Page 281 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

Reserve Estimates 449

decline-curve can be obtained. Gentry provided other curves to estimate a,, or
Equation 5-189 can be rearranged:

(5-1 96)

With the use of the semilog typecurves, caution must be exercised to ensure
that the interval being analyzed is indeed a hyperbolic curve [197]. Another
problem with the semilog plots is that an exact fit of the data may not be
possible; however, the techniques are relatively rapid.

Log-log (Fetkovich) Type-Curves

Conventional declinecurve analysis should be used only when mechanical
conditions and reservoir drainage remain constant and the well is producing at
capacity [197]. An advanced approach for decline-curve analysis, which is
applicable for changes in pressure or drainage, has been presented by Fetkovich
[267,268]. This technique, which is similar to the approach used in pressure
testing, involves log-log plots of q/ql (or qDd) versus ait (or t, ) for different
values of n (see Figure 5-146). As shown in this figure, a log-fog plot of the
dimensionless rate and dimensionless time can identify transient data and/or
depletion data, the Arps’ equations given in Table 5-33 must only be applied
to rate-time data that indicate depletion [268]. Use of transient data in the Arps’
equations will result in incorrect forecasts that are overly optimistic.
The full-size Fetkovich type-curves can be ordered from the book order
department of SPE. The field data are plotted on tracing paper that has the
same log-log scale as the full-size type-curves. The log-log plot of flow rate and
time can be in terms of barreldday versus days, barrels/month versus months,
or barrels/year versus years, depending on the time interval being studied. Using
the best fit on the appropriate typecurve, a match point can be used to obtain
91 and ai for the actual data. The appropriate equation can then be used to
analyze the rate, time, and cumulative production behavior.

RESERVE ESTIMATES
Definltion and Classificatlon of Reserves

Definltions [269-2711
Crude Oil. This is defined technically as a mixture of hydrocarbons that existed
in the liquid phase in natural underground reservoirs and remains liquid at
atmospheric pressure after passing through surface facilities. For statistical
purposes, volumes reported as crude oil include: (1) liquids technically defined
as crude oil; (2) small amounts of hydrocarbons that existed in the gaseous phase
in natural underground reservoirs but are liquid at atmospheric pressure after
being recovered from dwell (casinghead) gas in lease separators*; and (3) small
amounts of nonhydrocarbons produced with the oil.

*From a technical standpoint, these liquids are termed “condensate”; however, they are commingled
with the crude stream and it is impractical to measure and report their volumes separately. All other
condensate is reported as either “lease condensate” or ‘plant condensate” and included in natural
gas liquids.