『Abstract
Geological storage of CO2 in plagioclase-bearing
sediments, granitic rocks and fractured basalts is expected to
have a variety of geochemical and geomechanical effects, including
mineral trapping and porosity-permeability changes. The amount
of CO2 trapped by mineralisation and the
extent of any changes in host rock transport or mechanical properties
depend on rock composition. To date, studies of mineralisation
have mainly been based on dissolution and precipitation experiments
performed at ambient surface conditions, or at slightly elevated
temperature and pressure. However, the reactions that occur under
in-situ conditions are poorly constrained. In an attempt to determine
the secondary phases formed in the plagioclase-CO2-water
system under such conditions, we performed a set of batch experiments,
at temperatures in the range 200-300℃, using carbon dioxide pressures
from 0.4 to 15 MPa. We investigated reaction of anorthite and
albite, which are expected to precipitate calcite and kaolinite,
and dawsonite and quartz, respectively. However, in most of our
experiments, reactions precipitating clays (kaolinite and smectite
or illite), boehmite, and a Mg,Ni,Fe-hydrotalcite-like phase dominated,
the nickel being derived from the reaction vessel. Little or no
carbonate and no dawsonite were detected. Additional feldspar-free
control experiments employing Mg-rich brine showed that carbonates
would be preserved in our feldspar experiments if formed, but
that a carbonate substrate is needed to promote precipitation.
Since the secondary phases formed in our plagioclase experiments
are stable under a wide range of conditions we expect that they
will also form under CO2 storage systems
with suitable fluid-rock composition. From our results we conclude
that clay precipitation can proceed or accompany carbonate precipitation,
depending on the availability of carbonate substrate. Though an
artifact in our experiments, in the context of subsurface alkaline
waste injection, hydrotalcite may play a positive role here by
immobilising heavy metals, while carbonates will mineralise the
CO2.
Keywords: CO2 sequestration; Plagioclase
feldspar; CO2-rock interaction; Mineralisation;
Heavy metals』
1. Introduction
2. Methodology
2.1. Preparation of samples and solutions
2.2. Experimental design and procedure
3. Results
3.1. Anorthite-CO2-water/brine experiments
3.1.1. SEM observations on anorthite grain surfaces
3.1.2. XRD data
3.1.3. Infrared spectra
3.1.4. Thermogravimetric analyses
3.1.5. FEG EPMA results
3.2. Albite-CO2-water/brine experiments
3.3. Magnesite precipitation experiments
4. Discussion
4.1. Feldspar experiments - identity and formation of secondary
phases
4.2. Comparison with previous work on plagioclase
4.3. Significance of the carbonate control experiments
4.4. Implications
5. Conclusions
Acknowledgements
References