global picture of storage potential, and to establish the SRMS as a robust and authoritative reporting mechanism for storage resources. The catalogue is a six-year project funded by the Oil and Gas Climate Initiative, with technical assessments undertaken by the Global CCS Institute and Storegga. It is expected that by 2025, the catalogue will have assessed all countries across the globe.

The SRMS classifications are shown in Appendix figure 3. The Global CCS Institute in partnership with Storegga developed a series of guiding questions to help users classify their storage resources correctly. There are four major resource classes in the SRMS – these are Stored, Capacity, Contingent, and Prospective resources. Each class implies a different level of commercial maturity, with Prospective resources being the least mature and Stored being the most mature. Together, these make up the total storage resource base.

Appendix figure 3: The Storage Resource Management System classification system for CO₂ storage resources. Follow the question flow chart (blue boxes) to guide your resource classification.

Source: OGCI et al. (2022)

The third annual assessment cycle (“Cycle 3” in OGCI et al. (2022)) was completed in March 2022 and added approximately 1,000 gigatonnes of CO₂ (GtCO₂) of storage resources to the global resource base, which stands at 13,954 GtCO_2.

Appendix figure 4: Results from assessment cycle 3 of the CO₂ Storage Resource Catalogue

Source: OGCI et al. (2022)

Assessment cycle 3 increased the number of storage sites to 852 and the number of assessed countries to 30. Appendix figure 4 shows the total discovered and undiscovered storage resource. Just over 577 Gt of storage resources (or 4.1 per cent of the total global resource base) have been discovered – meaning they have been proven with subsurface data such as a well and seismic surveys. Unfortunately, only a very small fraction of the total global storage resource base can be considered commercial resources – just 253 MtCO₂ (or 0.002 per cent). Commercial resources must be ready for a storage operation to proceed and have:

• a legal and regulatory framework that enables CO₂ storage
• a thorough technical assessment and understanding of the storage complex
• a notional project development plan
• no significant barrier causing delay in development of the project.

The order of magnitude difference between sub-commercial resources and commercial resources suggests a significant opportunity exists to explore, develop, and appraise storage resources globally (Appendix figure 5). The CO₂ Storage Resource Catalogue can only use data in the public domain, so classifications in Appendix figure 5 likely underestimate resource commerciality because companies tend to keep their CCS project information private.

Appendix figure 5: CO₂ storage resources (which are associated with storage projects) by country and SRMS maturity class

Source: OGCI et al. (2022)

In February 2022, Santos became the first company to officially claim ownership of (or “book”) CO₂ storage resources using the SRMS system (4). It has booked 100 Mt of storage resources in the Cooper Basin of Australia ahead of its Moomba CCS Project, which has reached its final investment decision (FID). Santos booked nine Mt of 2P (proved plus probable) resource and 91 Mt of contingent (2C) resource.

REFERENCES

1. IPCC. IPCC Special Report on Carbon Dioxide Capture and Storage [Internet]. Cambridge; 2005 [cited 2022 Aug 31]. Available from: https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_wholereport-1.pdf
2. CarbFix. CarbFix: How it works [Internet]. 2022 [cited 2022 Jul 14]. Available from: https://www.carbfix.com/how-it-works
3. OGCI, Global CCS Institute, Storegga. CO2 Storage Resource Catalogue Cycle 3 Report [Internet]. 2022 Mar [cited 2022 Aug 29]. Available from: https://www.ogci.com/wp-content/uploads/2022/03/CSRC_Cycle_3_Main_Report_Final.pdf
4. Santos. Positioned for Success: Annual Report 2021 [Internet]. 2021 [cited 2022 Aug 8]. Available from: https://www.santos.com/wp-content/uploads/2022/02/2021-Annual-Report.pdf

*The Arthit project (Thailand) was added to the database after project number and capacities were finalised for this report and consequently this project is not included in relevant totals.