Two-phase boundary layer formation in a semi-infinite porous slab

Similarity profiles of pressure and saturation are analysed which result from one-dimensional planar withdrawal of fluid from a porous region initially containing a two phase mixture of steam and water. Approximate expressions are derived for the evolution of pressure and saturation profiles, and boundary-layer changes in saturation are identified. The existence of a similarity variable implies that the saturation conditions for the reservoir tend with time either to having both phases flowing; or to a single phase vapour. In particular, the nonlinear nature of the governing equations implies that infinitesimal changes in pressure can produce finite changes in saturation. The two mechanisms responsible for saturation changing with time involve local changes in energy storage in rock and fluid; together with spatial variations in flowing enthalpy. The latter mechanism occurred relatively slowly in the examples treated, and was responsible for boundary-layer formation when one phase was initially immobile. Dimensional analysis reveals that when a boundary layer develops, the underlying equations involve essentially only one dimensionless parameter which is typically small, being associated with the ratio of the energy density of the mobile phase relative to the total energy density.