We identify a new category of natural hydrothermal sys tems i n t ermed i at e between liquid- and vapor-dominated. This category is characterized by a "vapliq" vertical pressure profile, which is nearly vaporstatic in the shallower portion of the system, and nearly boiling-point-for-depth at depth. The prototype of these systems is the geothermal field of Los Azufres, Mexico. To explore the thermohydrological conditions conducent to this type of system, we propose a 1-D vertical scenario based on generally accepted conceptual models of liquid- and vapor-dominated geothermal reservoirs. We use the corresponding mass and thermal energy transport equations to establish that a necessary condition for the existence of 2-phase hydrothermal systems is that the absolute value of the vertical thermal flux must exceed &inl a parameter that depends only on the values of the pressure and of the thermal conductivity at the boiling point of the system. The values of &in are typically 1-4 times the average terrestrial flux. We also find that geothermal systems in which convective heat t r a nspo r t is accomplished by the well-known heat-pipe mechanism can exist only if the corresponding heat flux exceeds and the permeability at the boiling point of the system is smaller than kgmax, a parameter that depends, only on the values of the pressure and of the thermal conductivity at the boiling point. Ty ical values bf kgmax are 1-3x10- P8 m2, suggesting a reason for the fact that all vapor-dominated systems are associated with very-low matrix permeability formations. Applying these insights, and the mass and heat transport equations to Los Azufres, we conclude that a contrast of 1-3 orders of magnitude exists between the vertical permeability at the boiling point and that corresponding to the vapor-dominated portion of the system. We propose that similar permeability contrasts may be responsible for the characteristic composite pressure profiles observed in other vapliq systems.

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Copyright 1988, Stanford Geothermal Program: Readers who download papers from this site should honor the copyright of the original authors and may not copy or distribute the work further without the permission of the original publisher.