Page 198 - Principles of Applied Reservoir Simulation 2E

Part II: Reservoir Simulation 183

18.4 Deciding on a Match

There are several ways to decide if a match is satisfactory. In all cases,
a clear understanding of the study objectives should be the standard for making
the decision. If a coarse study is being performed, the quality of the match
between observed and calculated parameters does not need to be as accurate as
it would need to be for a more detailed study. For example, pressure may be
considered matched if the difference between calculated and observed pressures
is within ±10% draw down. The tolerance of ±10% is determined by estimating
the uncertainty associated with measured field pressures and the required quality
of the study. A study demanding greater reliability in predictions may need to
reduce the tolerance to ± 5% or even less, but it is unrealistic to seek a tolerance
of less than one percent. The uncertainty applies not to individual well gauge
pressures, which may be measured to a precision of less than one percent, but
to estimates of average field or region pressure from two or more well tests. The
latter error is generally much larger than the precision of a single well test. In
any event, model-calculated pressure trends should match field or region pres-
sure performance.
Another sensitive indicator of the quality of a history match is the match
of WOR, GOR, or water cut. Three factors need to be considered: breakthrough
time, the magnitude of the difference between observed and calculated values,
and trends. Adjustments in the model should be made to improve the quality of
each factor. Saleri [1993] has observed that a match of the field is more easily
obtained than a match of individual well performance. Indeed, he notes that
matching every well is virtually impossible. As a rale of thumb, the field match
may be valid for a year or more without updating, and we can expect the well
match to be valid for up to six months without updating. Deviations from this
rule will vary widely, and will depend on the type of system modeled and the
alignment of the interpreted model with reality. Indeed, gas reservoirs without
aquifer influx may be accurately modeled for the life of the field, while a gas
reservoir with complex lithology and water influx may never be satisfactorily
matched.