wAbstract
@Reactions between CO2-charged brines and
reservoir minerals might either enhance the long-term storage
of CO2 in geological reservoirs or facilitate
leakage by corroding cap rocks and fault seals. Modelling the
progress of such reactions is frustrated by uncertainties in the
absolute mineral surface reaction rates and the significance of
other rate limiting steps in natural systems. Here we use the
chemical evolution of groundwater from the Jurassic Navajo Sandstone,
part of a leaking natural accumulation of CO2
at Green River, Utah, in the Colorado Plateau, USA, to place constraints
on the rates and potential controlling mechanisms of the mineral-fluid
reactions, under elevated CO2 pressures,
in a natural system.
@The progress of individual reactions, inferred from changes in
groundwater chemistry is modelled using mass balance techniques.
The mineral reactions are close to stoichiometric with plagioclase
and K-feldspar dissolution largely balanced by precipitation of
clay minerals and carbonate. Mineral modes, in conjunction with
published surface area measurements and flow rates estimated from
hydraulic head measurements, are then used to quantify the kinetics
of feldspar dissolution. Maximum estimated dissolution rates for
plagioclase and K-feldspar are 2~10-14 and 4~10-16
mol m-2 s-1, respectively.
@Fluid ion-activity products are close to equilibrium (e.g. ’Gr for plagioclase between -2 and -10 kJ/mol) and
lie in the region in which mineral surface reaction rates show
a strong dependence on ’Gr. Local variation
in ’Gr is attributed to the injection and
disassociation of CO2 which initially depresses
silicate mineral saturation in the fluid, promoting feldspar dissolution.
With progressive flow through the aquifer feldspar hydrolysis
reactions consume H+ and liberate solutes to solution
which increase mineral saturation in the fluid and rates slow
as a consequence.
@The measured plagioclase dissolution rates at low ’Gr
of 2~10-14 mol m-2 s-1 would
be compatible with far-from-equilibrium rates of `1~10-13
mol m-2 s-1 as observed in some experimental
studies. This suggests that the discrepancy between field and
laboratory reaction rates may in part be explained by the differences
in the thermodynamic state of natural and experimental fluids,
with field-scale reactions occurring close to equilibrium whereas
most laboratory experiments are run far-from-equilibrium.
Keywords: carbon sequestration; kinetics; feldspar dissolution;
Gibbs free energy; Green River; Navajo Sandstonex
1. Introduction
2. Geology of Green River
@2.1. Natural CO2 reservoir systems
@2.2. Reservoir geology
3. Field sampling and experimental method
@3.1. Sample collection and analytical method
4. Hydrology
@4.1. Water sources for springs
@4.2. CO2 sources for springs
5. Calculating reactions and reaction rates
@5.1. Method - introduction
@5.2. Correction for brine inputs
@5.3. Flow paths
@5.4. Reaction progress
@5.5. Calculation of reaction rates
6. Discussion and results
@6.1. Flow paths, CO2 injection and chemical
evolution
@6.2. Reaction rates
7. Approach to equilibrium - ’Gr
@7.1. Reaction rate - equilibrium relationships
@7.2. Uncertainties in reaction rates and solution saturation
states
@7.3. Comparison of reaction rates with laboratory and field estimates
8. Conclusions
Acknowledgements
Appendix A. Uncertainty analysis
Appendix B. Supplementary data
References