Understanding diagnostic plots for well-test interpretation

In well-test analysis, a diagnostic plot is a scatter plot of both drawdown and its logarithmic derivative versus time. It is usually plotted in log–log scale. The main advantages and limitations of the method are reviewed with the help of three hydrogeological field examples. Guidelines are provided for the selection of an appropriate conceptual model from a qualitative analysis of the log-derivative. It is shown how the noise on the drawdown measurements is amplified by the calculation of the derivative and it is proposed to sample the signal in order to minimize this effect. When the discharge rates are varying, or when recovery data have to be interpreted, the diagnostic plot can be used, provided that the data are pre-processed by a deconvolution technique. The effect of time shift errors is also discussed. All these examples show that diagnostic plots have some limitations but they are extremely helpful because they provide a unified approach for well-test interpretation and are applicable in a wide range of situations.