Page 72 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

60 Reservoir Engineering

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0 400 800 1200 1600 Moo 2400
SATURATION PRESSURE, psi

Figure 5-41, Surface tension of several crude oils [Ql].

temperature is usually somewhat more pronounced than is observed for surface
tension. Although no quantitative relation is observed, the general trend suggests
lower interfacial tensions for the higher API gravity crudes. However, in studies
with a crude oil containing large amounts of resins and asphaltenes, different
effects of temperature on interfacial tension were observed when measurements
made at aerobic conditions were compared to anaerobic tests [93]. Interfacial
tension between the crude and reservoir brine showed a decrease with an
increase in temperature under aerobic conditions, whereas at anaerobic con-
ditions, interfacial tension increased with increasing temperatures. This dif-
ference in behavior was attributed to oxidation of the stock tank oil in the
aerobic tests. At conditions of reservoir temperature and pressure, interfacial
tension of the live reservoir oil was higher than the stock tank oil. The study
concluded that live reservoir crude should be used in measurements of interfacial
properties and that if stock tank oil is used, at least the temperature should
correspond to reservoir conditions.
Figure 543 shows the effect of dissolved gas and pressure on the interfacial
tension of three oil-water systems [89]. For each system, interfacial tension
increases as the amount of dissolved gas increases, but drops slightly as pressure
is increased above the bubblepoint.
Surface and interfacial tensions are important in governing the flow of fluids
in the small capillaries present in oil-bearing reservoirs. The capillary forces in
oil or gas reservoirs are the result of the combined effect of surface and
interfacial tensions, pore size distribution, pore shape, and the wetting properties
of the hydrocarboqhxk system.