Page 90 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

78 Reservoir Engineering

oil is flowing) and no permeability to water. At this point, the core can be closed
in for about 1,000 hours to allow sufficient time for wettability changes to occur.
Then the core is flooded with water in an unsteady-state test, or fixed ratios of
water and oil are injected in the steady-state test. If water continues to be the
wetting phase, the relative permeability to water (which will be only a function
of saturation) will be the same during the drainage and imbibition cycles. (The
importance of wettability on relative permeabilities will be discussed in the next
section of this chapter.) Aa the water is injected into the oil-flooded core, kw
increases while k, decreases. Not all of the oil can be displaced from the core,
regardless of the water throughput (at modest flow rates or pressure drops),
and this is referred to as the waterflood residual oil saturation, Sor.
Similar observations apply to gas-oil relative permeability data as displayed
in Figure 5-55. Typically, the gas-oil relative permeabilities are plotted against
the total liquid saturation, which includes not only the oil but also any connate
water that may be present. In the presence of gas, the oil (even if connate water
is present) will be the wetting phase in preference to gas. As a result, the k,
curve from gas-oil flow tests resembles the drainage krw curve from oil-water
flow tests. As seen in Figure 5-55, the irreducible gas saturation (also called
the equilibrium or critical gas saturation) is usually very small. When gas

Liquid saturation=S,,+S,, %

Flgure 5-55. Gas-oil relative permeability data [20].