Mineralization of basalts in the CO2–H2O–H2S system

• Basalts react in CO2–H2S–H2O system to form pyrite and carbonate.
• Reactivity with H2S is correlated with measured Fe(II)/Fe(III) ratios in the basalts.
• Metal substituted carbonates were identified as reaction products.

Basalt samples representing five different formations were immersed in water equilibrated with supercritical carbon dioxide containing 1% hydrogen sulfide (H2S) at reservoir conditions (100 bar, 90 °C) for up to 3.5 years. Surface coatings in the form of pyrite and metal cation substituted carbonates were identified as reaction products associated with all five basalts. In some cases, high pressure tests contained excess H2S, which produced the most corroded basalts and largest amount of secondary products. In comparison, tests containing limited amounts of H2S appeared least reacted with significantly less concentrations of reaction products. In all cases, pyrite appeared to precede carbonation, and in some instances, was observed in the absence of carbonation such as in cracks, fractures, and within the porous glassy mesostasis. Armoring reactions from pyrite surface coatings observed in earlier shorter duration tests were found to be temporary with carbonate mineralization observed with all the basalts tested in these long duration experiments. Geochemical simulations conducted with the geochemical code EQ3/6 accurately predicted early pyrite precipitation followed by formation of carbonates. Reactivity with H2S was correlated with measured Fe(II)/Fe(III) ratios in the basalts with more facile pyrite formation occurring with basalts containing more Fe(III) phases. These experimental and modeling results confirm potential for long term sequestration of acid gas mixtures in continental flood basalt formations.