Page 151 - Origin and Prediction of Abnormal Formation Pressures

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION PRESSURES 127

TABLE 5-3
Borehole seismic techniques (after Fertl and Chilingarian, 1987, table IV, p. 30)

Synthetic seismogram
Detailed correlation information between well log data and recorded seismograms. Transformation of
acoustic and density log data into a seismic trace format for direct comparison with surface or borehole
seismic recordings.
Velocity survey
Precise tie between seismic reflection travel times and known depths within a well. Useful for identifying
events on surface seismic displays corresponding to specific geologic formation boundaries.
Acoustic calibration
Velocity survey results adjust acoustic log values into the domain of macroscopic seismic measurements, so
that synthetic seismograms derived from acoustic logs will accurately match field near-surface structures.
Offset VSP
Enhanced resolution of the VSP technique achieved laterally away from the borehole. Converted wave
modes deduce additional lithologic properties.
Predicted acoustic log
Inversion of seismic trace data. Prediction of rock conditions, such as overpressure zones or porosity zones,
below the drill bit. The required velocity trend at the well location is derived from a combination of
acoustic log data, seismic velocity survey, and surface seismic velocity analysis results.
Salt proximity survey
Specialized format of vertical seismic profile. Determines shape of a salt dome flank. Instead of using
reflected waves, this technique depends on observing the direct arrival time of waves that travel through the
salt dome. This configuration solves the structural interpretation problem that has always been very difficult
when using only surface seismic measurements.

the rate of sedimentation (mainly during Pliocene-Quaternary time), and (4) tectonic
movements of the Earth's crust.
Assuming that filtration of water through the seals obeys Darcy's law, the following
equations can be obtained:
If R > Rcr:

9
Ka = Pa = 1 + Ahs 9 ~.C~b (R- Rcr)] (5-1)
Pn (kw/#s) gpwh
If R _< Rcr:

Ka -- 1
where Ka is the coefficient of abnormal pressure (i.e., the ratio of abnormal pressure,
pa, to the normal hydrostatic pressure, p,, at the same depth h, kw is the permeability
of the seal to water; Ahs is the thickness of the seal, #s is the viscosity of water, )~
is the coefficient showing what portion of the total water is expelled from the rocks
through the seals by vertical filtration, C is the consolidation coefficient of the rocks, q~
is the porosity, g is the gravitational acceleration, Pw is the average density of water, R
is the sedimentation rate (e.g., during the Pliocene-Quaternary), and Rcr is the critical
sedimentation rate.