Page 31 - Origin and Prediction of Abnormal Formation Pressures

14 E.C. DONALDSON, G.V. CHILINGAR, J.O. ROBERTSON JR. AND V. SEREBRYAKOV

True gas-in-place
.o estimate
=IN==
a
Erroneous
gas-in-place
estimate
i
i
g
i
i i
",,,,i 2 i i
9 li~
Overestimate
Cum. gas producti( )n
Fig. 1-7. Typical p/z versus cumulative production behavior for an overpressured gas reservoir in sand-shale
sequence. (Modified after Fertl and Chilingarian, 1977, fig. 4, p. 35.)

.E
Q.
"4--
(1)
N
\\
\i~.~
~ 1 x'" 2 "~3

Time risk, cost
Fig. 1-8. Generalized trends of key drilling factors in hydrostatic and overpressured environments: 1
= hydrostatic pressures" 2 = overpressures; and 3 = severe overpressures. (Modified after Fertl and
Chilingarian, 1987, fig. 13, p. 37.)

ECONOMICS IN OVERPRESSURE ENVIRONMENTS
Exploration in normal-pressure environments generally shows predictable trends for
time, cost, and risk. Presence of abnormal pressures, especially superpressures, however,
is a very critical factor. Time, cost and risks can increase drastically, greatly affecting
the profit. This is clearly shown in Fig. 1-8.
Based upon the shale resistivity ratio method (Fig. 1-9) and regardless of measured
formation pressure gradients, the following conclusions were drawn by Timko and Fertl
(1971) and Fertl and Chilingarian (1976) for the shale-sand sequences (but not massive
carbonate sections).