Cement carbonation: can it help or hinter CO2 storage? - presentation given by Chris Rochelle at the UKCCSRC Cardiff Biannual Meeting, 10-11 September 2014

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Cement carbonation: Can it help or hinder CO2 storage?
Chris Rochelle

Boreholes and the issues with them Focus on cement and carbonation reactions Evidence form laboratory studies Evidence from field observations Evidence from other sources (other cements and natural systems) Which areas remain uncertain?
Overview

Boreholes cut through the natural impermeable rocks above a CO2 store.
Thus engineered seals may provide routes for the migration of CO2 regardless of the sealing capacity of the caprock.
Boreholes

Potential leakage pathways
•
Acidic CO2 and alkaline cement can react quickly.
•
Potential leakage routes could be through the engineered parts of the well (cement: c, e; steel: d).
•
Or along the interfaces between different parts of the well (cement/steel: a, b; cement/rock: f).
•
Poor well construction / completion could introduce leakage routes, or they could be created during field operations.
Gasda et al. (2004)

Sometimes wells do fail …
Poor well sealing risks uncontrolled escapes of CO2, which for large releases may be difficult and expensive to rectify.
Image courtesy of G. Hatziyannis, IGME

Potential scenarios
Worst case:
CO2 degrades cement and steel => migration
Steel casing
Cement
Rock
Carbonation of bulk cement, but leaching along interfaces
Best case:
Limited carbonation => better sealing

Carbonation reactions
•
Borehole cements are commonly based on Portlandite cements, with additives designed to improve properties (e.g. flow, strength, chemical, temperature etc).
•
Volumetrically the most important phases in Portland cement are portlandite (Ca(OH)2) and calcium silicate hydrates (CSH).
•
These and other phases will undergo carbonation reactions producing one or more of the polymorphs of CaCO3 (calcite, aragonite and vaterite), plus other phases such as amorphous silica.
•
Potentially the volume of solids might change.

Carbonation reactions
•
Straightforward for portlandite: Ca(OH) 2 + CO2 => CaCO3 + H2O
•
More complex for CSH phases: 2Ca9H22Si6O218(OH) 8.6H2O + 8CO2 jennite/CSH(2) gel => 2Ca5Si6O16(OH)2.9.5H2O + 8CaCO3 + 4.5H2O tobermorite / CSH(I) gel 2Ca5Si6O16(OH)2.9.5H2O + 5CO2 => 5CaCO3 tobermorite / CSH(I) gel + 6SiO2 + 10.5H2O

Short-term diffusional processes
Lab experiments:
•
Intact cement has low perm., so CO2 ingress likely to be diffusion-controlled.
•
Blocks of cement (±steel) reacted for up to several years.
•
In batch-type experiments CO2 ingress is by diffusion.
•
Cement altered, but sample not destroyed.
•
One or more reaction fronts are formed.
•
Carbonation does not change size of cement block, but increases its density.
Steel
10 mm
Fully carbonated cement
Partly carbonated cement
(110 days, 30°C, 80 bar)
Relict reaction fronts

Localised volume changes
Some evidence that the carbonation front is associated with localised solids volume changes and microcracking, which may help CO2 ingress.
But also see sealing of cracks by secondary carbonates.
CaCO3 replaced CSH matrix
Enhanced microporosity through CSH dissolution, locally associated with shrinkage microfractures, but also higher-density carbonates
2 mm
Altered cement with shrinkage cracks
Unaltered cement
28 mm
Unaltered cement

Short-term flow processes
100
10
1
0.1
0.01
0.001
Flow rate (ml/hour)
1000
4000
3000
2000
0
Time (min)
Highly porous channel
Lab experiments:
•
What happens to a faulty cement/steel interface?
•
Significant flow along imperfections caused by poor bonding.
•
Some evidence for a reduction in fluid flow due to a degree of self healing.
•
Possible plugging due to formation of Ca carbonate precipitates (or over longer times, steel corrosion products - Fe-oxides, Fe-carbonate).
•
Positive observation (for limited amounts of fluid flow).
30 day
test
IFP data, CO2GeoNet

Sealing versus leakage
-
Progressive plugging of sample for first 9 days of CO2-rich water flow.
-
But permeability goes up again with continued flow of CO2-rich water.
-
Could continued flow cause leaching to exceed precipitation?
Permeability drops initially
Perm. increases upon prolonged flushing with CO2-rich water
0.1
0.01
1
0.001
Time (min)
1000
10000
Water permeability (mD)
Increase after debonding
Initial permeability
•
After 2 weeks water flow, self healing allows recovery of initial low permeability (through continued hydration of cement or carbonate precipitation).
•
But continued flow causes dissolution of initially-precipitated carbonate, and an opening up of flow paths.
•
Degree of carbonate saturation important.
IFP data, CO2GeoNet

Evidence from field operations
•
There is a generally good performance history of wells containing CO2-rich fluids.
•
But most wells designed to last a few 10s of years, so borehole seals only tested over short timescales.
•
Storage of CO2 requires containment for 1000s years.
•
It is important to extend studies of short-term processes to timescales relevant to performance assessments.
•
Samples of borehole cement recovered after 30 years in the SACROC site show carbonation along interfaces, but many features are comparable with laboratory samples.

Fluid flow along interfaces
Laboratory sample (IFP)
Steel casing
Cement + carbonate rim (∼1-2 mm)
Sound cement with veins (50mm)
Shale caprock
Cement Shale
Carey et al., 2007
CO2 migration along interface
(5-10mm wide ‘orange zone’)
CO2 migration along interface
Sidewall core from well (3 m above base of caprock)
15 mm

Evidence from natural systems
•
To help understand long timescale processes, consider natural cement minerals that have undergone natural carbonation over 1000s years (natural analogues).
•
Studied analogue sites in Northern Ireland.
•
Metamorphosed Cretaceous chalks / limestones (heated by Tertiary dolerite intrusions):
•
Flint nodules were converted to high temperature calcium silicates (e.g. larnite [β-Ca2SiO4], spurrite [Ca5(SiO4)2CO3]) in the Tertiary.
•
Cooling led to groundwater ingress.
•
Calcium silicates hydrated to ‘cement-type’ minerals (CSH etc).
•
Uplift and erosion allowed atmospheric CO2 / equilibrated surface water to react with the CSH-rich nodules.
•
The latter have undergone carbonation reactions since the retreat of glacial ice (c. 10 ka).

Northern Ireland natural analogues
Scawt Hill
Carneal plug
Belfast
Altered chalk / limestone
c. 250 m
SE
NW

Scawt Hill analogue site
•
Find ‘rinds’ of carbonate minerals that appear to have protected the remaining cement minerals from further reaction.
•
Some nodules full of soft gel-like CSH, others are zoned and retain evidence of high temperature phases.
•
Observations positive in terms of sealing.
•
(But concs of dissolved C only low, more akin to edge of CO2 plume).
Metamorphosed chalk / limestone
Weathered nodule (just 1cm ‘rind’ of carbonation)
Altered flint nodule with carbonation ‘rind’
Ca7Si6(CO3)O18.2H2O

Other sources of information
Radwaste disposal - multi barrier concept with cement backfill

Carbonation of cement backfill
•
Some repository concepts use large quantities of cementitious materials, but degradation of organic material in the waste will produce CO2 that will react with these materials.
•
Carbonation will reduce pH buffering, impact radionuclide immobilisation, and may alter cement permeability and strength.
•
Backfill cement higher permeability than borehole cement, but has very similar mineralogy (mainly portlandite and CSH), so many reactions similar.
•
Repository performance timescales 106 years, far longer than for CCS.
•
See similar effects of carbonation:
•
Carbonated cement remains intact, but does not change in overall size.
•
Increase in density (by up to about 8%) on CO2 uptake.
•
Development of carbonation reaction fronts.
•
Noticeable reduction in permeability.

Impact on permeability
Permeability reduction around reaction front – beneficial in terms of sealing.
Carbonated
Non-carb.

Complex reaction fronts
•
Detailed mineralogical characterisation shows that the process of cement carbonation creates 3 main zones (2 complex reaction fronts).
•
Main reactions upon carbonation: portlandite => calcite/aragonite/vaterite; CSH => calcite/aragonite/vaterite + silica gel.
•
Other elements complicate things (Al in CASH phases => alumina gel).
•
Chloride can also react (e.g. forming Friedel’s Salt, hydrocalumite).

Reactions at fronts
•
Reaction proceeds with a zone of porosity enhancement and Ca leaching, followed by a region of carbonate precipitation.
•
Aragonite and vaterite dominate initial CaCO3 precipitation, but over time (i.e. behind the main reaction front) calcite dominates.
•
In the fully carbonated cement, secondary carbonate lines voids (such as gas bubbles), occurs as small widely-distributed grains, and as a 3D ‘chickenwire’-like network.
3D network of thin, high density carbonate precipitate, surrounding silica-rich regions of low density and higher porosity.
Gas bubble, lined and partially filled with secondary carbonate.
Carbonate precipitation
Porosity enhancement
CO2
Ca2+

Overview of observations (1 of 2)
•
Reactions in the bulk cement are relatively rapid:
•
Dissolution => release of Ca, a porosity increase, and diffusion of Ca towards the source of CO2 (i.e. against CO2 flow direction).
•
Precipitation of silica and a range of carbonates (calcite may dominate in the long term), sealing some of the porosity generated.
•
Carbonates may form a complex 3D series of sheets (‘chicken-wire’ texture), which may be effective at reducing permeability.
•
Other secondary minerals (such as Cl- or SO4-rich phases) may further seal permeability.
•
Limited carbonation may form ‘rinds’ that protect the rest of the cement.
•
Reactions along interfaces appear to have some ability to seal flow paths, but this is limited, and dissolution will progressively increase if there is a flow of CO2-rich water.
•
So a high quality initial cement job is key, and limited cement carbonation may be beneficial.

•
Experiments designed to simulate in-situ conditions and realistic scenarios. But note that we are trying to simulate slow processes within just a few weeks.
•
We see similar behaviour (qualitatively) between small laboratory tests and >10s years, multi-metre scale observations of carbonated well cement.
•
Comparable observations have also been made of material from naturally-carbonated cement minerals reacted for >1000s years (natural analogues).
•
Such similarities for systems from a range of temporal and spatial increase our confidence in long-term predictions of borehole cement behaviour.
Overview of observations (2 of 2)

•
Bulk cement: Quantify rates of progression of reaction fronts and whether this decreases over time (i.e. sealing improves).
•
Study more ‘old’ carbonated samples (natural samples, archaeological samples, well cements).
•
Identify what controls the microcracks around the carbonation front seen in the lab experiments.
•
Interfaces: Quantify potential for ‘self healing’ of leaks, and the limits to acceptable (short-term) flow.
•
Wells: Develop new ways to monitor cement behaviour, both during the injection phase and in the longer term.
•
Leaks: Develop effective remediation strategies once leakage detected.
Potential future research areas

Thank You
Acknowledgements:
•
EC for funding the CO2GeoNet Network of Excellence.
•
EC and NDA for funding the project ‘FORGE” ‘Fate Of Repository Gases in Europe’.
•
NERC for funding the CRIUS project.

Image courtesy of G. Hatziyannis, IGME

Permeability testing
•
Test 5cm x 5cm cores of cement held in a Teflon sheath.
•
Accurately measure pressures and flow rates at the inlet and outlet ends of the cores to derive flow rates (pre- and post-CO2).

Example - NRVB flow tests
Consistent initial hydraulic permeability data
1000x decrease in hydraulic permeability after reaction with dissolved CO2
Consistent gas permeability data when using either CO2 or N2
Halving of gas permeability after reaction with gaseous CO2

•
Predict a zone of reaction rather than a single front (as per the experiments). Highlights the importance of kinetics in reactions.
•
Predict the formation of very narrow carbonate precipitation fronts due to the complex interplay of flow, reaction and back-diffusion of Ca2+ ions (similar process to experiments, but length scale is larger.
1D coupled modelling of reactive flow
At 100 hours
At 100 hours
CO2
CO2

Cement carbonation: can it help or hinter CO2 storage? - presentation given by Chris Rochelle at the UKCCSRC Cardiff Biannual Meeting, 10-11 September 2014

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Cement carbonation: can it help or hinter CO2 storage? - presentation given by Chris Rochelle at the UKCCSRC Cardiff Biannual Meeting, 10-11 September 2014