Page 38 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 2

ORIGIN OF ABNORMAL FORMATION PRESSURES

G.V. CHILINGAR, J.O. ROBERTSON JR. and H.H. RIEKE III

INTRODUCTION

Interstitial (intergranular or formation) fluid pressures, either above or below the
hydrostatic pressure, occur around the world under a wide range of geological condi-
tions. Any pressure that is either above or below the hydrostatic pressure is referred to
as an abnormal formation pressure. Pressures above the hydrostatic pressure are often
referred to as abnormally high (AHFP) or surpressures. Pressures below the hydrostatic
pressure may be referred to as either abnormally low (ALFP) or subpressures.
The object of early formation analysis of abnormally pressured zones was primarily
to predict and identify these zones prior to drilling into them. This need for prior knowl-
edge was motivated by the economic losses that were often experienced by suddenly
drilling into an unrecognized abnormally pressured region. Attention must be paid to
pore fluid and rock stresses in sedimentary sequences, because the knowledge of vertical
and lateral stress patterns in a depositional basin is helpful in evaluating its history and
development. A thorough quantitative understanding of compaction mechanics, the rela-
tionship between the total overburden stress, effective stress, and pore stress (pressure)
in fine-grained clastics is required to recognize the potential development of abnormally
high pressured formations. Possible origins of abnormally pressured formations are
presented in Table 2-1 (surpressured) and Table 2-2 (subpressured).
Throughout the world, literature is filled with examples of abnormally high pressured
formations, recorded at depths of a few hundred feet to that greater than 20,000 ft. As
shown in Table 2-1, the processes often responsible for the generation of abnormally
high formation pressures (AHFP) can be grouped into three categories: (1) changes in
the rock pore volume, (2) changes in the fluid volume within the pores, and (3) changes
in the fluid head. All three of these mechanisms require changes that occur faster than
the formation is able to drain-off the excess pressure.

Definitions

The following terms (definitions) are used in this chapter (loads, stresses and
pressures).
Stress: the total pressure (force per unit area, a) acting at a point.
Effective stress, ~c: that part of the load (force per unit area) that is not counteracted by
other forces and is available to cause compaction.
Hydrostatic gradient, Gh: the pressure exerted by a column of water per unit of depth.
Hydrostatic pressure, Ph: the pressure exerted at the bottom of a water column or at a