Page 196 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 181

between the tubing and the casing. This result was confirmed by running a
cement bond log and finding a leak in the wellbore interval adjacent to the gas
cap. Gas from the gas cap was entering the welibore and causing the larger than
expected production GOR. This effect can be modeled by a variety of options,
depending on the degree of accuracy desired: for example, it could be modeled
by altering productivity index (PI) in the well model or by designing a near
wellbore conceptual model and preparing pseudorelative permeability curves,
The choice of method will influence the predictive capability of the model. Thus,
a pseudo-relative permeability model will allow for high GOR even if the well
is recomputed, whereas the PI could be readily corrected at the time of well
recompletion to reflect the improvement in wellbore integrity.
Map adjustments may also be necessary. This used to be considered a last
resort change because map changes required substantial effort to redigitize the
modified maps and prepare a revised grid. Pre-processing packages and
computer-aided geologic modeling are making map changes a more acceptable
history match method. In the case of geostatistics, a history matching process
may actually involve the use of several different geologic models. Each geologic
model is called a stochastic image or realization. Additional discussion of
geostatistics is presented in Chapter 11.
Toronyi and Saleri [1988] present a detailed discussion of their approach
to history matching. It is noteworthy because they provide guidance on how
changes in some history match parameters affect matches of saturation and
pressure gradients. A summary is presented in Table 18-2. It shows, for example,
that a change in pore volume can effect pressure as it changes with time. As
another example, relative permeability changes are useful for matching saturation
variations in time and space. Notice that fluid property data are seldom changed
to match field history. This is because fluid property data tend to be more
accurately measured than other model input data.
History matching must not be achieved by making incorrect parameter
modifications. For example, matching pressure may be achieved by adjusting
rock compressibility, yet the final match value should be within the set of values
typically associated with the type of rock in the formation. In general, modified
parameter values must be physically meaningful.