Page 89 - Geology of Carbonate Reservoirs

70 PETROPHYSICAL PROPERTIES OF CARBONATE RESERVOIRS
a

TABLE 3.1 Typical Contact Angle and Interfacial Tension Values

Interfacial Tension
System Contact Angle (dynes/cm)
Air – brine 0 ° 72
Air – mercury 140 ° 480
Crude oil – water 0 ° 35

3
a Typical fluid density ranges, according to Vavra et al. (1992) , are gas = 0.00073 – 0.5 g/cm , oil = 0.51 – 1.0

3
3
g/cm , and brine = 1.0 – 1.2 g/cm .
Source : Vavra et al. (1992) .
⎛ σ w/o cos θ w/o ⎞
P cR = P cL⎜ ⎝ σ ⎟ ⎠
a/m cos θ a/m
where P cR is capillary pressure in the reservoir brine – oil system, P cL is air – mercury
capillary pressure from the laboratory, σ w/o is the brine – oil interfacial tension, σ a/m

is air – mercury interfacial tension, θ w/o is the contact angle of the brine – oil – solid

system, and θ a/m is the contact angle of the air – mercury – solid system. Under ideal

conditions the values for contact angles and interfacial tensions should be deter-
mined in the laboratory. These measurements are difficult and costly, and according

to Vavra et al. (1992) , the values in Table 3.1 are commonly used instead.
3.2.3 Height of Oil Column Above Free - Water Level
Once the air – mercury capillary pressure data are converted to brine – oil values, the
height of oil above the free - water level ( P c = 0) can be calculated. This method can

be used to predict saturation for any type of fluid in different kinds of reservoir

pore systems at any chosen height in the reservoir. The expression for calculating
the height of the oil column is
h = P c
.
0 433(ρ b − ρ o )

where h is the height of the oil column in feet above the free - water level, P c is the
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is the specific density of brine in g/cm
capillary pressure value for brine – oil in psi, ρ b
3
is the specific density of oil in g/cm at ambient conditions,

at ambient conditions, ρ o
and 0.433 is the pressure gradient (specific weight) in pure water expressed as

psi/ft.
3.2.4 Evaluating Seal Capacity
If capillary pressure data are available on rocks from sealing formations, the
maximum height of hydrocarbon column that can be confined by the seal before

leakage is given by
P dS −
h max = P dR
.
0 433(ρ w − ρ hc )