Page 173 - Origin and Prediction of Abnormal Formation Pressures

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

unlimited number of precise and accurate pressure measurements during a single trip
into the borehole. Applications include: (1) the analysis of naturally fractured reservoirs;
(2) pulse testing techniques to establish reservoir continuity; (3) qualitative estimation
of permeability in low-permeability formations; (4) reservoir management in producing
fields; (5) detection of fluid interfaces from vertical pressure profiles; and (6) verification
(calibration) of other overpressure indicators.

CONCLUSIONS

Two groups of quantitative methods for the analysis and prediction of abnormally
high and abnormally low formation pressure zones were described. The methods of
the first group are based on general geologic and tectonic information and may be
used prior to drilling. The methods of the second group are based on the use of
geological, geophysical and drilling-related information accumulated in the process of
drilling. Using equations of a general form, zones of abnormal pressure can be located
using resistivity, density, sonic time travel, gamma-rays and neutron-gamma logs. In
these methods, almost all log responses, except radioactivity in the abnormally high
pressured zones, are related to porosity. A decrease in natural radioactivity in the
abnormally high pressured zones is related to a decrease in 4~ content in the regions
with nonequilibrium compaction.
The following conclusions have been reached by Fertl and Chilingarian (1987):
(1) Industry-wide experience shows that costly misinterpretations are best avoided
by studying a combination of several pressure indicators. Not all of them, however, can
always be used or are necessarily needed in any one drilling application.
(2) Pulsed neutron capture (PNC) logs can be used to detect and quantitatively
evaluate the overpressure environments.
(3) Empirical correlations between r-shale values as a function of the true vertical
depth and magnitude of formation fluid pressure gradients and/or equivalent mud
weight requirements can be established for a given geological area.
(4) Provided Z-derived normal compaction trendlines can be easily derived, the
equivalent depth method allows reliable quantitative formation pressure estimates.
(5) PNC logs allow the monitoring of short- and long-term pressure depletion of, and
concurrent shale water influx into, hydrocarbon-bearing reservoirs.

BIBLIOGRAPHY

Alexandrov, B., 1987. Abnormally High Formation Pressures in Oil and Gas Basins. Nedra, Moscow, 215
PP.
Blokh, A., 1969. Water Structure and Geological Processes. Nedra, Moscow, 216 pp.
Chilingarian, G.V. and Vorabutr, P., 1981. Drilling and Drilling Fluids. Developments in Petroleum Science,
11. Elsevier, Amsterdam, 767 pp.
Daniel, W.L. and Fertl, W.H., 1984. Logging high-angle, long-reach boreholes. Oil Gas J., Dec.: 103-108.
Dellinger, T.B., Graveley, W., Tolle, G.C. and Sexton, T.H., 1983. Field testing to extend reach of directional
wells. Oil Gas Eur. Mag., 9(2): 14-16.