HR: 1340h
AN: U43C-1379    [Abstracts]
TI: Experimental alteration of peridotite during injection of CO2-rich fluids
AU: * ANDREANI, M
EM: andreani@msem.univ-montp2.fr
AF: Laboratoire de Geosciences CNRS - Universite Montpellier 2, Place Eugene Bataillon, Montpellier, 34090, France
AU: LUQUOT, L
EM: luquot@msem.univ-montp2.fr
AF: Laboratoire de Geosciences CNRS - Universite Montpellier 2, Place Eugene Bataillon, Montpellier, 34090, France
AU: GOUZE, P
EM: philippe.gouze@msem.univ-montp2.fr
AF: Laboratoire de Geosciences CNRS - Universite Montpellier 2, Place Eugene Bataillon, Montpellier, 34090, France
AU: GODARD, M
EM: marguerite.godard@gm.univ-montp2.fr
AF: Laboratoire de Geosciences CNRS - Universite Montpellier 2, Place Eugene Bataillon, Montpellier, 34090, France
AU: GIBERT, B
EM: benoit-gibert@gm.univ-montp2.fr
AF: Laboratoire de Geosciences CNRS - Universite Montpellier 2, Place Eugene Bataillon, Montpellier, 34090, France
AB: CO2 sequestration by direct injection into geological formation is a highly regarded option for reducing greenhouse gas emissions. Carbonation of ultramafic rocks is the most efficient reaction to trap CO2 into stable carbonate phases. Thus, widespread ultramafic bodies represent an immense storage capacity by long-term hydrothermal processes (slow-spreading ridges) and by industrial injection of CO2-enriched fluids. A critical issue for durability of both processes is the prediction of consequences of fluid circulation/injection on rock properties. Yet, fundamental parameters characterizing in situ reactions of CO2-enriched fluids in ultramafic rocks and their consequences on hydrodynamic properties are lacking. We investigate these processes by scaled rock sample percolation experiments. We present results of experiments performed using sintered grounded dunite (96.5% olivine, 2% diopside, 1.5% spinel) flooded at constant specific discharge (0.6 ml/min) with CO2-enriched fluid up to PCO2 = 95 bars, under a confined pressure of 120 bars and a temperature of 160C. Permeability stabilizes after an increase period of 1h. The chemical composition of the outlet fluid is dominated by Si and is depleted in Mg relative to stoechiometric dissolution of olivine during the whole experiment. SEM imaging of the central part of the core sample reveals an increase in porosity starting in the vicinity of the fluid inlet and propagating toward the core outlet. Behind this reaction front, diopside grains are preferentially altered. A porous and poorly crystallized material is observed around diopside and olivine grains. EDS-TEM is required to characterize the neoformed material. Mg-enriched carbonates develop locally on olivine grains, preferentially in the vicinity of diopside. Olivine and carbonate are separated by a porous interface made of a poorly crystallized Si- rich gel and nanograins of magnetite. Gel composition varies between talc and serpentine stoechiometry. These results show that carbonation is rapid and overcome serpentinization under those conditions (xCO2 = 0.8). The existence of the porous reactive interface promotes fluid renewal at the olivine surface, and allows reaction to persist despite the carbonate formation. Pressure loss (permeability), measured across the sample, is constant during the experiment independently on the fluid-rock mass transfers. Probably the permeability increase due to the increase of the nanoporosity by dissolution is balanced by the increasing pressure loss induced by the gel formation. Long-lasting experiments are presently run to test the limit of this process.
DE: 1012 Reactions and phase equilibria (3612, 8412)
DE: 1039 Alteration and weathering processes (3617)
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3614 Mid-oceanic ridge processes (1032, 8416)
DE: 5112 Microstructure
SC: Union [U]
MN: 2007 Fall Meeting