CARBONATE PRECIPITATION IN AN EXPERIMENTAL SUPERCRITICAL CARBON DIOXIDE-BRINE-ROCK SYSTEM

The CO2-H2O system exhibits complex multiphase behavior over a wide temperature and pressure range. The overlap with important geochemical processes has not been broadly evaluated. Carbonate precipitation from multiphase (subcritical H2O and supercritical CO2) fluid-rock reactions was evaluated in a series of experiments conducted in a flexible cell hydrothermal apparatus. In one experiment, a 5.5M NaCl brine-rock system was reacted for 32 days to approach steady state, then injected with supercritical CO2 and allowed to react an additional 45 days. In a separate experiment, the brine-rock system reacted for 77 days without supercritical CO2. In a third experiment, the rock was reacted with 5.5M Mg-NaCl brine for 59 days. Supercritical CO2 was then injected and the system allowed to react an additional 80 days. In all experiments, arkose (microcline + oligoclase + qtz + bt) plus illite-rich shale were part of the reactive system.

The brines present a range of reaction potential among rock, brine, and supercritical CO2. On one hand, carbonate may readily precipitate from Mg-rich brine containing aqueous carbonate ion buffered by coexisting supercritical CO2 (Kaszuba et al., 2003). On the other, Na-rich brine, initially devoid of divalent cations, can only derive the cations needed for carbonate precipitation by reaction with silicates. Magnesite precipitated in the Mg-NaCl brine-supercritical CO2-rock experiment whereas magnesite and siderite precipitated in the NaCl brine-supercritical CO2-rock system. Magnesite crystals in both experiments are large, up to a few mm in length, and occur as individual grains and splays. Euhedral magnesite precipitated in Mg-NaCl brine, indicating equilibrium growth. Pitted crystal faces of magnesite in the NaCl brine experiment indicate disequilibrium and are interpreted as dissolution of early-formed magnesite. Siderite occurs as individual crystals, 200-250 mm in diameter, on shale. Euhedral texture, indicative of equilibrium, is interpreted as precipitation of siderite after early-formed magnesite. Occurrence of pitted magnesite and euhedral siderite may represent a paragenetic sequence for carbonates. This sequence is not predicted by geochemical models, indicating complexity of geochemical reactions among supercritical CO2, brine, arkose, and shale.