Gravitational stability of water over steam in vapor‐dominated geothermal systems

Vapor‐dominated geothermal systems exist in a most extraordinary state: a condensate or water layer several hundred meters thick overlies a main steam zone of undetermined thickness. Why doesn't the water fall down? We show that a heavy fluid phase overlying a light phase of the same fluid in a porous medium can be stable provided the permeability in the vicinity of the interface is not too large. The special behavior of a phase change interface in response to disturbances of the boundary account for the stabilization. Heavy fluids overlying light fluids in a porous medium are unstable, irrespective of permeability, if the fluids are immiscible. Application of the stability analysis to vapor‐dominated geothermal systems shows that the condensate layer can be stabilized if the permeability near its base is no larger than about 40 nm2 (0.04 millidarcy).