Influence of Mg2+ on CaCO3 precipitation during subsurface reactive transport in a homogeneous silicon-etched pore network

• Published in: Geochimica et cosmochimica acta 2014-06, Vol.135, p.321-335
• Main Authors: Boyd, Victoria, Yoon, Hongkyu, Zhang, Changyong, Oostrom, Mart, Hess, Nancy, Fouke, Bruce, Valocchi, Albert J., Werth, Charles J.
• Format: Article
• Language: English
• Subjects: contamination; Environmental Molecular Sciences Laboratory; GEOSCIENCES; INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY; microfluiidics; precipitation
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2014.03.018

Calcium carbonate (CaCO3) geochemical reactions exert a fundamental control on the evolution of porosity and permeability in shallow-to-deep subsurface siliciclastic and limestone rock reservoirs. As a result, these carbonate water–rock interactions play a critically important role in research on groundwater remediation, geological carbon sequestration, and hydrocarbon exploration. A study was undertaken to determine the effects of Mg2+ concentration on CaCO3 crystal morphology, precipitation rate, and porosity occlusion under flow and mixing conditions similar to those in subsurface aquifers. This was accomplished by promoting CaCO3 precipitation through the mixing of two solutions flowing parallel to each other in a microfluidic pore structure, containing uniform concentrations of dissolved Ca2+ and carbonate (CO32−), and systematic variations in the concentration of Mg2+. Raman spectroscopy indicates that all three polymorphs of CaCO3 (calcite, aragonite, and vaterite) were present under all experimental conditions. Coordinated brightfield imaging results show the morphology of calcite with increasing Mg2+ progressed from blocky and dogtooth approximately 10–80μm in size, to anhedral spheroidal approximately 5–30μm in size. The morphology of aragonite with increasing Mg2+ progressed from shrubs and fuzzy dumbells to spheroidal, and the size increased from approximately 5–60μm to 20–200μm. Recrystallization was observed in all experiments, but more so at low Mg2+, in which many small microcrystals dissolved and re-precipitated as one or a few larger calcite crystals. Analysis of brightfield images indicates calcite is the most abundant polymorph under all conditions. However, the area of pore space with aragonite increased from 20% at the highest Mg2+ concentration. The initial apparent precipitation rate of mineral polymorphs with no Mg2+ present was 2.5 times greater than when 40mM Mg2+ was added, and large (20–200μm) aragonite crystals formed primarily near to and below the center mixing zone with increasing Mg2+ concentration. Pore-scale modeling results are consistent with experiments, and indicate that all three polymorphs are thermodynamically favorable, with calcite and aragonite being the most favorable and having similar saturation ratios (SR>100). The influence of Mg2+ on mineral precipitation rates is consistent with previous studies showing that calcite precipitation rates decrease with increasing Mg2+ concen