HR: 0800h
AN: MR11A-0926    [Abstracts]
TI: Reaction-Induced Permeability Change in Thermally Cracked and Deformed Aplite: Importance of Reactive Surface Area and Mineralogy
AU: * Tenthorey, E
EM: eric.tenthorey@anu.edu.au
AF: Australian National University, RSES Mills Rd., Canberra, ACT 0200 Australia
AB: This experimental study investigates hydrothermal reactions in a granitic system and attempts to quantify how such reactions affect hydrologic properties, namely specimen permeability. Of specific interest is the evolution of permeability under variable differential stress conditions, from the compactional and dilatancy regimes to that of shear failure. Under these different stress conditions, reactive surface area will vary, possibly affecting the rate and absolute magnitude of permeability change. Experiments were conducted using a Paterson gas apparatus capable of independently controlling confining pressure (Pc), pore pressure (Pp) and axial load. Most experiments were conducted at Pc=100 MPa and Pp=50 MPa with temperatures of 200-600$\deg$C. Under isostatic conditions, permeability was observed to increase with temperature due to increased thermal cracking at grain boundaries. As differential stress was increased in each experiment, permeability was first observed to decrease, presumably due to crack closure. Upon continued loading to higher stresses, dilatancy resulted in significant permeability enhancement. In later experiments, permeability was allowed to evolve at each stress level and was observed to decay by an exponential function of the form k $\alpha$ 1-$\mu$(1-exp$^{(-rt)}$)$^{2}$, suggesting a precipitation type mechanism for the observed permeability change. The rate constant AŸA›A›ƒ_sAªA.ƒ_orAŸA›A›ƒ_sAª? progressively increased up to 500$\deg$C, but was much smaller in the 600$\deg$C experiment, indicating a possible change in the precipitating mineral assemblage as suggested by experimental studies in the KNASH system. Overall, reaction rates were enhanced during dilatancy and after rupture, an observation suggesting a negative feedback effect, in which enhanced mineral precipitation moderates permeability generation during episodes of deformation. The nature of fluid flow in such systems is crucial to the formation of porphyry metal deposits and also plays a major role for the development of some geothermal energy projects. These experimental results will also have important implications for precipitation-induced sealing in granitic fault zones, where hydrologic changes might alter various physical properties of the fault.
DE: 5104 Fracture and flow
DE: 5114 Permeability and porosity
DE: 5139 Transport properties
DE: 3630 Experimental mineralogy and petrology
SC: Mineral and Rock Physics [MR]
MN: 2004 AGU Fall Meeting