Page 192 - Petroleum Geology

those for increasing water saturations, imbibition curves. From the previous
discussion of irreducible water saturation, it will be evident that a drainage
experiment is terminated at irreducible water saturation (as it must be), and
an imbibition experiment is terminated at the irreducible oil saturation. This
is reached when the oil saturation is insufficient to maintain a continuous oil
phase through the pore space, and the capillary pressure that exists in the oil
droplets so formed is insufficient to drive them from one pore to the next,
through the constrictions or “throats”.
There are several points of interest in relative permeability curves. First,
the sum of the relative permeabilities is always less than unity. Secondly, the
drainage relative permeability to water may become negligibly small before
the irreducible water saturation is reached. Thirdly, the drainage relative per-
meability to oil is close to unity at irreducible water saturation (it has even
been claimed to be greater than unity). From a practical reservoir engineering
point of view, such curves can be used to predict production rates, water./oil
ratios, and water cut (proportion of water in total production or yield) at
various saturations.
The reason for the high drainage relative permeability to oil at irreducible
water saturation is that the oil in a water-wet sand (for example) is excluded
from the pendular spaces, which contribute little to the flow of a single liquid.
The reason for the very low relative permeability to water at low saturations
above the irreducible is that the water is denied access to the central pore
space and must flow in thin streams or films around the grains - paths of
much greater tortuosity. The reason for the sum of the relative permeabilities
being less than unity is that each liquid interferes with the other, and at satura-
tions between the two irreducible limits, the tortuosity of each component
is greater than the tortuosity of a single-phase flow. That these two tortuosities
are unequal is shown by the feature that the relative permeabilities are equal
at water saturations greater than 50%, commonly 60-65%.
For production purposes we are more interested in imbibition curves that
will predict well behaviour as water saturations increase. In migration studies,
we shall be more interested in drainage curves. Figure 8-12 shows the predicted
waterloil ratio and water cut derived from the imbibition relative permeability
curves of Fig. 8-12.

Mechanics of production

The energy of a petroleum reservoir comes from the expansion of the
water and/or the gas to replace the volume of petroleum produced. If water
expansion below the oil/water contact is the main driving force, the reservoir
is said to have water drive; if gas expansion above the gas/oil contact is the
main driving force, the reservoir is said to havegas drive. Because it is necessary,
as we shall see, to conserve the natural reservoir energy, gas is never produced
intentionally when it exists as a gas cap to an oil reservoir that is economically
producible.