Re: [asa] geo engineering

From: Rich Blinne <rich.blinne@gmail.com>
Date: Tue Jul 22 2008 - 17:54:06 EDT

On Tue, Jul 22, 2008 at 2:25 PM, David Campbell <pleuronaia@gmail.com>
wrote:

> One carbon sequestration scheme claimed
> that injecting it into basalt deposits would create carbonate, but I
> don't know the details-I suspect it's not overly efficient as a way to
> produce carbonate.

See the details here in today's PNAS:

http://www.pnas.org/content/105/29/9920.full

Carbon dioxide sequestration in deep-sea basalt

Developing a method for secure sequestration of anthropogenic carbon dioxide
in geological formations is one of our most pressing global scientific
problems. Injection into deep-sea basalt formations provides unique and
significant advantages over other potential geological storage options,
including (i) vast reservoir capacities sufficient to accommodate
centuries-long U.S. production of fossil fuel CO2 at locations within
pipeline distances to populated areas and CO2 sources along the U.S. west
coast; (ii) sufficiently closed water-rock circulation pathways for the
chemical reaction of CO2 with basalt to produce stable and nontoxic (Ca2+,
Mg2+, Fe2+)CO3 infilling minerals, and (iii) significant risk reduction for
post-injection leakage by geological, gravitational, and hydrate-trapping
mechanisms. CO2 sequestration in established sediment-covered basalt
aquifers on the Juan de Fuca plate offer promising locations to securely
accommodate more than a century of future U.S. emissions, warranting
energized scientific research, technological assessment, and economic
evaluation to establish a viable pilot injection program in the future.

...

Deep-sea basalt offers a unique environment for CO2 sequestration that
combines both vast volumes of seawater-filled pore space and Mg-Ca silicate
rocks (18). Within deep-sea basalt aquifers, the injected CO2 mixes with
seawater and reacts with basalt, both of which are rich in alkaline-earth
elements. The release of Ca2+ and Mg2+ ions from basalt will form stable
carbonate minerals as reaction products (19, 20). Takahashi et al. (21)
present a general geochemical model for mineral trapping in basalt. Recent
laboratory experiments demonstrate the potential for rapid carbonate
precipitation in fresh continental flood basalt (22). Dissolution and
precipitation reactions in deep-sea basalt can proceed in fluid-filled
fractures and pores at rates equal to or greater than measured in the
laboratory (22, 23). Carbonate precipitation over time may alter in situ
porosity and permeability within basalt aquifers, however, and thus
progressively decrease the CO2-basalt reaction rate to a finite limit.
Although natural weathering processes in deep-sea basalt precipitate
pore-filling carbonates, fractured and permeable basalt crust extends for
millions of years before its porosity has been appreciably filled (24).
Land-based experiments provide some insight into these effects, but
estimating the in situ rates and accelerated effects, if any, of carbonate
precipitation in basalt are difficult to predict without deep-sea CO2
injection experiments. Matter et al. (25) conducted a small-scale injection
experiment in mafic rocks to investigate the in situ rates of reaction. Two
processes, mixing between the injected solution and aquifer water and the
release of cations from water-rock dissolution, were found to neutralize the
introduced carbonic acid within 200 h of injection (25). Long residence
periods for fluids in the ocean crust (e.g., >500 years) would therefore
provide 104-fold longer times for dissolution of basalt and release of Ca2+
and Mg2+ ions from the formation of carbonate minerals.

...

CO2 injected into deep-sea basalt is a supercritical fluid, and may be mixed
with and dispersed into the aquifer through turbulent mixing processes and
displacement of the aquifer fluid. Fluid-rock chemical reactions will
proceed rapidly on surfaces of fractured basalt and within pore spaces. For
selected injection targets in basement reservoirs that exist below 「2,700 m
water depth and are covered by 200 m or more of sediment, both gravitational
and stratigraphic trapping will occur as well as geochemical trapping. Fig.
4 illustrates the extent of the region on the Juan de Fuca plate that
satisfies these bathymetric and sediment thickness constraints. We restrict
the region to avoid natural fluid inflow/outflow areas within 20 km of
seamounts, the Juan de Fuca ridge, the Cascadia trench, and the Blanco and
Mendocino fracture zones. By using the high, model-constrained estimate of
40 m/year lateral flow rate in the shallow crust, this restriction sets a
500-year buffer around potential natural outflow zones on the Juan de Fuca
plate to further protect against the possibility of long-term CO2 leakage to
the seafloor. We compute 「78,000 km2 in the region that meet these depth and
geologic conditions. Assuming that a channel system dominates the
permeability over one-sixth of the upper 600 m of basement (38), we estimate
that this area contains 7,800 km3 of highly permeable basalt. Given an
average channel porosity of 10% (33, 38), we can calculate that 780 km3 of
potential pore volume will be available for CO22 is injected to fill this
volume, and it remains in liquid form (CO2 density 「1 g/cm3, or 0.27 g
C/cm3), the total storage capacity for injected CO2 in this area is 208 Gt
of carbon. If all of the CO2 becomes fixed as carbonate (CaCO3 density 「2.7
g/cm3, or 0.36 g C/cm3), with complete acid neutralization reaction with
basalt, this reservoir could hold 「250 Gt of carbon. Increasing the sediment
cover to −300 m thickness will decrease the area by 12% to 68,500 km2 and
the volume to 685 km3 for this region. For example, at the current annual
emission rate of 1.7 Gt of carbon per year by the United States (5), the
basement on the Juan de Fuca plate alone would provide sufficient CO2
sequestration capacity for 122C147 years, depending on whether all of the
injected CO2 converts to carbonate. Given its proximity to the U.S. west
coast, however, a more realistic scenario may be to assess the Juan de Fuca
reservoir as a sequestration option for CO2 sources from western states, via
pipeline transport. Of course, if this becomes technologically and
economically feasible, the reservoir would fill over a considerably longer
time than estimated for U.S. emissions from the entire country.

Rich Blinne
Member ASA

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Received on Tue Jul 22 17:54:39 2008

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