A Method To Account for Afterflow at the Pulsing Well During Pulse Tests

The nature of wellbore storage is such that afterflow during a pulse test can affect the reservoir pressure performance and can lead to the calculation of erroneous performance and can lead to the calculation of erroneous values for formation transmissibility and storage. This is most likely to occur when the wells of interest are close together or when after flow persists for a long time relative to the pulse length. This article describes a technique that was developed to account for the effects of after flow at the pulsing well during pulse testing of a small production pilot. The technique is not general because it requires that a computer-generated simulation of each pulse test be made. An application of the method is given. Introduction In carrying out a pulse test, we introduce a pressure disturbance into a reservoir by alternately increasing and decreasing the flow rate at the pulsing well in a known manner. The pressure at the responding well is monitored, and, if the wells are in pressure communication, the pressure distrubance eventually will affect the pressure at the responding well. Since the form and the duration of the flow, rate disturbance are known, and since the mathematics that describe the pressure behavior of fluid-beefing reservoirs are well understood, the pulse test pressure response can be predicted. Several methods are available to calculate values for formation transmissibility and storage within a pulse-tested reservoir. Although all real reservoirs are heterogeneous, the models for deriving these techniques assume that the reservoir is ideal. When the wells of interest are far apart or when the duration of after flow is short relative to the pulse length, the effects of wellbore storage on the pulse test results will be slight. If, on the other hand, the pulsing well and the responding well are close together or if after flow persists for a tong time, the effects of wellbore storage on the pulse test results may be substantial. The work described here began during the analysis phase of a series of pulse tests that were run in a small phase of a series of pulse tests that were run in a small pilot test area. Computer simulations of the tests showed pilot test area. Computer simulations of the tests showed that the method of Mondragon and Menzie would not compensate adequately for the strong effects of after flow on test results. Description of the Method Since a series of injection/falloff tests had been run in the pilot area, it was possible to obtain values for the ratio of formation transmissibility to the wellbore storage capacity, /F, at each well by type-curve matching techniques. Using this parameter, we can determine the after flow vs. time profiles that would occur during the pulsing-well shut-in periods and incorporate them into a computer simulation of each pulse test. A typical pulsing well-flow profile showing after flow during the shut-in period is profile showing after flow during the shut-in period is illustrated in Fig. 1. Given that the pulsing wells were observed to go on vacuum soon after shut-in and given that the wellbore storage capacity for these wells during the on-vacuum condition should be approximately two orders of magnitude larger than it would be during injection SPEJ p. 519