The bottomhole pressure measurements acquired during well tests sometimes contain oscillating components associated with tides. When present in well test data, tidal pressure oscillations may mask reservoir related transient pressure behavior. Hence, from pressure transient analysis point of view the tidal signal is a parasitic feature that has to be identified and removed from the test pressure data. At the same time, the tidal signal itself depends on formation properties. For example, amplitude of tidal oscillations depends on formation compressibility. Therefore, identification of tidal signal in downhole pressure data provides a means for monitoring the reservoir compressibility changes associated with reservoir fluid movements and long-term production. Compared to the pressure variation during a well test, the amplitude of the tidal pressure signal is normally very small. This makes it very difficult to identify the tidal signal in the test pressure data. In this paper, we present a special algorithm for identification of tidal signal in downhole test pressure data. Introduction It has been observed that sometimes the pressure data acquired during pressure buildup tests contain a low-amplitude oscillating signal superimposed on a monotonic pressure trend associated with pressure buildup. In many cases the oscillating signal can be traced to tides. Tidally induced oscillations are distinguished by a characteristic pattern reflecting combination of several frequencies present in the signal. For offshore wells, the characteristic pattern of tidal oscillations can be easily observed in the pressure data measured by a pressure gauge placed on sea floor. Fig. 1 presents an example of sea floor pressure oscillations obtained during a well test. The well rate during this test sequence is shown in the lower part of Fig. 1. The pressure in Fig. 1 oscillates within a 1.5 psi pressure range. The pressure oscillations on the sea floor are induced by changing water level, the ocean tide.1 The pressure oscillations on the sea floor are transmitted through overburden to the reservoir. The oscillating pressure signal at the reservoir depth, however, has much smaller amplitude and may be shifted (delayed) in time compared to the signal on sea floor. The amplitude attenuation, the ratio of the amplitude in the reservoir to the amplitude on sea floor, is a function of total reservoir compressibility2,5 and the time delay is a function of formation permeability.3,4 Identification of tidal signal in the downhole pressure data measured during a well test thus offers a means for determining in situ formation compressibility and formation permeability. Hence, the tidal signal present in downhole pressure data contains information about reservoir properties and to recover this information one needsa reliable technique for identifying the tidal signal in downhole pressure data andthe correlations that relate tidal signal characteristics, attenuation factor and time shift, to formation properties. Development of reliable correlations should be based on rock mechanics principles and requires careful calibration work. Even without such correlations, monitoring evolution of tidal signal characteristics over the life of a well provides the opportunity for monitoring the reservoir compressibility changes associated with reservoir fluid movement and long-term production.
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