『Abstract
Calcite and magnesite dissolution rates were measured at 60℃,
30 atm pCO2, 0.1 M NaCl, and pH from 4.95±0.05
to 5.60±0.05 as a function of organic (acetate, oxalate, malonate,
succinate, phthalate, citrate, EDTA) and inorganic (sulphate,
phosphate, borate, silicate) ligand concentration in the range
of 10-5 to 10-2 M. These conditions can
be considered as boundary model environments for sedimentary oil-field
basins of underground CO2 storage. Experiments
on dissolution of magnesite powders (100-200 μm) and calcite crystal
planes were performed in a batch reactor with in-situ pH measurements
and under controlled hydrodynamic conditions using the rotating
disk technique. At 60℃ in circumneutral solutions in the presence
of 0.02 M NaHCO3 and 30 atm pCO2
(pH = 4.95), calcite dissolution is weakly affected by the presence
of ligands: the rates increase at the maximum by a factor of 2
and, at 0.01 M ligand concentration in solution, the order is:
silicate<citrate<NaCl〜borate<malonate<EDTA<sulphate<acetate. The
order of ligand effects on calcite dissolution at pH = 5.55 (0.1
M NaHCO3, 30 atm pCO2)
is: phosphate<NaCl<citrate<acetate<succinate<malonate<phthalate<EDTA.
Magnesite dissolution rates at 60℃, 30 atm pCO2
and 0.02 M NaHCO3 (pH = 4.95) were weakly
affected by the presence of acetate, silicate, borate and NaCl
but increase in the presence of sulphate, EDTA, citrate and oxalate.
These ligand-affected rates were rationalized using a phenomenological
equation which postulates the Langmuirian adsorption of a negatively-charged
or neutral ligand on rate-controlling surface sites, presumably>MeOH2+ (Me=Ca, Mg). Proposed equations
of rate-ligand concentration dependencies can be directly incorporated
into reaction transport codes. Results obtained in this study
demonstrate that both magnesite and calcite reactivity is not
appreciably affected by acetate, oxalate, citrate, succinate,
sulphate, and phosphate that are most likely present in deep carbonate
aquifers at the physical and chemical conditions pertinent to
CO2 geological sequestering sites. The concentration
of ligands necessary to increase the rates by a factor of 3 to
10 is on the order of 0.01 M. Such a high concentration is unlikely
to be encountered in deep sedimentary basins. Therefore, as a
first approximation, reactive transport modelling of dissolution
induced by CO2 injection in carbonate rocks
does not require to explicitly account for the effect of dissolved
organics.
Keywords: Calcite; Magnesite; Dissolution; Kinetics; Carbon dioxide;
Phosphate; Sulphate; Borate; Silicate; Acetate; Oxalate; Malonate;
Succinate; Phthalate; Citrate; EDTA; pCO2』
1. Introduction
2. Experimental methods
3. Results
4. Discussion: Modeling of ligand-affected carbonate mineral dissolution
5. Conclusions and geological applications
Acknowledgments
Appendix A. The list of experiments performed in this study
References