Page 158 - Origin and Prediction of Abnormal Formation Pressures

134 G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ AND J.O. ROBERTSON JR.
(A) Abnormally-High Pressure

Log r Log p Log R Log Ln, Log In~ , Log A,~
...... ..... L .......
-- n "1~
"t "I, me
s|.

:1,
im

(B) Abnormally-Low Pressure

Log Log p Log R Log I~,~ Log In~ Log Az
t
,,,, . . . . . . . . . . . . . . . .

9 ~.
, 9 ~

;.;.-

Fig. 5-5. Well-log responses in zones of (A) abnormally high and (B) abnormally low pressures. 1
= Abnormally high formation pressure (AHFP) in reservoir rock; 2 = shales; 3 = limestone; 4 --
sandstone; and 5 = abnormally high and abnormally low pressures in shales. (Modified after Dobrynin and
Serebryakov, 1989, fig. 54, p. 112.)

CALCULATION OF ABNORMAL PORE PRESSURE DURING DRILLING
In normally pressured zones, all log responses related to porosity, when plotted on a
semilogarithmic paper, form straight lines (i.e., the vertical axis is log (x), where x is
a geophysical parameter such as resistivity, sonic travel time, density, and gamma-ray
and neutron log responses). In zones of abnormally high and abnormally low pressures,
however, the magnitudes of the responses change significantly (Fig. 5-5). Abnormal
pressure, therefore, can be detected on wireline logs (also logging while drilling).