Page 88 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 3

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS

A. GUREVICH, G.V. CHILINGAR, J.O. ROBERTSON and E AMINZADEH

INTRODUCTION

Although the study of formation pressures has a history of more than 50 years,
still not all aspects and phenomena are investigated thoroughly enough and taken
into account while studying many oilfields. For example, pressure increase caused
by vertical gas migration and liquid-gas redistribution may be a major factor during
periods of intense gas generation and migration. Compaction of clays depends not only
on the overburden or/and geodynamic loading but also on reduction in strength and on
temperature change.
A better understanding of natural phenomena allows extending the scope of pressure
prediction methods. It is useful to emphasize that almost all mechanisms that produce
pressure deviation from the hydrostatic one were well known some 50 and more years
ago in physics, soil mechanics, geochemistry, etc. Regretfully, the knowledge that lies
beyond the immediate scope of traditional petroleum geology, has not been used fully
enough.
Pressure distribution and abnormality are caused by both gravitationally non-equi-
librium distribution of fluid density (free convection) and changes in fluid compression
(forced convection), that are generated and influenced by different factors. Therefore,
to achieve better correlation, pressure may be divided, with a reasonable accuracy,
into the free convection and forced convection components and each component may
be separately correlated with corresponding factors. Sets of characteristics, separately
presenting ability of fluid-filled rock to change pressure under external influence, the
external influence itself, and permeability, should be used for correlation.
In general, reliability of pressure prediction methods, including geophysical methods,
is still not high. In subduction and orogeny regions (Kucheruk and Lustig, 1986) or
where vertical fluid migration is intensive, an error in pressure determination may be
high. For example, in Azerbaijan, where vertical migration of fluids is very active
and pressure distribution differs from that which would be produced by compaction
processes only, well-log methods do not guarantee reliable results (Melik-Pashaev et al.,
1983). Carstens (1980) showed, using empirical data, that high shale porosity does not
necessarily coincide with pressure abnormality. Carstens and Dypvik (1981) indicated
that clays in the abnormal pressure zone of geological section (about 13,000 ft and
deeper) in the Viking graben (North Sea) are well consolidated, although pore pressures
reach 0.8 of the overburden. Carstens (1980) emphasized that under such circumstances
usual abnormality indicators (higher sonic transit time, lower electric resistivity, low
mechanical strength of formations and the d-exponent factor) may be misleading and