What are the Time Scales of Carbonate Mineral Sequestration of CO2 in the Subsurface?
A Department of Earth and Space Sciences Colloquium presented by Carl Steefel, Center for Nanoscale Control of Geologic CO2 & Lawrence Berkeley National Laboratory
Thursday, October 04, 2012
4:00 PM - 5:00 PM
Mineral trapping of CO2 in the subsurface is acknowledged to be the most secure form of sequestration, but some studies have suggested that the process is extremely slow, perhaps on the order of 10,000 years or more. But what are the arguments for these long time scales based on? Certainly part of it has to do with the slow dissolution rates of silicates needed to provide a source of cations (Ca2+, Mg2+, and Fe2+) and alkalinity for carbonate precipitation. Rates of dissolution for many silicates are very slow (e.g., albitic plagioclase and chlorite), while other silicate minerals (anorthitic feldspar, olivine) dissolve appreciably faster. Determining which mineral is rate-limiting in the case of the faster dissolving silicates (dissolving silicate or precipitating carbonate), however, is not always straightforward without a careful analysis of dissolution and precipitation as a coupled process, as pointed out previously by Steefel and Van Cappellen (1990) and Maher et al. (2009). We are investigating coupled dissolution and precipitation in microfluidic experiments similar to single pore environments in which magnesite replaces forsterite, a case where rates of dissolution and precipitation are of similar magnitude. A second explanation for the slow rates of carbonate mineral sequestration in the models for CO2 sequestration is more subtle, and follows from the assumption that the reservoir within which carbonate mineral precipitation might occur is well-mixed and therefore characterized everywhere by the low pH values typical of brine in equilibrium with supercritical CO2. But our analysis of physically and chemically heterogeneous subsurface materials, especially during the residual trapping stage, suggest that local chemical microenvironments can develop in which pH, alkalinity, and cation concentrations rise sufficiently high that substantial carbonate can precipitate. This effect has now been observed experimentally in flow/diffusion cell capillary tube experiments involving forsterite dissolution and magnesite precipitation. The effect is further investigated with reactive transport modeling of reservoir sandstone from the Cranfield site based on connected mineral surface area and porosity maps. Preliminary experiments and modeling both suggest that carbonate precipitation can be significant on the time scale of tens of years.
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