Page 153 - Hydrocarbon

140 Reservoir Fluids

by subtraction
P o P w ¼ðr r Þgh ¼ P c
w o
and remember that
2s cos y
P c ¼
r t
This is consistent with the observation that the largest difference between the
oil–water interface and FWL occurs in the narrowest capillaries, where the capillary
pressure is greatest. In the tighter reservoir rocks, which contain the narrower
capillaries, the difference between the oil–water interface and the FWL is larger.
If a pressure-measuring device were run inside the capillary, an oil gradient
would be measured in the oil column. A pressure discontinuity would be apparent
across the interface (the difference being the capillary pressure), and a water gradient
would be measured below the interface. If the device also measured resistivity, a
contact would be determined at this interface, and would be described as the OWC.
Note that if oil and water pressure measurements alone were used to construct a
pressure–depth plot (Figure 6.30), and the gradient intercept technique was used to
determine an interface, it is the FWL which would be determined, not the OWC.
The difference between the OWC and the FWL is greater in tight reservoirs,
and may be up to 100 m difference. A difference between GOC and free oil level
exists for the same reasons, but is much smaller, and is often neglected.
For the purpose of calculating oil in place in the reservoir, it is the OWC, not
the FWL, which should be used to define to what depth oil has accumulated. Using
the FWL would overestimate the oil in place, and could lead to a significant error in
tight reservoirs.

6.2.9.3. Saturation–height relationships
Saturation is the proportion of one fluid phase in a pore system to the total amount of
fluid. Initially, the pores in the structure are filled with water. As oil migrates into the
structure, it displaces water downwards, and starts with the larger pore throats where
lower pressures are required to curve the oil–water interface sufficiently for oil to
enter the pore throats. As the process of accumulation continues, the pressure
difference between the oil and water phases increases above the FWL because of the
density difference between the two fluids. As this happens the narrower pore throats
now begin to fill with oil, and the smallest pore throats are the last to be filled. When
no more water is able to be removed, the reservoir is at irreducible water saturation.
The reservoir is composed of pores of many different sizes, and can be compared
to a system of capillary tubes of widely differing diameters, as shown in Figure 6.31.
The narrowest capillaries determine the level above which only the irreducible
(or connate) water remains. Typical irreducible water saturations are in the range
10–40%. The largest capillaries determine the level below which the water
saturation is 100%, that is the OWC. Between the two points there is a gradual
change in the water saturation, and the interval is called the transition zone. The
height of the transition zone depends on the distribution of pore sizes, but can be