Page 89 - Reservoir Formation Damage

72 Reservoir Formation Damage

on the amount of the oil and water affine clay minerals existing in the
formation, and the morphology of illite which behaves hydrophilic in
fibrous form and hydrophobic in platy form.

End-Point Saturations

The end-point saturations determine the mobile fluid saturation range
for the flow functions. The end-point saturations,^, S t, and 5 or, for an
oil-gas-water system represent the connate water, trapped gas, and residual
oil saturations which vary as a result of the packing of particles during
formation damage. The values of these quantities are larger for ordered
packing of particles (~40%) and smaller for disordered packing of par-
ticles (-10%). They can be correlated with permeability. For a given type,
however, they decrease by increasing permeability or porosity. For
example, as shown by Collins (1961), the connate water saturation
decreases linearly with increasing logarithmic permeability in sandstones.
Thus, the end-point (also known as irreducible, residual, or immobile)
fluid saturations can be approximated by:

= aj- :j = gas, oil, water (4-5)
S rj
where j denotes the gas, oil, or water phases, r denotes the end-point
saturation condition, K is permeability and a, and bj are some empirically
determined parameters.

Alteration of the Flow Functions: Capillary
Pressure and Relative Permeability

Capillary pressure and relative permeability vary by (1) the pore
surface properties including wettability, end-point saturations and contact
angle, and (2) the net overburden stress effecting the tortuosity, porosity
and interconnectivity of pores. Marie (1981) points out that capillary
pressure and relative permeability are complicated functions of the
properties of the fluids and porous media. By dimensional analysis of an
oil-water system in porous media, Marie (1981) has shown that these flow
functions can be correlated by means of the pertinent dimensionless
groups as:

(4-6)