In situ carbonation of peridotite for CO2

storage

In a paper published in the Proceedings of the National Academy of Sciences, Peter B. Kelemen and Jürg

Matter from Columbia University describe how the natural process of carbonation of a mineral, peridotite,

could be enhanced to capture and store billion of tonnes of CO2 every year from the atmosphere.

A type

Peridotite is a naturally occuring rock formation

present in Oman, as well as other locations

around the world including Papua

New Guinea, New Caledonia and along the

east coast of the Adriatic Sea.

The researchers found that hundreds of

thousands of tonnes of CO2 are naturally

stored in Oman alone every year, and suggest

ways in which this process could be enhanced

to store more than a billion tonnes.

“Peridotite carbonation can be accelerated

via drilling, hydraulic fracture, input of

purified CO2 at elevated pressure, and, in

particular, increased temperature at depth,”

said Professor Kelemen.

“In fact, after an initial heating step,

CO2 pumped at 25 or 30 ｰC can be heated

by exothermic carbonation reactions that

sustain high temperature and rapid reaction

rates at depth with little expenditure of energy.”

This means the reactions would be selfsustaining,

and could potentially store all the

CO2 emissions from power stations around

the world.

Enhancing the capture process

The rock formations in Oman store CO2 as

magnesium and calcium carbonate and

dolomite (a mineral composed of calcium,

magnesium, carbon and oxygen) in a network

of underground veins.

Olivine ((Mg,Fe)2SiO4) one of the

main constituents of peridotite, reacts with

groundwater containing dissolved CO2,

forming carbonates that increase the volume

of the rock by up to 44%, causing fractures

to appear.

The fractures allow more water to penetrate

and increase the speed of the reaction.

The authors propose that fracturing

techniques used in the oil and gas industry

to allow oil to flow more easily could be

used to increase the volume of rock exposed

to CO2 and allow more of the gas to react.

Another key factor that affects the rate

of carbonation is the temperature of the rock.

The authors calculate that the optimal temperature

for carbonate formation is 185 ｰC.

After fracturing, the rock could be heated

to this temperature by injecting hot fluid.

This could increase the rate of carbonation

by up to a million times, the authors say.

As dissolved CO2 in surface water cannot

be supplied rapidly enough to keep pace

with the enhanced carbonation rates, a pure

stream of CO2 or a CO2 rich mixture of fluid

would then be injected.

As the carbonation reaction is exothermic,

the scientists calculate that an initial

heating step is all that would be required to

maintain the higher rates of CO2 capture at

depth.

This method could store over a billion

tonnes of CO2 in a cubic kilometre of rock,

the authors say.

Capturing CO2 from seawater

An alternative process could avoid prolonged

pumping of fluid and the use of purified

CO2.

In Oman, New Caledonia, and Papua

New Guinea, peridotite is present beneath a

thin veneer of sediment offshore, beneath the

sea bed. Here, peridotite could be drilled and

fractured, and a volume could be heated using

the method described above.

Little heating would again be required,

as the temperature at the bottom of a 5km

borehole is already 100 ｰC.

Seawater could then be pumped into

the well, where it would heat up and the dissolved

CO2 would react with the peridotite

raising the temperature further.

The seawater could then rise back to the

surface through another well, several kilometres

from the first, through convection.

The CO2 depleted seawater would absorb

more CO2 from the atmosphere, reducing

overall world concentrations of the gas.

The authors calculate that this method

could store only around ten thousand tonnes

of CO2 in a cubic kilometre of rock, due to

the limited concentration of CO2 in seawater,

but at relatively little cost.

Conclusion

The authors have described a process that

provides a way to permanently store CO2

in mineral deposits available in several locations

around the world.

One method involves transport and injection

of concentrated CO2, in a similar

way to currently proposed plans to store

CO2 in aquifers or depleted oil and gas

fields.

The other method could provide a way

for CO2 from the atmosphere to be extracted

and stored without the energy penalty of

capturing from point sources such as power

stations.

Although this method would store considerably

less CO2 by volume of rock, it

would be relatively cheap to implement, and

since the rock does not have to be heated,

could be employed on a very large scale.

Further studies needed

According to the paper, the reactions studied

are virtually impossible to replicate in a lab.

More elaborate models combined with field

tests will be required to evaluate and optimise

the method.

For example, it is difficult to predict the

consequences of hydraulic fracturing of peridotite,

plus cracking associated with heating,

hydration, and carbonation, in terms of permeability

and reactive volume fraction, say

the authors.

“Large-scale field tests should be conducted,

because the proposed method of enhanced

natural CO2 sequestration provides

a promising potential alternative to storage

of supercritical CO2 fluid in underground

pore space,” said Professor Kelemen.

We know that we've got lots of Peroditite throughout the Ring.