| Abstract |
A steady state flow under action of gravity with water-vapour phase transition is considered. It is assumed that water layer lies over the layer of superheated vapour in a stratum that locates between two parallel high permeability domains saturated with water and vapour, respectively. When fluid flows from one high permeability domain to another one through the stratum, phase transition occurs at the water-vapour interface. Under the assumption of smallness of advective energy transfer as compared with the conductive one, the stationary distributionof the characteristics in the stratum with water-vapour phase transition is obtained. Consequently, the problem is reduced to the investigation of the boundary value problem at the unknown interface. The solution is shown to describe both vaporization and condensation processes in reservoirs. It is also found that three different solutions can exist for the same boundary conditions for the pressure and temperature functions. Investigation of normal stability of the interface shows that stable configuration corresponds to the unique solution whereas unstable configuration arises when steady state solution is not unique. It is found that stable water over steam configuration can exist when permeability value is about 1 millidarcy. This value is greater by the order than the critical value in Schubert-Straus example of the geothermal system. The fairly high value of the permeability makes it possible to explain the existence of a wide class of stable natural geothermal reservoirs where vapour layer underlies the water one. |