Page 119 - Principles of Applied Reservoir Simulation 2E

104 Principles of Applied Reservoir Simulation

Figure 11-5 shows a random sampling from a discrete probability-
distribution. A running average is also plotted. The figure shows that the running
average does not stabilize, or approach a constant value, until at least 20 trials
have been completed. This is a large number of realizations if history matching
is needed for each realization. Indeed, it would be an unacceptably large number
of realizations, in most cases, because of the time it takes to perform a history
match,
Multiple realizations can also confuse people who are not closely involved
with the modeling process because they do not have a single picture of the
reservoir. On the other hand, the use of multiple realizations makes it possible
to quantify the uncertainty associated with our limited knowledge of properties
distributed spatially throughout the field. Table 11-3 summarizes the advantages
and concerns associated with geostatistics. There is no established procedure
for selecting one or more realizations for history matching from a set of
geostatistically derived realizations. One procedure is described by Rossini, et
al. [1994], An application of reservoir geostatistics in the context of a
multidisciplinary study is presented by Wang, et al. [1998].

Table 11-3
Geostatistics

Advantages Concerns
+ Realism + Cost and confusion of multiple realizations
4 Quantifies uncertainty + History matching still necessary to account
for model discontinuities such as channeling
4 History matching complicated by factors such
as probabilistically generated heterogeneity

Exercises

Exercise 11.1 (A) Run EXAM1.DAT and record the final time, final pressure
and initial oil volume (B) Multiply the volume of the reservoir in EXAM 1 .DAT
by 0.5,10 and 100. This can be done by altering the gridblock size (see Chapter
24.1.1). Make a table showing the final time, final pressure, and initial oil
volume for each case. (C) How does the change in volume affect the pressure