The transition from hydrostatic to greater than hydrostatic fluid pressure in presently active continental hydrothermal systems in crystalline rock
- Geological Survey, Menlo Park, CA (United States)
Fluid flow at hydrostatic pressure (P{sub h}) is relatively common through fractures in silicic and in mafic crystalline rocks where temperatures are less than about 350-370C. In contrast, pore-fluid pressure (P{sub f}) > P{sub h} has been encountered at the bottom of 3 geothermal exploration wells that attained temperatures >370C (at Larderello, Italy, at Nesjavellir, Iceland, and at The Geysers, California). Chemical sealing by deposition of minerals in veins appears to have allowed the development of the high P{sub f} encountered in the above wells. The upper limit for the magnitude of P{sub f} that can be attained is controlled by either the onset of shear fracturing (where differential stress is relatively high) that reopens clogged veins, or the hydraulic opening of new or old fractures (at relatively low values of differential stress). The brittle-plastic transition for silicic rocks can occur at temperatures as high as 370-400C in tectonically active regions. In regions where high-temperature geothermal systems develop and persist, it appears that either strain rates commonly are in the range 10{sup {minus}12} to 10{sup {minus}13}, or that silicic rocks in the shallow crust generally behave rheologically more like wet quartz diorite than wet Westerly granite.
- OSTI ID:
- 5462776
- Journal Information:
- Geophysical Research Letters (American Geophysical Union); (United States), Vol. 18:5; ISSN 0094-8276
- Country of Publication:
- United States
- Language:
- English
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58 GEOSCIENCES
HYDROTHERMAL SYSTEMS
RESERVOIR PRESSURE
BRITTLE-DUCTILE TRANSITIONS
CALIFORNIA
CONTINENTAL CRUST
DEPOSITION
FLUID FLOW
GEOLOGIC FRACTURES
GEOTHERMAL FLUIDS
GEOTHERMAL WELLS
HYDRAULIC FRACTURING
ICELAND
ITALY
MINERALS
PORE PRESSURE
RESERVOIR TEMPERATURE
SEALS
STRAIN RATE
TECTONICS
DEVELOPED COUNTRIES
DEVELOPING COUNTRIES
EARTH CRUST
EUROPE
FLUIDS
FRACTURING
GEOLOGIC STRUCTURES
GEOTHERMAL SYSTEMS
ISLANDS
NORTH AMERICA
USA
WELLS
150301* - Geothermal Exploration & Exploration Technology- Geophysical Techniques & Surveys
580000 - Geosciences