HR: 0800h
AN: H11B-0488    [Abstracts]
TI: Stochastic Analysis of Precipitation/Dissolution and Aperture Alteration in Variable Aperture Fractures Under Gradient-Reaction Conditions
AU: * Chaudhuri, A
EM: abhijit.chaudhuri@colorado.edu
AF: University of Colorado, Boulder, Campus Box428, Department of Civil Engineering, Boulder, CO 80309-0428,
AU: Rajaram, H
EM: hari@colorado.edu
AF: University of Colorado, Boulder, Campus Box428, Department of Civil Engineering, Boulder, CO 80309-0428,
AB: Precipitation and Dissolution reactions within fractures alter fracture apertures, which in turn affects their flow and transport properties. Different types of aperture alteration patterns occur in different flow and reaction regimes. One class of regimes encountered in geological systems is the "gradient reaction" regime, where fluids are essentially in chemical equilibrium with a mineral everywhere, but precipitation-dissolution reactions are driven by solubility gradients that result from variations in temperature or salinity. In many such cases, the solubility gradient is invariant over very long periods of time, and largely unaffected by medium alteration. For instance in a sparsely fractured rock mass, heat transfer is largely conduction-dominated, due to the large heat capacity of the rock, and not significantly modified by fluid flow or the feedback between aperture alteration and fluid flow. Similar behavior has also been postulated during the emplacement of ocean-bed methane hydrates. We present a stochastic analysis to develop equations for the evolution of the mean aperture, aperture variance, spectrum/covariance and effective transmissivity under gradient-reaction conditions. The stochastic analysis consistently predicts (i) a runaway growth of transmissivity in the case of dissolution and (ii) a much slower rate of transmissivity decrease in a variable-aperture fracture than in a parallel-plate fracture. In the case of dissolution, an increase in initial aperture variance leads to a faster rate of transmissivity growth, while in the case of precipitation it leads to a slower rate of transmissivity reduction. Dissolution leads to an enhancement of anisotropy in the aperture correlation structure, with more persistent correlation in the direction of flow. The behavior is opposite in the case of precipitation. The predictions of the stochastic analysis are verified based on high-resolution Monte-Carlo simulations in computer-generated random initial aperture fields. We discuss potential applications of our results to natural and engineered geological processes incuding hypogene karstification, methane hydrates and geothermal systems. We also present preliminary results from ongoing work evaluating the role of convective heat transfer and hydromechanical coupling on aperture alteration in a gradient reaction regime
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1829 Groundwater hydrology
DE: 5104 Fracture and flow
DE: 8010 Fractures and faults
DE: 8045 Role of fluids
SC: Hydrology [H]
MN: 2007 Fall Meeting