Page 154 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 139

Transition zones complicate the identification of fluid contacts because
the definition of fluid contact is not universaily accepted. For example, water-oil
contact may be defined as the depth at which the capillary pressure is zero (the
free water level). The WOC depth can be identified using a Repeat Formation
Test by finding the point of intersection between the oil-phase pressure and the
water-phase pressure. By contrast, water-oil contact may be defined as the
deepest point in the reservoir at which a well can still produce water-free oil.
The different definitions of contact result in differences in the transition zone
model, so it is important to know which definition is applicable and who has the
authority to judge the validity of the model. In some cases, it may be necessary
to prepare models with both definitions and treat one definition as the base case
while the other definition is viewed as a sensitivity.
The proper way to include capillary pressure in a model study is to correct
laboratory measured values to reservoir conditions. This is done by applying
the correction:

where y is interfacial tension (IFT) is wettability angle [Amyx, et al., 1 960]. The
problem with the correction is that it requires data that are often poorly known,
namely interfacial tension and wettability contact angle at reservoir conditions.
Rao and Girard [1997] have described a laboratory technique for measuring
wettability using live fluids at reservoir temperature and pressure. Alternative
approaches include adjusting capillary pressure curves to be consistent with well
log estimates of transition zone thickness, or assuming the contact angle factors
out. If laboratory measurements of IFT are not available, IFT can be estimated
from the Macleod-Sugden correlation for pure compounds orthe Weinaug-Katz
correlation for mixtures [Fanchi, 1990].

14.3 Viscous Fingering

Viscous fingering is the unstable displacement of a more viscous fluid
by a less viscous fluid. The fingering of an injection fluid into an in situ fluid
can influence reservoir flow behavior and adversely impact recovery. It is