Capillary desaturation experiments produce disconnected
(trapped) ganglia of mesoscopic sizes intermediate between
pore size and system size.
Experimental evidence for interactions between
these mesoscale clusters during desaturation
is analyzed and discussed within the
established microscopic and macroscopic laws of Newton,
Young-Laplace and Darcy.
A theoretical expression for capillary number correlations
is introduced that seems to have remained unnoticed.
It expresses capillary desaturation curves in terms
of stationary capillary pressures and
The theoretical expression shows that the plateau
saturation in capillary desaturation curves may
in general differ from the residual
nonwetting saturation defined through the
saturation limit of the main hysteresis loop.
Hysteresis effects as well as the difference between wetting
and nonwetting fluids are introduced into
the analysis of capillary desaturation experiments.
The article examines experiments with different
desaturation protocols and discusses the existence of
a mesoscopic length scale intermediate between
pore scale and sample scale.
The theoretical expression is derived entirely within the
existing traditional theory of two-phase flow in
porous media and compared to a recent experiment.