Page 51 - Primer on Enhanced Oil Recovery

42 Primer on Enhanced Oil Recovery

case the rock is water wet and initially is fully saturated with water (S w 5 1). At the
initial drainage very small pressure is needed to extract approximately half of the
connate water. As the water is drained further and the saturation goes below 0.3 the
capillary pressure starts to rise sharply. One needs to apply significant external pres-
sure to counterbalance the capillary pressure to continue drainage. Sharp capillary
pressure rise means that more and more external pressure is needed to extract smal-
ler and smaller amounts of water. At certain point one reaches the pressure at which
drainage becomes unsustainable for various reasons. This is unreducible water satu-
ration. If at this point the rock is brought in contact with water then water will be
drawn into the rock it is an imbibition process. However, in the case presented,
the saturation would only reach the maximum at 0.2.
Remember, at this point, there is no external pressure to push water into the rock
at this point. If we apply now external pressure then the external pressure will coun-
terbalance the capillary pressure and the imbibition will continue. At first the imbi-
bition continues relatively easily but then, after approximately 0.55 saturation
value, the pressure needs to be increased quite sharply to push in smaller and smal-
ler amounts of water. In the example provided we cannot get water saturation above
approximately 0.65. The secondary drainage will create a hysteresis loop for water
flow through the rock at a variable water saturations.
The explanation of the observed behavior is relatively simple. As saturation
decrease only smaller and smaller pores would contain wetting phase and one needs
more and more force to counterbalance the capillary force and extract smaller and
smaller amounts of liquid.
The same is perfectly applicable to oil saturation in a rock. The behavior is more
complicated for the multiphase system, but, generally, the behavior is similar. One
only needs to remember that much also depends on the saturation history and the
speed of saturation changes.
It can be seen that in the dynamic situation it is impossible to have full saturation
and it is impossible to recover all liquid from the rock. There is always residual sat-
uration there is always oil and connate water left in the reservoir. Up to the point
we can reduce residual oil saturation by either changing wettability or by decreasing
interfacial tension but there is always oil left in the reservoir beyond economical
recovery.
Oil extraction is a dynamic situation liquids should flow though the intercon-
nected pores and individual phase saturation is usually below 100%. In water-wet
rock some pressure gradient needs to be applied for water to flow into the bigger
pores, while making oil to flow into bigger pores requires some pressure in oil wet
formation.
Partial permeability depends on the saturation too. In a simple model system
water partial permeability k rw and oil partial permeability k ro dependences for a
water-wet rock can be as shown on Fig. 5.5. It is possibly to say in general that
the permeability for a phase will be at the highest at the highest phase saturation
and that the permeabilities would be reduced to zero for the unreducible
saturation.