This paper shows that the decline-curve analysis approach does have a solid fundamental basis. The exponential decline is shown to be a longtime solution of the constant-pressure case. The constant-pressure infinite and finite reservoir solutions are placed on a common dimensionless curve with all the standard "empirical" exponential, hyperbolic, and harmonic decline-curve equations. Simple combinations of material balance equations and new forms of oil well rate equations for solution-gas drive reservoirs illustrate under what circumstances specific values of the hyperbolic decline exponent (1/b) or "b" should result.
Log-log type curve analysis can be performed on declining rate data (constant-terminal pressure case) completely analogous to the log-log type curve matching procedure presently being employed with constant-rate case pressure transient data. Production forecasting is done by extending a line drawn through the rate-time data overlain along the uniquely matched or best theoretical type curve. Future rates are then simply read from the real time scale on which the rate-time data is plotted. The ability to calculate kh from decline-curve data by type curve matching is demonstrated.
This paper demonstrates that decline-curve analysis not only has a solid fundamental base, but provides a tool with more diagnostic power than has previously been suspected. The type curve approach provides unique solutions upon which engineers can agree, or shows when a unique solution is not possible with a type curve only.