Page 111 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 91

In the reviewed publications, there were no special analysis and experimental
confirmation of the plasticity of shales at deep horizons. The conclusions on plasticity
seem to have been made indirectly on the basis of observed deformations of walls in
the wellbore. It is not apparent that shales at temperatures above 100~ can be plastic,
because at such temperatures mineral grain surfaces have no or almost no bound water,
which could reduce dramatically the grain-to-grain friction. Deformational behavior of
shales in the geologic section of the Azerbaijan formations with abnormal pressures
should be studied thoroughly. Possibly, one can stabilize these shales by means of
special formulation of drilling muds and, thus, reduce the required mud density. Also
possibility of using direct electric current (DC), in conjunction with various chemicals,
for electrochemical stabilization of heaving and sloughing shales should be thoroughly
investigated (see Chilingarian, 1991, p. 293).
It is always better to drill at the lowest possible wellbore pressure to achieve a
higher penetration rate. For geologic sections where shales with higher pressures are
interbedded with reservoir sands with significantly lower pressures this may be crucial.
Therefore, it is necessary to separately evaluate contributions of shale plasticity and
pressure abnormality to the deformation of wellbore walls. In the case of high shale
plasticity, special composition of drilling mud may be used instead of a higher mud
density. This can provide a significant increase in the penetration rate and, therefore,
reduce costs.
In a well that is being drilled through shales, with clays still retaining colloidal
properties and maintaining thixotropy, there is a possibility that vibrations of the drilling
tools and tubing may also contribute to fluidization of shales in the wellbore walls.
In some wells, although the drilling mud weight was less than that calculated to be
necessary to balance the pore pressure in shales, no problems were encountered. In the
reviewed papers no analysis has been presented of the difference between shales with
and without problems. The properties of shales and the drilling procedures in both cases
should be thoroughly investigated. The emphasis should be on the relative roles of (a)
pressure abnormality, that causes or increases plasticity or other deformational properties
of shale, and (b) shale plasticity and other properties indigenous to the shale itself.
It is important to determine the relative contributions to deformation of shales made
by (a) the abnormal water and gas pressure in shale and (b) the plasticity of the shale
itself at different mineral compositions and temperatures. The effect of temperature on
the amount of bound water and, thus, the rheology of shales in the upper part of the
geologic section (temperature below 40-50~ also should be investigated.
(3) In the reviewed papers there were no thorough analyses and direct estimations of
the precision of pressure determination by well-logging data. Whether or not calculated
pressures are always or usually close enough to the actual values is not clear.
During the geologic history, pressure abnormalities in shales and permeable forma-
tions can increase after some decline. In such a case, a definite relation between the
pressure and porosity will cease to exist. Vertical fluid migration and increase in the oil
and, especially, gas column heights can raise the pressure. Excess pressures due to the
fluid column height also could reverse the pressure change trend.
Vertical paths for the upward fluid migration and pressure redistribution (i.e.,
currently active and buried mud volcanoes, fractured zones, and faults) are numerous