Economic Potential of DACCS and Global CCS Progress
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.
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GLOBAL CCS
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