Experimental Perspectives of Mineral Dissolution and Precipitation due to Carbon Dioxide-Water-Rock Interactions

This brief review has demonstrated the considerable breadth of experimental data that is available for mineral dissolution and precipitation due to CO 2-water-rock interactions as well as the diversity of laboratory techniques used to acquire these data. In particular, a sizeable dataset quantifying the dissolution rates of minerals likely to be of significance for Geological Carbon Storage has been assembled. We hope this review has also demonstrated that further laboratory experiments, particularly kinetic measurements, are needed for conditions close to those likely to be encountered in CO2 reservoirs. Here, pore fluids are likely to be already close to equilibrium with their host rocks prior to CO2 injection, and will respond primarily to the lowering of pH which results as CO2 dissolves into pore waters. Despite limitations in our understanding, a number of important conclusions can be drawn about CO2-water-rock interactions, particularly with respect to relative rates, that may be of value for the design of sequestration schemes. The fastest minerals to respond to the changes in fluid chemistry induced by CO2 injection are calcite, anhydrite and dolomite. When present in the reservoir, these minerals serve to raise pH on a timescale of days in response to dissolution of CO2 in formation waters, and as a result may further inhibit the slower response of silicate minerals to injection. Nevertheless, silicate reactions may take place within the lifetime of an injection site. Conversion of feldspars to secondary clay minerals is likely to be an important reservoir reaction, but at present the rates of such reactions are difficult to predict because they may be more dependent on the rate of precipitation of the secondary phases than on feldspar dissolution. This is important, because the secondary minerals may reduce permeability significantly and if this happens on the timescale of the original injection, it could reduce reservoir capacity. Impurities co-injected with CO2 may provide new sources of acidity as well as dissolved solutes that may promote precipitation of new minerals and additional growth of preexisting minerals. Along main flow paths exhibiting high permeability, fluid composition will likely be dominated by the injected fluid; more protected areas exhibiting reduced flow may instead retain rock-dominated fluids. Over time, mineral dissolution and precipitation may progressively modify flow pathways and shift the types of reaction taking place. These processes will collectively determine the interplay between reaction kinetics and mass transport processes in CO2 reservoirs. Copyright © 2013 Mineralogical Society of America.