Page 417 - Petrophysics 2E

WATER-OIL-ROCK INTERFACIAL ACTMTY 385

r
%
- CORE QLICONE WETTABILITY
0.64s
0
1
:: 90- 2 0.02 0.176
#
3 0.2 -0.222
4 2.0 -1.250
-1.335
Curvoo cut off at woR-ioo
_,,,._ ._ -.....-..... -.---.--d

I I

There are a number of other influences that disturb the normal trend
of relative permeability curves, and they must not be confused with
the effects due to wettability alone [72]. The relative flow of fluids
is a function of pore size distribution; therefore, any change of this
distribution due to blocking will change the relative permeability curves.
The overburden pressure applied to cores in the laboratory changes
pore-size and pore-throat size distributions, reducing the size of the larger
pores, which changes the porosity as well. Furthermore, smaller pore
sizes may increase the irreducible water saturation in water-wet rocks and
the residual oil saturation; thus, the mobile oil saturation is decreased.
An increase of temperature causes the wettability to change to a more
water-wet system. Thus, core floods for the determination of relative
permeabilities should be conducted at simulated reservoir conditions of
overburden pressure, pore pressure, and temperature for the resulting
relative permeability curves to be representative of conditions in the
reservoir [69-721.
The saturation history of the water-oil-rock system (or core) has a
fundamental influence on the equilibrium wetting condition of the rock.
Oil reservoirs are generally assumed to have been filled with water,
which was displaced into a trap by migrating oil. Thus oil reservoirs
tend to be preferentially water-wet, although several major oilfields have
been found to be oil-wet and heavy oil deposits ('MI < 20) are generally
oil-wet. The wettability of the reservoirs probably changed gradually
from strongly water-wet to some degree of intermediate wettability and
finally to oil-wet as polar compounds in the oil diffused to the interface
and adhered to the rock surface. Thus oilfield rocks exhibit all degrees
of wettability from strongly water-wet to strongly oil-wet [3, 46, 65,
73, 741. Low-molecular-weight compounds and gas in the crude oil
may accelerate the deposition of polar compounds by the deasphalting