Pressure Detrending for Harmonic Pulse Test Data Preprocessing

Harmonic Pulse testing was developed as a form of well testing that can be applied during ongoing production or injection operations. A pulsed signal is superimposed to the background pressure trend thus no interruption of well and reservoir production is required before and during the test. The pulsed pressure and rate signal analysis is performed in the frequency domain; to this end, the pressure and the rate signals need to be decomposed into harmonic components. The derivative of the harmonic components in the frequency domain can then be analyzed similarly to a conventional well test. In practice the interpretability of the derivative of the harmonic components can be significantly improved if the pressure trend to which the pressure pulses are superimposed is removed, i.e. a detrending of the pressure data is performed prior to well test interpretation. In the present paper, the results obtained after applying different detrending methodologies to pressure data recorded during pulse tests in different reservoir conditions are presented and discussed. Analyses on synthetic test data proved that polynomial detrending is effective in removing the pressure trend induced by field depletion and constant well interference but cannot deal with transient effect related to preexisting rate history or ongoing production changes. Conversely, some of the detrending algorithms based on a heuristic approach are very effective to remove both. Moreover, detrended data can be further regularized by excluding anomalous cycles from the analysis, i.e. cycles that do not respect the designed test periodicity, such as in the case of well interference and/or temporary interruption of the pressure pulses during the execution of the test. The adoption of an effective detrending strategy can considerably improve the quality of the pressure data obtained from harmonic pulse tests and thus the test interpretability. Therefore, it offers the possibility of overcoming the limitation of applicability due to the difficulty of imposing a regularly pulsing rate for the whole test duration (typically lasting several days). This makes harmonic pulse tests very attractive for well performance monitoring, as in gas storage fields.