THERMAL CONSTRAINTS ON THE SEALING EFFICIENCY OF THE CAPROCK OVERLYING THE MEDICINE LAKE HYDROTHERMAL SYSTEM
|Authors:||Colin F. Williams and Frederick V. Grubb|
|Conference:||Stanford Geothermal Workshop||Session:||Geology|
|Abstract:||Medicine Lake volcano, a large Quaternary volcano located within the northern California segment of the Cascades Range, has been identified as the possible site of a 48.8 MW powerplant to be operated by the California Energy General Corporation. Recent research by the USGS and other institutions has focused on evaluating and understanding the sealing efficiency of the caprock overlying the hydrothermal system. The caprock consists of hydrothermally altered volcanic rock that isolates the hydrothermal system from shallow, low-temperature ground water. Water-level and fluid pressure measurements in shallow (<200 m) water wells, intermediate depth (up to 1400 m) temperature-gradient holes, and deep (up to 2600 m) exploratory geothermal production wells demonstrate that the hydraulic seal produced by the caprock results in a hydraulic head difference of more than 200 m between the shallow ground water system and the lower pressure geothermal reservoir. Equilibrium temperature measurements acquired from three exploratory production wells have been combined with thermal conductivity measurements on core samples to reveal three thermal regimes characterizing the Medicine Lake system. The upper 100 to 400 m are nearly isothermal, reflecting the rapid downward flow of cold (<20 oC) meteoric water through the shallow subsurface. From the base of this isothermal zone to depths as great as 800 m, temperatures increase steadily with depth, reflecting constant, conductive heat flow. This interval of conductive heat flow corresponds to the zone of argillic alteration identified by J. Hulen and S. Lutz of EGI as the caprock of the hydrothermal system. Below this interval the nearly isothermal conditions of the geothermal reservoir predominate, with temperatures exceeding 250 oC.
Mathematical models for the thermal effects of vertical water flow within the Medicine Lake system indicate that downward flow in the shallow subsurface is rapid, reaching rates of approximately 50 cm/yr, a result consistent with observations of this "rain curtain" effect in other young volcanic settings. In addition, the thermal models indicate that all of this flow terminates in the upper 100 meters of the zone of argillic alteration. Variations in heat flow below this point are best explained by conductive transients resulting from the evolving physical state and geometry of the hydrothermal system.