Page 113 - Origin and Prediction of Abnormal Formation Pressures

ORIGIN OF FORMATION FLUID PRESSURE DISTRIBUTIONS 93

DEFINITIONS OF TERMS AS USED IN THIS CHAPTER

Hydrostatic (or normal)pressure: a pressure in a formation equal to the weight of
the actual column of water from the measurement point to the surface or to the
reference plane at an average elevation of the relief of area under consideration. The
latter is much more convenient in many cases (Gurevich et al., 1987). Inasmuch
as the groundwater table is nearly always not horizontal, the hydrostatic formation
pressure is an ideal one. It always differs from the actual pressure and is used as a
reference value only. The hydrostatic pressure gradient is obtained from the specific
weight of fluid and, therefore, varies with depth according to the actual fluid density
distribution.
Abnormal pressure: a pressure that noticeably exceeds the hydrostatic one.
Subnormal pressure: a pressure that is noticeably lower than the hydrostatic one.
Free convection of a fluid: a flow of fluids caused only by non-equilibrium (in the field
of gravity) distribution of their densities.
Forced convection of afluid: a flow of fluids caused only by changes in fluid compres-
sion.
Pressure prediction: indirect determination of pressure before penetrating the reservoir
by a well and measuring reservoir characteristics.
Pressure detection (indirect determination): indirect determination of pressure from
values of other reservoir characteristics measured in a well penetrating the reservoir.

CONCLUSIONS

The major conclusions can be summarized as follows.
(1) The authors recommend using their approach to a statistical correlation of
pressure abnormality with geologic and physical characteristics: to subdivide pressure
value into free and forced convection components and to correlate them with the factors
that actually cause pressure increases.
(2) A complete list of factors that cause the forced convection of fluids includes some
important elements not mentioned previously by many investigators. They include: the
piezo-convection effect, a major factor in areas and periods of intensive gas generation
and migration; dependence of sand and clay compaction on both compacting force and
reduction of rock matrix strength; and clay compaction caused by temperature increase.
(3) Sets of geologic and physical parameters that can be correlated with pressure
increase are recommended for the cases of compaction and upward fluid migration
domination. These parameters present separately the ability of fluid-filled rock to change
pressure under an external influence, the external influence itself, and permeability of
formations.