A Novel Approach to Experimental Studies of Mineral Dissolution Kinetics

Zhu, Chen

Indiana University

关键词: Carbon Dioxide;    58 Geosciences;    Rock-Fluid Interactions;    Chemical Reaction Kinetics;    54 Environmental Sciences;   

DOI  :  10.2172/949962
RP-ID  :  None
RP-ID  :  FG26-04NT42125
RP-ID  :  949962

美国|英语

来源: UNT Digital Library

【 摘 要 】

Currently, DOE is conducting pilot CO{sub 2} injection tests to evaluate the concept of geological sequestration. The injected CO{sub 2} is expected to react with the host rocks and these reactions can potentially alter the porosity, permeability, and mechanical properties of the host or cap rocks. Reactions can also result in precipitation of carbonate-containing minerals that favorably and permanently trap CO{sub 2} underground. Many numerical models have been used to predict these reactions for the carbon sequestration program. However, a firm experimental basis for predicting silicate reaction kinetics in CO{sub 2} injected geological formations is urgently needed to assure the reliability of the geochemical models used for the assessments of carbon sequestration strategies. The funded experimental and theoretical study attempts to resolve this outstanding scientific issue by novel experimental design and theoretical interpretation of silicate dissolution rates at conditions pertinent to geological carbon sequestration. In this four year research grant (three years plus a one year no cost extension), seven (7) laboratory experiments of CO{sub 2}-rock-water interactions were carried out. An experimental design allowed the collection of water samples during experiments in situ and thus prevented back reactions. Analysis of the in situ samples delineated the temporal evolution of aqueous chemistry because of CO{sub 2}-rock-water interactions. The solid products of the experiments were retrieved at the end of the experimental run, and analyzed with a suite of advanced analytical and electron microscopic techniques (i.e., atomic resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), electron microprobe, X-ray diffraction, X-ray photoelectron spectroscopy (XPS)). As a result, the research project probably has produced one of the best data sets for CO{sub 2}-rock-water interactions in terms of both aqueous solution chemistry and solid characterization. Three experiments were performed using the Navajo sandstone. Navajo sandstone is geologically equivalent to the Nugget sandstone, which is a target formation for a regional partnership injection project. Our experiments provided the experimental data on the potential CO{sub 2}-rock-water interactions that are likely to occur in the aquifer. Geochemical modeling was performed to interpret the experimental results. Our single mineral (feldspar) experiments addressed a basic research need. i.e., the coupled nature of dissolution and precipitation reactions, which has universal implication to the reaction kinetics as it applied to CO{sub 2} sequestration. Our whole rock experiments (Navajo sandstone) addressed the applied research component, e.g., reacting Navajo sandstone with brine and CO{sub 2} has direct relevance on the activities of a number of regional partnerships. The following are the major findings from this project: (1) The project generated a large amount of experimental data that is central to evaluating CO{sub 2}-water-rock interactions and providing ground truth to predictive models, which have been used and will inevitably be increasingly more used in carbon sequestration. (2) Results from the feldspar experiments demonstrated stronger coupling between dissolution and precipitation reactions. We show that the partial equilibrium assumption did not hold in the feldspar hydrolysis experiments (Zhu and Lu, submitted, Appendix A-2). The precipitation of clay minerals influenced dissolution of primary silicate in a much stronger way as previously envisioned. Therefore, our experimental data indicated a much more complex chemical kinetics as it has been applied to carbon sequestration program in terms of preliminary predictive models of CO{sub 2}-rock-water interactions. Adopting this complexity (strong coupling) may influence estimates of mineral trapping and porosity/permeability for geological carbon sequestration. In general, our knowledge of the coupling of different reactions is poor, and we must consider the uncertainties resulting from our poor knowledge on this regard. (3) Our experimental results concur with previous findings that the role of dissolved CO{sub 2} is mostly to acidify the brine, but not change the mechanisms of reactions. This conclusion is based on careful paired experiments with and without CO{sub 2}. (4) We observed strong chemical reactions between CO{sub 2} acidified brine with the Navajo sandstone. The laboratory experiments were conducted at a higher temperature (200 C) than that in the field ({approx}90 C) in order to induce measurable chemical changes in the laboratory. However, field conditions are more acidic and reaction time is much longer (1000 years versus 10-80 days in the laboratory). Therefore, the conclusions on extensive reactions are relevant. We observed extensive dissolution of feldspars, and precipitation of clay minerals.