ABSTRACT
The inverted decline curve, or IDC, is the plot of the inverse of a well's producing rate (1/q) against the log of its producing time. If the well's producing pressure history is known, this curve can be used to determine reservoir permeability, the skin effect, and the area being drained by the well. For a hydraulically fractured well, the length of an infinite conductivity fracture which is equivalent to the actual finite conductivity fracture can also be determined.
This technique uses equations which fit the constant pressure transient radial flow solution. Also, polynomials have been developed which match pre-pseudoradial flow production from hydraulically fractured wells with dimensionless fracture conductivities of 0.01, 0.1, 1, 5 and 500. For pseudosteady state flow periods, a depletion or a pseudosteady-state equation can be used to match the performance of the well. By this technique, the well's entire production history becomes the equivalent of a very long drawdown test. This method is especially useful for low permeability reservoirs.
Advantages of this method over type curves include: greater resolution because the scale of the y-axis is linear rather than logarithmic, the capability of using superposition to account for pressure changes, the ability to include the derivative in the analysis, and better determination of permeability and skin effect because, as in a Homer plot, the fonner is dependent on the slope and the latter on the intercept. The entire production history, including pre-radial, radial, and pseudosteady state flow, can be used in the analysis.
