Page 236 - Origin and Prediction of Abnormal Formation Pressures

209

Chapter 9

PREDICTION OF ABNORMALLY HIGH PRESSURES IN PETROLIFEROUS
SALT-BEARING SECTIONS

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

INTRODUCTION
Thick salt-bearing sequences form sealing complexes in many oil regions around the
globe. Serving as seals, these complexes often include oil, gas and brine accumulations
with abnormally high formation pressure (AHFP). Forecasting AHFP in evaporites
is difficult. The reason for that is the absence in the evaporites of transition zones,
which are typical for clastic seals (Dobrynin and Serebryakov, 1989). Consequently, in
evaporite sequences there are no indications of approaching the overpressured intervals
(such indicators are common in clastic sequences). Logging techniques in this type of
section are not applicable. Reservoir pressure within these formations changes abruptly,
with no gradual transition. Fluid accumulations in evaporites are sporadically developed
over the area and are mostly associated with zones of weakness due to salt intrusion
(fracturing, development of sand lenses, etc.). Even an increase in drilling rate is not
always diagnostic in the evaporite sequences. Overpressured fluid accumulations are
usually encountered immediately underneath the salt section, which is drilled easily
and at a high rate. It is very difficult in such an environment to observe an increase in
drilling rate caused by a decrease in rock strength (zones of fracturing) or by a decrease
in differential pressure between the borehole and the reservoir. The situation is often
further obscured due to the intentional decrease in drilling rate in salt sections with the
purpose of eliminating the borehole deviation.
In evaporites, especially in salt-beating sections, it is much more difficult to detect
a temperature increase on approaching the AHFP than it is in clastic sections. Another
complicating factor in evaporite seals is their non-uniform gas-saturation, which is not
the case in the clastic seals.
Brine shows are commonly associated with the overpressured zones in evaporites.
Substantial brine shows are common in geologically young, Mesozoic and Cenozoic,
salt-bearing sections. Older Paleozoic sediments, with more compacted and catageneti-
cally altered rocks, display oil and gas shows accompanied with much less intense brine
shows.
The southeastern Dnepr-Donets Basin in the Ukraine (see Fig. 9-1) is an example
of a region where the Lower Permian evaporites form a regional seal for a gas-beating
section. Little knowledge of the gas-occurrence conditions within this region and a
lack of overpressure forecast methodology here have resulted in numerous gas shows,
kicks and blow-outs (Zilberman, 1972; Zilberman and Zilberman, 1978). Dangerous