Abstract
Production data are usually analyzed using typecurves. Production data are typically variable rates, whereas typecurves are typically "constant rate" solutions. Therefore, "superposition" must be used to convert the multiple rates into their constant rate equivalent, before typecurves can be used to determine the flow regime.
For linear flow the kernel of the superposition function is square-root-time, for radial flow it is log-time and for boundary-dominated flow it is material-balance-time. When superposition is used, the appropriate function must be used. In practice, the dominant flow regime dictates the choice. However, the flow regimes can be determined only after typecurve matching. This creates a self-referencing procedure.
In many situations, using square-root-time superposition makes the data look like linear flow, while using log-time makes it look like radial flow, and using material-balance-time makes it look like boundary dominated flow. This is self-defeating and can easily lead to the wrong diagnosis of flow regimes. In other situations, the choice of superposition function does NOT change the shape of the data, and the diagnosis tends to be unique. Numerous variable rates are investigated in an attempt to identify when superposition is safe, or which function distorts data the least.
Superposition functions are more forgiving for the early time data, and therefore they are less likely to change the shape of the data significantly in well test analyses, whereas in production data analysis the focus is more on the late time data. Using an inappropriate superposition function can result in the wrong diagnosis of flow regimes and the wrong interpretation of production, and very critically, the wrong forecast of reserves. The choice of superposition function must be an "informed choice" by the analyst.
