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The Geophysical Aspects of Abnormal Fluid Pressures
Often the determining factor in our ability to successfully drill through an abnormally-pressured formation is our knowledge of where to expect the top of the overpressure in the section. To locate large abnormally pressured formations, the petroleum industry is now using geophysical data and techniques. Seismic common-depth-point data are being processed to generate plots of interval velocity versus depth which grossly approximate velocity surveys or integrated acoustic logs. Lower velocity departures from the normal trend in a predominantly sand-shale section are generally good indicators of undercompacted and overpressured sediments.
Another more qualitative approach to abnormal pressure detection involves gravity data. An abnormally-pressured sand-shale section exhibits lower bulk densities than a normally-pressured section at the same depths. Bouguer gravity anomalies are often expressions of this density contrast. Other techniques may be needed to help distinguish an overpressured shale mass from a salt mass since salt and shale have about the same bulk densities. When the seismic and gravity approaches have been integrated together with all geological and drilling data, abnormally pressured zones have been drilled efficiently and safely.
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