Geoscience Reference
In-Depth Information
Velocity (m/s)
Fig. A3.5
Sonic velocity from a calibration well with a fitted compaction trend. (This figure and
figs. A3.6
-
A3.10
courtesy Petro-Canada.)
A simple relation that is often used to describe how overpressure affects seismic velocity is Eaton's
P
p
=
S
v
−
(
S
v
−
P
h
)(
V
obs
/
V
norm
)
3
.
Here
P
p
is the pore pressure,
S
v
is the total vertical stress and
P
h
is the hydrostatic pressure. At a given
depth
z
(ft),
P
h
is the pressure due to a column of water from the surface and is typically given by 0.44
z
psi, though the exact value of the multiplier depends on the salinity assumed.
S
v
is the pressure due to
the column of material actually present and is given in psi by 0.43
is the average density
(g/cm
3
) from surface to depth
z
(ft).
V
norm
is the seismic velocity on a normal shale compaction trend,
typically defined from sonic logs in normally pressured sections in neighbouring wells, and
V
obs
is
the actual observed seismic interval velocity, typically derived from stacking velocities.
To apply this in practice, the first step is to define the normal compaction trend. The simplest
way to do this is to use sonic logs in wells where there are pressure measurements that demonstrate
hydrostatic pressures are present.
Figure A3.5
shows a velocity log from such a well. Some outlying
values have been excluded by means of the polygon shown, and within the polygon shale intervals
have been identified by the use of other logs and selected for the trend calculation (shown in red). The
fitted compaction trend is almost linear over this interval, but is in fact a function which ensures that
the velocity gradient will decrease with depth. In other cases it may be necessary to refine the analysis
ρ
z
, where
ρ
