Page 248 - Origin and Prediction of Abnormal Formation Pressures

220 V.I. ZILBERMAN, V.A. SEREBRYAKOV, M.V. GORFUNKEL, G.V. CHILINGAR AND J.O. ROBERTSON JR.

lower in the case of gas migration along faults and fractures. Conductivity of faults and
fractures may have increased during the periods of neo-tectonic activity.
Thus, the reservoir pressure can be extrapolated from the sub-salt productive for-
mations into the overlying sealing sequences (Fig. 9-5). A model of the massive gas
accumulation and local gas accumulations (secondary traps) in the sealing sequences
recombines the two into a single reservoir system, where the pressure changes with
depth according to the weight of the gas column (Fig. 9-5). This model can be used to
forecast AHFE
The AHFP prediction technique based on the above model and on the forecast outline
as shown in Fig. 9-5 has been used since 1975. Its efficiency was proven in 62 wells
drilled through the evaporite sequence in the Melikhov, Medvedov and Krestishchen-
skoye fields of the DDB (Ukraine). Prior to the application of this technique, serious
problems (gas shows, blow-outs and open gushers) arose in these fields. As a result of
these complications, three wells did not reach their intended depth and four wells were
abandoned for mechanical reasons.
In addition to the above-described method, one should keep in mind the findings
of Fertl and Chilingarian (1987) that in predicting overpressures caused by tectonic
activity, the following predictive techniques can be used in the presence of shales: (1)
increase in the resistivity of shales on approaching AHFP; (2) decrease in the acoustic
interval transit time (Us ft -1 ) in shales; (3) increase in the bulk density of shales; and (4)
decrease in the pulsed neutron capture cross-section of shales, etc.
The reverse is true in the case of undercompacted shales in thick sand-shale
sequences.

CONCLUSIONS
In the case of AHFPs in the evaporite sequences, it is necessary to:
(1) prepare a good salt-plug outline at an early exploration stage;
(2) outline the AHFP zones;
(3) avoid drilling expensive wells outside the GWC;
(4) determine the boundaries between the salt bodies and the hydrocarbon accumula-
tions and delineate AHFP zones using computer models;
(5) quantitatively determine AHFPs;
(6) study the resistivity and density of associated shales (Fertl and Chilingarian, 1989).

BIBLIOGRAPHY

Anikiyev, K.A., 1971. Forecast of Abnormally High Reservoir Pressure and Improvement of Oil and Gas
Drilling. Nedra, Leningrad, 167 pp.
Dobrynin, V.M. and Serebryakov, V.A., 1989. Geological and Geophysical Techniques of Forecasting
Abnormally High Reservoir Pressure. Nedra, Moscow, 287 pp.
Durmishyan, A.G., 1973. On syngenetic and epigenetic nature of abnormally high reservoir pressure
(AHRP) in the subsurface. Neftegazov. Geol. Geofiz., 3: 50-53.