Page 89 - Origin and Prediction of Abnormal Formation Pressures

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

cannot be used for pressure detection. Therefore, new approaches, that can form a basis
for new and more reliable methods for different geological environments are necessary.
They can only be developed by more thorough and detailed in-depth analysis of causal
relations between pressure distribution and the complete set of geological, physical,
and physicochemical factors. Such an analysis can lead to new mathematical and other
formal methods for establishing quantitative relations between parameters of pressure
distribution and geological environment.
A new approach to the empirical study of non-hydrostatic pressure and pres-
sure-changing mechanisms in fields and regions was suggested by Gurevich et al.
(1987). The essence of this approach is in dividing the pressure value into the free
convection and forced convection components and correlating these components sep-
arately with those geologic parameters and factors that cause each component. Such
statistical analysis of the already existing field data will provide better basis for pre-
drilling pressure prediction. At the same time it will be helpful to classify the geologic
patterns of regions and formations to develop separate techniques for determination
of pressure from combinations of drilling and geophysical data. The additive pressure
components and geologic and physical characteristics and parameters that correspond to
them separately can make a reliable basis for pressure prediction and detection expert
systems, a powerful tool in the oil and gas exploration (Aminzadeh, 1991).
This approach of pressure subdivision into two physically different components was
radically new but the work was discontinued and the writers had no possibility to apply
this approach. The authors of this chapter continue this line of research.
The purpose of this chapter is to summarize concisely current understanding of
pressure distribution origin, to indicate areas that have not been fully studied and that
have, therefore, a potential for new practical development, and to outline a possible new
approach to the solution of the abnormal and subnormal pressure problem.

FACTORS CAUSING FLUID FLOW AND PRESSURE DISTRIBUTIONS
Flow of underground fluids (water, oil, and gas) and distribution of their pressures
are just two sides of the same coin and must be considered together. Distributions of
fluid flow and pressure are determined by a superposition of distributions of (1) factors
causing fluid flow and (2) permeability of rocks.

Factors of fluid flow and pressure distribution and changes

Factors of fluid flow and pressure may be divided into two separate groups: (1) those
that cause free convection, and (2) those that cause forced convection. This division
is natural from the point of view of physics. Abnormal and subnormal pressures are
caused only by the forced convection factors in flat countries and by both free and
forced convection factors in areas with broken ground relief, especially in intermontane
areas. It may be of no importance for mathematical simulation of flow and pressure
distribution. But it is important, even crucial sometimes, for statistical analysis of
relations between distributions of pressure and geologic parameters and characteristics,