GaMin’11 – an International Inter-laboratory Comparison for Geochemical CO2 - Saline Fluid - Mineral Interaction Experiments

Due to the strong interest in geochemical CO2-fluid-rock interaction in the context of geological storage of CO2 a growing number of research groups have used a variety of different experimental ways to identify important geochemical dissolution or precipitation reactions and – if possible – quantify the rates and extent of mineral or rock alteration. In this inter-laboratory comparison the gas-fluid-mineral reactions of three samples of rock-forming minerals have been investigated by 11 experimental labs. The reported results point to robust identification of the major processes in the experiments by most groups. The dissolution rates derived from the changes in composition of the aqueous phase are consistent overall, but the variation could be reduced by using similar corrections for changing parameters in the reaction cells over time. The comparison of experimental setups and procedures as well as of data corrections identified potential improvements for future gas-fluid-rock studies

Dissolution rates of talc as a function of solution composition, pH and temperature.

International audienc

Probing the surface properties of weathered silicate minerals to better understand their reactivity

While we expect conventional reactive transport simulations to provide reliable estimations of the evolution of fluid-rock interactions over time scales of centuries and even more, recent experimental studies showed that they could hardly be satisfactorily used on simplified systems (e.g. batch experiments on single minerals), on time scales of weeks [1]. As emphasized elsewhere [1, 2], the reasons for such inconsistencies have to be sought in the nature of the rate laws used in the geochemical codes, which heavily rely on our description of the fundamental mechanisms involved during water-mineral reactions. In that respect, the present ongoing work aims at gathering some of our recent findings in the dissolution kinetics of a series of Al-free silicates, in relation to the physicochemical properties of their surfaces after/during hydrothermal weathering. A first still unresolved issue that we are addressing is the effect of ubiquitous silica-rich layers which form on silicate minerals. While µm-thick silica coatings formed on the surface of wollastonite crystals without significantly affecting their dissolution rate, we observed that nm-thick silica coatings fully passivate the surface of olivine crystals [1, 3]. We will show how the use of microscopic (STEM, HTEM) [3] and spectroscopic (ToF-SIMS, XPS) techniques helped us to unravel these paradoxical properties, and which chemical parameters could influence the textural features of the layers. A different (or supplementary) mechanism possibly responsible for unexpected decreases of silicate dissolution rate at "far-from-equilibrium" conditions (e.g. diopside, [4]) was proposed to arise from the surface topography of the dissolving crystals and the occurrence (or absence) of etch pits [5]. We will show how the in situ monitoring of the dissolving surface of diopside as a function of fluid saturation state in a HAFM flow-cell (e.g. [6]) is allowing us to address this question. [1] Daval et al (2010) Proceed WRI-13, 1, 713-716 [2] Zhu (2009) Rev Mineral Geochem, 70, 533-569 [3] Daval et al (2009) Am Mineral, 94, 1707-1726 [4] Daval el al (2010) Geochim Cosmochim Ac, 74, 2615-2633 [5] Arvidson & Luttge (2010) Chem Geol, 269, 79-88 [6] Saldi et al (2009) Geochim Cosmochim Ac, 73, 5646-565

The role of fluid chemistry on permeability evolution in granite: Applications to natural and anthropogenic systems

International audienceEfforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO 2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipitation of secondary mineral phases, the products of fluidrock interactions, within the fracture network can significantly reduce the permeability of the overall system, whereas mineral dissolution can enhance reservoir permeability. The coupling between these two competing processes dictates the long-term productivity and lifetime of geothermal reservoirs. In this study, we simulate the conditions within a geothermal system from induced fracturing to the final precipitation stage. We performed batch and flow-through experiments on cores of the Carnmenellis granite, a target unit for geothermal energy recovery in Cornwall (UK), to understand the role of mineral dissolution and precipitation in controlling the permeability evolution of the system. The physico-chemical properties of the cores were monitored after each reaction-phase using ICP-OES, SEM, hydrostatic permeability measurements, and X-ray Computed Tomography. Results show that permeability evolution is strongly dependent on fluid chemistry. Undersaturated alkaline fluids dissolve the most abundant mineral phases in granite (quartz and feldspars), creating cavities along the main fractures and generating pressure-independent permeability in the core. Conversely, supersaturated alkaline fluids, resulting from extended periods of fluid-rock interactions, promote the precipitation of clay minerals, and decrease the permeability of the system. These results suggest that chemical dissolution during geothermal operations could generate permeable pathways that are less sensitive to effective stress and will remain open at higher pressures. Similarly, maintaining the circulation of undersaturated fluids through these granitic reservoirs can prevent the precipitation of pore-clogging mineral phases and preserve reservoir permeability in granite-hosted geothermal systems