167 Geochemistry

Tuesday, 18 October 2005
1:30 PM - 5:30 PM

REACTIVITY OF DAWSONITE IN CARBON SEQUESTRATION

Paper number: 167-4
Program: Technical Sessions
Time: 2:15 PM – 2:30 PM

Authors

Abstract

Although prominent in a host of modeling studies, dawsonite (NaAlCO3(OH)2) rarely occurs in nature and is not a precipitant in experimental simulations of a carbon repository (e.g., Kaszuba et al., 2003, 2005). Given the potential importance of dawsonite in the performance of a geologic carbon sequestration site, the stability of this mineral under physical-chemical conditions relevant to geologic storage and sequestration of carbon requires critical examination. Short-term experiments demonstrate that dawsonite is readily synthesized from gibbsite and kaolinite in concentrated NaHCO3 solutions. A series of hydrothermal fluid-mineral experiments using flexible cell hydrothermal apparatus (Dickson cells) were used to assess the long-term geochemistry and reactivity of dawsonite in more geologically reasonable solutions: moderately saline (0.05 molal) NaHCO3 solution with synthetic dawsonite at 50 and 75oC, 200 bars; 1 molal NaHCO3 brine with Georgia kaolinite at 75oC, 200 bars; and 1 molal NaCl brine with Georgia kaolinite at 75oC, 200 bars. In this last experiment, the brine-mineral system was reacted at pressure and temperature for ~ 45 days to approach steady state, then injected with supercritical CO2 and allowed to react an additional 45 days. Reacted fluid was periodically sampled and analyzed for pH, SiO2, Al, Na, Cl, and CO2. Select samples were analyzed for colloidal particle-size-distribution and concentration.

At temperature-pressure conditions expected for a carbon repository, dawsonite dissolves to yield 10-100 times more Al in NaHCO3 solution than predicted using existing thermodynamic data. Al-hydroxide colloids, a significant fraction of which are smaller than 0.45 microns, developed in solution. The colloidal fraction that is less than 0.45 microns, a commonly used filter size, accounts for at least some of this Al surplus. The Al surplus and the colloid abundance also appear to be related to the NaHCO3 concentration. A number of computer calculations of carbon repository integrity predict the formation of dawsonite in response to CO2 injection. Dissolution of dawsonite and formation of mobile Al hydroxide colloids can undermine the reliability of these predictions. The geochemical behavior of Al will be a key to determining the potential of dawsonite to sequester carbon.