Hydrology and thermomechanics of liquid-dominated hydrothermal systems in Iceland Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/ms35tc96f

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  • Low-temperature hydrothermal activity in Iceland is apparently mostly controlled by dikes and fractures. Conventional methods of production data analysis are not readily applicable in cases of heterogeneous/anisotropic fracture dominated hydrothermal systems. Moreover, the dikes and fractures may control the heat uptake mechanism of low-temperature activity. The free-surface response functions of analytical reservoir models are presented and methods for analyzing production data on the basis of such models are developed. Based on a homogeneous and isotropic half-space model apparent permeability estimates of 0.7 millidarcy are obtained for two low-temperature systems in Tertiary strata in N-Iceland whereas estimates of 5-20 millidarcy are obtained for two systems in Quaternary strata in SW-Iceland. A vertical two-dimensional flow model is, however, more consistent with the apparent linear dike/fracture control of many hydrothermal systems and results in higher permeability estimates. Methods of simulating long term production data by simple lumped capacitor/conductor ladders based on only production/drawdown data are developed and the responses of analytical as well as real systems are shown to be easily simulated by such simple systems. The parameters of simulation ladders also provide information on global hydrological characteristics of hydrothermal systems. A possible dike/fault controlled source mechanism of low-temperature activity in Iceland is considered. This process involves the downward migration of open sections of unwelded quasi-vertical fractures resulting from cooling and contraction of the adjacent rock, in conjunction with vertical heat transfer in the fracture. The rate of downward migration is estimated and found to depend very strongly on the magnitude of the horizontal regional stress. Stress conditions may therefore determine whether a low-temperature system can evolve at a given location as well as determine the intensity of hydrothermal activity.
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