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Quantifying South Africa's carbon storage potential using geophysics

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Commentary
S Afr J Sci 2012; 108(9/10)
hp://www.sajs.co.za
Quanfying South Africa’s carbon storage
potenal using geophysics
Author:
David Khoza1,2
Aliaons:
1School of Cosmic Physics
Dublin, Dublin Instute
for Advanced Studies,
Dublin, Ireland
2School of Geosciences,
University of the
Witwatersrand,
Johannesburg, South Africa
Correspondence to:
David Khoza
Email:
davidkhoza@cp.dias.ie
Postal address:
5 Merrion Square, Dublin 2,
Dublin, Ireland
How to cite this arcle:
Khoza D. Quanfying South
Africa’s carbon storage
potenal using geophysics.
S Afr J Sci. 2012; 108(9/10),
Art. #1197, 2 pages.
hp://dx.doi.org/10.4102/
sajs.v108i9/10.1197
© 2012. The Authors.
Licensee: AOSIS
OpenJournals. This work
is licensed under the
Creave Commons
Aribuon License.
Along with many other nations, South Africa faces the challenge of curbing carbon emissions,
as coal-red power plants generate 92% of the total electricity used. The result of this generation
is the release into the atmosphere of 400 million tonnes of CO2 annually, which contributes to
the greenhouse gases that have a detrimental effect on global climate. Given the length of time
needed to implement renewable energy sources, an alternative solution to signicantly reduce
greenhouse gas emissions is to capture the CO2 produced by coal-red power stations and
store it in geological formations in the subsurface – a process broadly called carbon capture and
storage. In order to successfully achieve this sequestration, a mechanism must exist to monitor the
behaviour of the CO2 injected into the earth.
Accordingly, in 2009 the South African government established the Centre for Carbon Capture
and Storage,1 a division of the South African National Energy Development Institute. The Centre
is tasked with the research and technical development of carbon capture and storage. The
establishment of the Centre was followed in 2010 by the publication of an atlas1 which identies
and ranks potential CO2 storage sites, mostly in Mesozoic basins along the coast (Outeniqua,
Orange and Durban/Zululand basins), and to a lesser extent the Karoo Basin (Figure 1). The
theoretical study that led to the production of the atlas was based largely on a literature review
of all available boreholes and other geological information. What is needed is a more quantitative
way of imaging potential storage sites, and I aim to address this need here.
The main challenge in carbon capture and storage is identifying those localities and geological
settings within South Africa which have the greatest potential to store signicant volumes of
CO2. The capture and storage of carbon is analogous to how oil and gas are naturally trapped
in underground formations. Thus an ideal storage site must comprise a porous and permeable
medium (e.g. sandstone) where CO2 can be injected and stored, overlain by an impermeable cap
rock (e.g. shale) that will retain (by dissolution or adsorption) the injected material and prevent
it from moving or escaping into the atmosphere. Geological storage options for deep injection
of CO2 include depleted oil and gas formations, deep unmineable coal seams and deep saline
formations - the last offering perhaps the most potential in the South African context.
Page 1 of 2
FIGURE 1: Map showing the distribuon of potenal onshore and oshore CO2 storage sites and the locaons of magnetotelluric
staons in the Karoo Basin.
0 50 100 200 300km
Beaufort West
Lesotho
Johannesburg
Potenal CO2 storage
sites in Karoo Basin
Potenal CO2 storage sites in
oshore Mesozoic Basin
24°0’O”E 30°0’O”E
30°0’O”S
30°0’O”S
24°0’O”E 30°0’O”E
Magnetotelluric (MT) sites
Possible storage sites in the Main Karoo Basin
Drakensberg group_Basalts
Clarens formaon
Elliot formaon
Molteno formaon
Dwyka Group (Diamicte)
Sandstone and Mudrock-Rhythmite
Beaufort group
Whitelhill fm (Carbonaceaous Shale)
Mudrock-Rhythmite
Commentary
S Afr J Sci 2012; 108(9/10)
hp://www.sajs.co.za
One of the potential storage sites identied2 is in the Karoo
Basin (Figure 1). Given the reported low permeability and
porosity of the Ecca Group in the Karoo Basin, the potential
for CO2 storage in the region has been inferred as low1; more
quantitative work needs to be undertaken to determine
if this is the case. To this end, I am using a geophysical
remote sensing method, magnetotellurics (MT), to provide
quantitative estimates as to the storage potential of the Karoo
Basin. The MT technique is a deep imaging geophysical
technique whereby naturally occurring electric and
magnetic elds (in the frequency range 1000 Hz – 0.001 Hz)
are measured on the surface of the earth to determine the
resistivity structure of the subsurface (from a few hundred
metre to tens of kilometres). The resistivity of a rock formation
is a function of four parameters, (1) the porosity of the rock
that is occupied by a uid; (2) the degree of interconnection
of the uid; (3) the resistivity of the host rock; and (4) the
salinity of the groundwater. Thus, by knowing the resistivity
of a geological formation we can, in principle, determine
the rock properties (porosity and permeability) needed for
reservoir characterisation using Archie’s Law.2 This principle
is illustrated in Figure 2, which shows a porosity–resistivity–
salinity nomogram that can be used to estimate porosity from
bulk resistivity measurements. Thus, if one has temperature
and salinity measurements (for example from boreholes)
and resistivity from MT results to plot on a nomogram,
connecting the points with a line of best t would yield the
resistivity of the pore uid, which could in turn be used to
estimate porosity.
In southern Africa we have collected over 750 MT sites as part
of the highly successful Southern African Magnetotelluric
Experiment,3 in order to study the crustal and mantle structure
of the region. Figure 2 shows an example of a resistivity
response from one MT site in the Karoo Basin, which was
derived from the processing of recorded electromagnetic
responses. The two curves essentially represent apparent
resistivity variations as a result of induced electrical
current ow in directions parallel (transverse electric) and
perpendicular (transverse magnetic) to the north-west to
south-east prole. The abscissa represents the period in
seconds (the inverse of frequency in Hz) which is a proxy
for depth in kilometres. One can use the MT responses like
these collected in the Karoo Basin to characterise one of the
potential CO2 storage sites shown in Figure 1. The MT sites
are spaced at intervals of approximately 10 km – 15 km along
a north-west to south-east prole. The acquisition of MT data
is usually done along two-dimensional proles, and at each
site horizontal variations in electric and magnetic elds are
recorded, using non-polarising electrodes for the former
and magnetometers for the latter. For optimal resolution of
geological formations such as in the Karoo Basin, much more
detailed data is required. It is hoped that upon successful
characterisation of onshore storage sites using MT, data
acquisition will be extended to offshore basins.
Acknowledgements
The South African Centre for Carbon Capture and Storage is
thanked for funding this ongoing PhD study. Prof. Alan Jones,
Dr Mark Muller (both from the Dublin Institute for Advanced
Studies) and Dr Susan Webb (University of the Witwatersrand)
are thanked for their supervision of the project.
References
1. Cloete M. Atlas on the geological storage of carbon dioxide in South Africa.
Pretoria: Council for Geoscience; 2010.
2. Archie GE. The electrical resistivity log as an aid in determining some
reservoir characteristics. Pet Trans AIME. 1942;146:54–62.
3. Jones AG. Area selection for diamonds using magnetotellurics: Examples
from southern Africa 2010 [homepage on the Internet]. No date [cited
2012 May 10]. Available from: http://www.geophysics.dias.ie/projects/
samtex/Home.html
4. Botha J. F, Woodford AC, Chevallier LP. Hydrogeology of the Main Karoo
Basin: Current knowledge and research needs. WRC Report No TT 197.
Pretoria: Water Research Commission; 2003.
Page 2 of 2
FIGURE 2: (a) An example of magnetotelluric (MT) data showing the transverse electric (TE) and transverse magnec (TM) apparent resisvity and phase responses
ploed against increasing period, the laer being a proxy for increasing depth. (b) The porosity–resisvity–salinity nomogram is used to esmate porosity percentage
from bulk resisvity measurements.
104
103
102
101
100
180
90
0
-90
-180
Apparent resisvity (Ohm-m)Phase (degrees)
102 10-1 100 101 102 103
Period (s)
Typical MT response
Rock resisvity (Ohm-m)
Formaon factor
Temperature, cengrade
Water resisvity (Ohm-m)
Equivalent nacl, ppm
0.1
1.0
5.0
10
50
1000
500
0.5
10
100
1000
1
0.1
0.01
0.05
0.5
1
5
10
20
100 100 000
50 000
10 000
5000
500
1000
300
20
30
40
50
250
200
150
100
TE
TM
ab
formaon factor
resisvity of a saturated rock
resisvity of the pore uid
porosirty, m cementaon factor (varies between 1 and 2)
Archie’s Law
= F = Ø-m
F
Ø
ρO
ρW
ρO
ρW
... In view of this, Surridge and Cloete (2009) highlight that South Africa is investigating carbon capture and storage as a greenhouse gas mitigation measure, with a significant amount of sequestratable carbon dioxide emissions coming from the synthetic fuel industry. Khoza (2012) emphasises the need for monitoring the behaviour of injected CO2 in geological formations for successful carbon sequestration. ...
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... New methods for exploiting subsurface resources for energy purposes aside, the evolving nature of the energy industry also creates new uses for the subsurface. Energy generation results in substantial carbon emissions, especially in South Africa, where 92% of total electricity comes from coal-fired power plants, 180 which emit approximately 400 million tonnes of carbon dioxide (CO 2 ) annually. 181 Coal also appears to be a driving force for South Africa's energy economy in the foreseeable future. ...
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Atlas on the geological storage of carbon dioxide in South Africa
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It is apparent that South Africa will remain reliant on fossil fuels for the next few decades until other energy sources can gradually replace these high-carbon fuels. CCS is therefore regarded as a transition measure from fossil fuels to renewable/nuclear energies. Storage opportunities may be sought for smaller or shorter-term point sources, but also for larger or longer-term point sources. For example, a project that emits 1 Mt of carbon dioxide per year for 20 years would require proven storage in the order of 20 Mt (0.02 Gt), while a project that emits 30 Mt of carbon dioxide per year for 50 years would require proven storage of the order of 1 500 Mt (1.5 Gt). A previous study into South Africa's potential for storing carbon dioxide was undertaken at the behest of the then Department of Minerals and Energy by the CSIR and released during 2004. At a theoretical level, the study determined that South Africa could have substantial storage capacity in geological formations. 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Area selection using magnetotellurics examples from Southern Africa.
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Full-text available
9th International Kimberlite Conference, 2008 The electrical conductivity of the continental upper mantle is highly sensitive to ambient temperature (e.g., Jones et al., 2008; Ledo and Jones, 2005), to iron content (Mg) (Jones et al., 2008), to the presence of an interconnected conducting phase, such as a solid phase like graphite or sulphides (e.g., Duba and Shankland, 1982; Ducea and Park, 2000; Jones et al., 2003) or a fluid phase like partial melt (e.g., Park and Ducea, 2003), or to bound water through hydrogen diffusion (e.g., Hirth et al., 2000; Karato, 1990; Karato, 2006). Given these sensitivities, deep-probing magnetotellurics (MT) can aid in area selection for potential diamondiferous prospective regions by mapping regions of deep lithospheric roots and by mapping regions that possibly contain high quantities of carbon (Jones and Craven, 2004).The Southern African Magnetotelluric Experiment (SAMTEX) project is imaging the three-dimensional regional-scale geometry of the electrical conductivity of the continental lithosphere below southern Africa using the natural-source electromagnetic method magnetotellurics. Herein we present images of the electrical resistivity (inverse of conductivity) at various depths, and compare the inferred resistivities with seismic parameters at the same depths obtained from body wave and surface wave data from the Kaapvaal Seismic Experiment
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The Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics
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The usefulness of the electrical resistivity log in determining reservoir characteristics is governed largely by: (i) the accuracy with which the true resistivity of the formation can be determined; (2) the scope of detailed data concerning the relation of resistivity measurements to formation characteristics; (3) the available information concerning the conductivity of connate or formation waters; (4) the extent of geologic knowledge regarding probable changes in facies within given horizons, both vertically and laterally, particularly in relation to the resultant effect on the electrical properties of the reservoir. Simple examples are given in the following pages to illustrate the use of resistivity logs in the solution of some problems dealing with oil and gas reservoirs. From the available information, it is apparent that much care must be exercised in applying to more complicated cases the methods suggested. It should be remembered that the equations given are not precise and represent only approximate relationships. It is believed, however, that under favorable conditions their application falls within useful limits of accuracy.
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Hydrogeology of the Main Karoo Basin: Current knowledge and research needs
  • Jan 2003
  • J F Botha
  • A C Woodford
  • L P Chevallier
Botha J. F, Woodford AC, Chevallier LP. Hydrogeology of the Main Karoo Basin: Current knowledge and research needs. WRC Report No TT 197. Pretoria: Water Research Commission; 2003.