Page 71 - Practical Well Planning and Drilling Manual

Section 1 revised 11/00/bc 1/17/01 2:56 PM Page 47

Casing Design [ ]
1.4.4

2. Offset well records:
a) Pressures measured by sampling tools, e.g., repeat formation
tester (RFT) and modular formation dynamics tester (MDT)
b) Leakoff tests and frac job results will give infield stress levels
c) Drilling records showing drilling trends, gas levels in mud,
kicks, losses, and wellbore stability
d) Measurements of shale bulk density (from drill cuttings)
e) Resistivity logs that show a departure from the normal trend
with depth may indicate overpressures. Thus, a resistivity LWD
tool may give prior warning of a possible overpressured zone.
f) Sonic logs can give bulk densities (allow overburden stress
calculation) and maximum formation horizontal stresses. In
undercompacted formations, the sonic velocity will be lower; be
aware, however, that normally pressured fractured shales will also
produce lower sonic velocity and so this should be correlated with
resistivity log results.

Predicting overpressures from seismic surveys. Seismic surveys can
give quite a lot of useful information for overpressure prediction. Sonic
velocity through rock generally increases with depth as the formations
become more compacted. If this compaction stops increasing with
depth then the velocity will not increase with depth.
An empirical relationship was developed in 1968 by E. S.
Pennebaker, which relates the interval transit time (reciprocal of veloc-
ity) to pore pressure, lithology, depth, and age. This was found to be a
reliable indicator of overpressures in the Gulf Coast.
If this can be interpreted from the seismic, it may be then an indi-
cator of potential overpressure. As noted previously, normally pres-
sured fractured rock may produce the same result and this possibility
also must be considered.
A new technique that may show promise is amplitude variation
with bandwidth (AVB). The seismic wavelet is divided into bands of
frequencies. The amplitude of each of these frequencies in the return-
ing wavelet is compared with the original source signal. How the shape
of these bands of frequencies is changed may give leads on where
abnormal pore pressures are likely.
Structures may be seen that would indicate a column of a lighter
than saltwater gradient (e.g., oil or gas) from a lower, normally pres-
sured zone. As you move up the light column, overpressure (the pres-
sure above normal) will increase and maximize at the top.

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