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Log-log Diagnostic Plot • Plot the pressure and semilog derivative versus time on a log-log diagnostic plot. Use the appropriate time function based on the rate history of the injection period preceding the falloff. (See Appendix, page A-10 for time function selection) The log-log plot is used to identify the flow regimes present in the welltest. An example log-log plot is shown below: ,000o Example Log-log Plot R= Pressure Data +o0a Wellbore Storage Period Semilog Pressure Derivative Function a,00 El Transition period Radial Flow a ,o OUnitpe during e storage Derivative flattens 0.001 0.01 o., + 10 Elapsed Time(hours)-Tp=24.0 Identification of Test Flow Regimes • Flow regimes are mathematical relationships between pressure, rate, and time. Flow regimes provide a visualization of what goes on in the reservoir. Individual flow regimes have characteristic slopes and a sequencing order on the log-log plot. • Various flow regimes will be present during the falloff test, however, not all flow regimes are observed on every falloff test. The late time responses correlate to distances further from the test well. The critical flow regime is radial flow from which all analysis calculations are performed. During radial flow, the pressure responses recorded are representative of the reservoir, not the wellbore. • The derivative function amplifies reservoir signatures by calculating a running slope of a designated plot. The derivative plot allows a more accurate determination of the radial flow portion of the test, in comparison with the old method of simply proceeding 1%2 log cycles from the end of the unit slope line of the pressure curve. • The derivative is usually based on the semilog plot,but it can also be calculated based on other plots such as a Cartesian plot, a square root of time plot, a quarter root of time plot, and the 1/square root of time plot. Each of these plots are used to identify specific flow A-6