Page 166 - Well Logging and Formation Evaluation

156 Well Logging and Formation Evaluation

Pressure

Pc

Critical point
Liquid

Gas

Tc Temperature
Figure 11.1.1 P-T Diagram

nR = constant = P V T = P V T 2 . (11.1.2)
1
11
2 2
Hence, V 2 = V 1* (P 1*T 2 / (P 2 *T 1).
In the example, V 1 = 3000, T 1 = 423, P 2 = 2500, T 2 = 403K. Hence,
3
V 2 = (3000*403 / (2500*423) = 1.14m . A more correct equation for real
gases as encountered in reservoirs is that of van der Waals:

2
( pa V ) ( * V - b) = RT. (11.1.3)
+
This equation may be expressed in the form:

pV = nZRT (11.1.4)

where Z varies with pressure and temperature. In order to calculate Z, it
is first necessary to know, for the gas concerned, the values of the criti-
cal pressure and temperature defined by P c and T c . These are used to
determine the reduced pressure and temperature given by:

P r = P P c
(11.1.5)
T r = P P c .

The most commonly used experimentally determined correlation
between Z and P r /T r is that of Standing and Katz, shown in Figure 11.12.