Interpreting Pressure Transient Tests with Wellbore Properties Changing between Drawdown and Buildup

Abstract Pressure transient testing is an invaluable tool in reservoir and production engineering as it is often the primary source of information for not only reservoir permeability, volume, and pressure but also wellbore damage and flow capacity. Many techniques to analyze pressure transient test such as superposition, deconvolution, etc. all assume that wellbore properties stay constant throughout time, which can be false in reality. This research investigates the effects of changing wellbore storage and skin between drawdown and buildup have on conventional pressure transient test analysis. The study demonstrates the need to develop new techniques that account for changing wellbore properties behaviors, and proposes a possible method. In this study, two wellbore parameters were considered changing between drawdown and buildup: wellbore storage coefficient, and wellbore skin. The effects of changing wellbore properties between drawdown and buildup then could then be analyze by comparing each case of changing wellbore parameters to the corresponding case with unchanging wellbore parameters. In the case of changing wellbore storage coefficient, this study shows that although storage only has an effect on the early-time behavior of pressure transient data, and the endpoint pressure difference is often small, assuming unchanging storage coefficient often results in significant errors in analysis. This study demonstrates that the principle of superposition does not hold in the case of changing wellbore storage coefficient, and it is not possible to use deconvolution to analyze such a test. Similarly, in the case of changing skin, which only has an effect on the mid/late time behaviors of pressure transient data, can result in significant errors in conventional analysis. This study also shows that the principle of superposition does not hold, and it is not possible to use deconvolution in these cases. Finally, as conventional pressure transient test analysis techniques are insufficient, the study demonstrates the needs for new analysis methods for the case of changing wellbore properties.