About CCS

Injection & storage

Screening and ranking sedimentary basins

Different basins can be compared and ranked as suitable CO2 storage basins by considering the following factors.

Tectonic setting or seismicity

The tectonic setting or seismicity of a basin should be considered. Although some areas are seismically active, they also contain large petroleum accumulations which suggest that CO2 storage is possible, so they should not be eliminated as potential storage sites. Instead, the site characterisation should be undertaken with particular emphasis on the impact of seismic activity.

Basin size and depth

Basin size and depth give an estimate of the overall storage volume achievable. The sedimentary basin needs to be deep enough to store CO2 in a supercritical phase (a depth of approximately 800m is needed for this), but not so deep that injection well drilling would be excessively costly. Coal seams can be considered as storage at depths of 300-800m. Saline formations are suitable at depths between 800 and 3500m.

Faulting intensity

Faulting intensity influences the capacity and the containment of the site. Faults could seal individual reservoirs, so extensive faulting can break the reservoir into compartments.  Multiple wells may then need to be drilled to effectively use the storage capacity of the reservoir.

Hydrogeology

Hydrogeology describes the dynamic flow system in the reservoir rock and is used to assess the potential of confined but extensive saline formations for hydrodynamic trapping. To keep the CO2 in the reservoir for long enough to enable residual trapping, solution trapping or mineral trapping, the saline formation needs a slow flow rate and/or a long migration pathway. The permeability of the reservoir needs to be a balance between reasonable injection rates and a slow flow rate for the CO2 once in the saline formation.

Geothermal conditions

The geothermal conditions of a storage basin affect the density of the CO2. In colder basins, CO2 is denser, so that more CO2 can be contained in the same volume of rock.

Reservoir seal pairs

A suitable storage site will have good reservoir-seal pairs. The reservoir will have high porosity and good permeability, and the seal will have low permeability. One way to determine the possible existence of reservoir-seal pairs is through examining stratigraphic columns (click here to view diagram).

Evaporites provide the best caprock seals. They form when water evaporates, leaving behind a mineral sediment.

Coal seams and coal rank

Coal seams can adsorb significant amounts of CO2. Coalbed methane (CBM) production usually involves pumping groundwater from the seam to reduce the pressure and release the methane. In enhanced coalbed methane production (ECBM), a gas is used to displace methane from the coal bed. If the gas used is CO2, CO2 is stored by adsorption onto the surface of the coal. CO2 would only be able to be stored in coal seams that were uneconomic to mine because of their depth or quality. While coals at a greater depth have good adsorption capacity, they generally have low permeabilities.

Hydrocarbon potential

Rocks that are suitable for containing and producing oil and gas are likely to be suitable for storing CO2. The potential for storing CO2 will be dependent on the timing of possible hydrocarbon production.

Industry maturity

If there is a mature oil/gas industry in the area, there will be a larger amount of available information about the site. Most of the hydrocarbon and coal would have been discovered and there are likely to be depleted oil and gas reservoirs. Such areas are likely to have good infrastructure such as roads, pipelines and wells.

Location: onshore/offshore

Onshore CO2 storage sites have economic and technical advantages but may have land use and tenure issues.

Climate

Climate affects the surface temperatures, the depth of the water table and the ease of development of storage facilities.

Summary table of criteria for screening sedimentary basins.

Criterion

Increasing CO2 Storage Potential -->
Classes
1 2 3 4 5

1

Seismicity (tectonic setting)

Very high
(e.g. subduction)

High
(e.g. syn-rift, strike-slip)

Intermediate (e.g. foreland)

Low  (e.g. passive margin)

Very low (e.g. cratonic)

2

Size

Very small (<1000 km2)

Small (1000–5000 km2)

Medium (5000 –25000 km2)

Large (25000–50000 km2)

Very large (>50000 km2)

3

Depth

Very shallow (<300 m)

Shallow
(300–800 m)

 

Deep
(>3500 m)

Intermediate (800–3500 m)

4

Faulting intensity

Extensive

 

Moderate

 

Limited

5

Hydrogeology

Shallow, short flow systems, or compaction flow

 

Intermediate flow systems

 

Regional, long-range flow systems; topography or erosional flow

6

Geothermal

Warm basin (>40ºC/km)

 

Moderate (30–40ºC/km)

 

Cold basin (<30ºC/km)

7

Reservoir–seal pairs

Poor

 

Intermediate

 

Excellent

8

Coal seams

None

Very shallow (<300 m)

 

Deep
(>800 m)

Shallow
(300–800 m)

9

Coal rank

Anthracite

Lignite

 

Sub-bituminous

Bituminous

10

Evaporites

None

 

Domes

 

Beds

11

Hydrocarbon potential

None

Small

Medium

Large

Giant

12

Maturity

Unexplored

Exploration

Developing

Mature

Super-mature

13

Onshore/ offshore

Deep offshore

 

Shallow offshore

 

Onshore

14

Climate

Arctic

Sub-arctic

Desert

Tropical

Temperate

15

Accessibility

Inaccessible

Difficult

 

Acceptable

Easy

16

Infrastructure

None

Minor

 

Moderate

Extensive

Ranking prospective saline formations and petroleum reservoirs within a basin

Within a basin, sites be compared using a variety of factors.

Storage capacity

This is determined by looking at the size of the area, the thickness of the reservoir, the pore volume available and the density of CO2 at the relevant temperature and pressure.

>> More on estimating storage capacity

Injectivity potential

The permeability and porosity need to be considered to determine how easily the CO2 can be injected into the reservoir.

Site logistics

The site is examined for economic and technological feasibility. The distance of the CO2 source from the site will determine the length of pipeline needed. The depth of the storage reservoir will impact on the drilling depth for an injection well.

Containment

The likely effectiveness of a seal can be determined by its thickness, extent and lithology. Faulting size and intensity need to be considered. The migration distance should be considered particularly where structural trapping is absent.

Existing natural resources

The likely presence of oil, gas, coal or potable water and proximity to population centres or national parks will determine competing interests for a CO2 storage site.

>> Back to identification and selection of sites


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