Page 166 - Origin and Prediction of Abnormal Formation Pressures

142 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.

Total
Resistivity Radioactivity Radioactivity of 4~
2 5 ]0 2O 30 3 4 5 2 3 4
500 I l I I I I I ' I I I I I I

Q
1000
E

w
t-" 1500 o;Ao
,4,.=- o ~,1"r
o. O o O 9149
9
(D
2000 e'~,

"li
O

Fig. 5-10. Resistivity (p, ohmm) total radioactivity (lr~, imp/mincm3), and radioactivity of 4~ versus
depth (m) in shales in the abnormally high formation pressured (AHFP) Kharasavey oilfield, northwestern
Siberia. (Modified after Dobrynin and Serebryakov, 1989, fig. 40, p. 88.)

pulsed neutron generator, the Z'-measurement is made by analyzing the time rate of
decay of the thermal neutron population. The E-values in shale formations decrease
in a regular fashion with depth in normally compacted clastic sequences. Abnormal
formation pressures are, however, flagged by divergence from this normal Z-trend
(Fertl and Chilingarian, 1987).
Although this absorption cross-section is a nuclear measurement, the recorded log
response is similar in appearance to the induction log. As a result, the r-measurement
can be used in many geological applications in cased holes previously available only
from the open-hole resistivity logs.
Fig. 5-11 shows a useful application of PNC logs for quantitative formation pressure
evaluation (Fertl and Timko, 1970). Shale resistivity (R~h) versus depth for a well drilled
in Louisiana in 1946 is shown in Fig. 5-11A. All reservoir sands below 8200 ft have
been productive for at least 25 years. This particular well produced from the Klump
Series. After a casing collapse below 8100 ft, the plan called for placing the well back
on production by recompleting it in the Homeseekers 'A' sand at a depth of 9060 ft