Page 243 - Origin and Prediction of Abnormal Formation Pressures

PREDICTION OF ABNORMALLY HIGH PRESSURES IN PETROLIFEROUS SALT-BEARING SECTIONS 215

Delineation of an AHFP zone in evaporites, as a zone of tectonic weakness, is
a direct method of preventing complications at the stage of designing the drilling
plan. An attempt to forecast overpressured brine shows in relatively young deposits
by locating fractured zones in salt-bearing sequences (which also represent zones of
tectonic weakness) was successful. The fractured zones in that case were identified
from seismic data. Satisfactory results are obtained when the evaporite sequence is not
deformed by salt diapirs (Melik-Pashayev, 1973).
A problem with delineating the development of weakness zones with seismic data
in the DDB is that the correlation of reflection and refraction waves disintegrates on
approaching salt plugs. The salt plugs proper are areas of the total loss of correlation.
Only an approximate position of the salt plug and a general outline of an area where
correlation is lost is provided by the seismic data. For the delineation of AHFP zones
in such cases it is proposed to use the established pattern in the thickness changes of
the post-salt (above salt) rocks where the so-called indicator sequences are present.
Zilberman et al. (2000) stated that for some periods in the geologic evolution, the
rate of subsidence of the tectonic weakness zones was faster than the rate of regional
subsidence. This resulted in a greater thickness of accumulated sediments, which was
more pronounced each time when the sign of tectonic movement changed. These were
exactly the time intervals when the indicator sequences accumulated; their thicknesses
above the zones of weakness are 1.5 to 3 times that of the background thickness
(Fig. 9-4). Among the indicator sequences in the Dnepr-Donets Basin are the Upper
Permian Shebelinsk Formation, and Lower Jurassic, Lower and Upper Cretaceous,
Paleogene and Neogene deposits (Zilberman and Chernyakov, 1981).
Thus, the AHFP zones can be delineated with the help of indicator sequences, which
exhibit variation in thickness: greater thickness in the tectonic weakness zones above
salt plugs as compared to the thickness of the same deposits away from the salt plugs
and weakness zones. Zones of weakness represent the most mobile foci so that the
associated indicator sequences are the thickest. The proposed technique by Zilberman
and Chernyakov (1979) enables one to delineate an AHFP zone using the thickness of
a geologic section up to 1000 m above the possible AHFP interval. This technique also
makes it possible to forecast a prospect based only on core drilling. Each subsequent
well drilled over the prospect substantially improves the information and hence the
reliability of the forecast. An example of this would be the isopach map of Fig. 9-3
for the Lower and Middle (Bajocian) Jurassic sediments near the salt plug of the West
Krestishchenskoye gas-condensate field. The 100-m isopach map presented in this figure
delineates an AHFP zone.
Not only does an application of this technique provide an opportunity to delineate
the AHFP zones in the section and areally, but it also provides forecasting at the design
stage and during drilling. In addition, it enables a better casing string design and drilling
program to take into account otherwise unexpected pressure surges. The well location
relative to a danger zone is fine-tuned as indicator sequences are penetrated and their
thicknesses are measured. This technique is based on the analysis of available geologic
information and does not require substantial additional funds.
There is, however, a weakness in this technique, because of an intrinsic weakness
in the appraisal and development of the salt plug-trapped hydrocarbon accumulations.