Effect Of Fluid Influx On Drawdown Behavior
Authors: Van Poolen H.K. , Kunzman W.J. 
Published: December 2007

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Abstract
Anomalous behavior shown by certain gas wells during drawdown testing and some oil fields produced by the depletion type mechanism has been successfully simulated. The combined study involved laboratory experiments and an analog computer model.
The laboratory and computer models were designed to simulate drawdown behavior of gas wells in which the pressure suddenly increases at a constant production rate following a period of pressure decline. In some oil fields produced by depletion type mechanism, this type of behavior is characterized by a sudden increase in production during the early life of the field, following a history of steady production decline where well patterns and pumping conditions are maintained constant.
In this study, the following hypothesis is proposed to explain such anomalous behavior: Hydrocarbons are concentrated in permeable lenses which are separated by tight water-bearing formations. During normal production from a downstream lens, the pressure differential between lenses becomes large enough to overcome the entry pressure of the water-bearing formations with respect to the hydrocarbons present. When the entry pressure is overcome, communication exists between lenses. The result is a sudden release of energy from an upstream lens to a downstream lens.
During a period of drawdown testing on a downstream lens, this additional source of energy could cause the pressure humps observed on some tests. In oil fields produced by the depletion type mechanism, the additional energy would cause the production humps occasionally observed. It is possible that relative permeability effects may contribute to this anomalous behavior.

Introduction
During drawdown testing, a well is usually produced at a constant rate. Normally, pressures will monotonically decline. This decline depends on fluid characteristics, reservoir rock characteristics, and reservoir geometry and will vary for different reservoirs.
During drawdown tests of certain gas wells, operators observed that pressures would increase again with continued constant production rates. Fig. 1 shows the difference between normal and the anomalous drawdown behavior. Anomalous pressure drawdown curves are shown in Figs. 2 and 3. These tests were performed on a gas well. Table 1 summarizes the important data describing the drawdown anomalies. The producing rate of the test is given with the time of occurrence and pressure level of the minimum pressure prior to increase in pressure.
The subject formation has a limestone and shale lithology with production from porous limestone stringers which are usually less than 10 ft thick. Reservoir limits are usually determined by permeability pinchouts rather than structure. Reservoirs in the area are without a water drive.