Gravitational Stability of a Moving Water-Steam Interface in Hydrothermal Systems

The stability of a hydrothermal system is considered. In this system a water layer overlies a superheated-steam layer in a reservoir with relatively low permeability located between two parallel high-permeability strata. The solution of the steady-state bounded problem with a phase transition interface separating the water and steam zones is obtained on the assumption that the convective energy transfer is small as compared with the conductive transfer. An investigation of the normal stability of the phase transition interface shows that stable configurations almost always exist in the geothermal system considered on the permeability range bounded from above by the value k ∼ 10-15 m2. Thus, the criterion of predominance of conductive transfer, which is simultaneously the criterion of existence of the fundamental solution, turns out to be the criterion of stability of the phase interface in the geothermal system considered. The fairly high value of the permeability satisfying this criterion makes it possible to explain the existence of stable natural geothermal reservoirs in which a water layer overlies a steam layer.