VAPLIQ: Hydrothermal Systems in the Vertical Permeability of Los Mexico Geothermal Reservoir

Abstract

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.