Estimation and Removal of Tidal Effects From Pressure Data
- Yong Zhao (ConocoPhillips Co) | Albert C. Reynolds (University of Tulsa)
- Document ID
- Society of Petroleum Engineers
- SPE Journal
- Publication Date
- March 2009
- Document Type
- Journal Paper
- 144 - 152
- 2009. Society of Petroleum Engineers
- 5.6.4 Drillstem/Well Testing, 4.1.5 Processing Equipment, 4.1.2 Separation and Treating, 5.1.2 Faults and Fracture Characterisation
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We consider field pressure data from two wells, one offshore and one onshore. The onshore well is shut-in and experiences only a very small pressure decline due to interference, whereas in the offshore case, the late time buildup data appear to be severely corrupted by ocean tidal effects. The objectives in both cases are to determine the tidal signal that can be used to estimate formation compressibility and to remove the tidal signal from the measured data.
For the onshore well, we use a Savitzky-Golay (SG) filter to smooth the pressure data. Subtracting the smooth data from the measured signal yields a modified signal that ideally represents the earth tidal signal plus measurement error. As the main harmonic components of the earth tidal signal are known, we perform a least-squares fit of the modified pressure data with a series of sines and cosines that contain only the known frequencies of the earth tide. By subtracting the series obtained by least-squares fitting from the true signal, we remove the effect of the earth tide.
In the offshore case, the time span of data that are severely affected by tidal effects is too short to use the SG filter with confidence. As the reservoir pressure change due to the tides is well approximated by a damped and delayed version of the seafloor response, a least-squares fit of the data with a function that follows the underlying buildup trend plus an attenuated-delayed response of the measured seafloor pressure trace allows one to obtain a good estimate of the attenuation factor and time delay so that the tidal component of the bottomhole pressure change can be determined and removed from the measured pressure.
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