From straight lines to deconvolution: The evolution of the state of the art in well test analysis

Well test analysis has been used for many years to assess well condition and obtain reservoir parameters. Early interpretation methods (by use of straight lines or log-log pressure plots) were limited to the estimation of well performance. With the introduction of pressure-derivative analysis in 1983 and the development of complex interpretation models that are able to account for detailed geological features, well test analysis has become a very powerful tool for reservoir characterization. A new milestone has been reached recently with the introduction of deconvolution. Deconvolution is a process that converts pressure data at variable rate into a single drawdown at constant rate, thus making more data available for interpretation than in the original data set, in which only periods at constant rate can be analyzed. Consequently, it is possible to see boundaries in deconvolved data, a considerable advantage compared with conventional analysis, in which boundaries often are not seen and must be inferred. This has a significant impact on the ability to certify reserves. This paper reviews the evolution of well test analysis techniques during the past half century and shows how improvements have come in a series of step changes 20 years apart. Each one has increased the ability to discriminate among potential interpretation models and to verify the consistency of the analysis. This has increased drastically the amount of information that one can extract from well test data and, more importantly, the confidence in that information.