Page 146 - Origin and Prediction of Abnormal Formation Pressures

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Chapter 5

METHODS OF ESTIMATING AND PREDICTING ABNORMAL FORMATION
PRESSURES

G.V. CHILINGAR, V.A. SEREBRYAKOV, S.A. KATZ and J.O. ROBERTSON JR.

INTRODUCTION

Most of the pressure detection techniques described in the literature are based mainly
on the observation that many overpressured rocks are associated with undercompacted
shales. The greater the undercompaction, the higher the porosity, and the greater
the percentage of the load carried by the intergranular fluid. Undercompaction and
decompaction often result in the creation of higher porosity in comparison with a
normal compaction trend. Weller (1959) and Vassoevich (1960) noted that shales can
be used as a geologic manometer. They found interrelationships among the degree of
compaction, shale porosity, and overburden and formation pressures. There are different
shale porosity versus depth curves for various lithologic cross-sections and different
basins. The normal compaction trend must be known to estimate the correct abnormal
pressure for (1) each basin, (2) each oil and gas field, and maybe (3) each well (see
Rieke and Chilingarian, 1974).
A major problem when drilling new wells in an abnormally pressured area is that
of identifying an abnormally pressured formation prior to drilling into it. Drilling
unexpectedly into an abnormally pressured formation (AHFP) can result in costly
drilling expenses. These costs can be avoided through the observation of a combination
of several formation-pressure indicators. These include surface and borehole seismic,
drilling, and well logging data. Although several such indicators should be monitored at
the wellsite, not all data are usable or necessarily required in any one drilling operation.
Drilling operations in overpressured environments should incorporate the following
steps: (1) planning the well- gathering and interpretation of all known information
of the region in which the well is to be drilled; (2) preparation of a preliminary
drilling program with educated guesses about the potential of encountering abnormally
pressured formations; and (3) review and revision of the drilling program as the well is
being drilled. One should always have contingency plans prepared if an abnormally high
pressured formation is encountered so that all decisions can be made quickly. It should
be remembered that drilling operations, drilling fluids (muds), and casing constitute
major costs in a drilling program. Also, a review of the regional fracture pressure
gradient, as well as the pore-pressure variations, should be estimated prior to drilling
and continuously monitored while drilling the well in order to design an effective and
economical drilling mud program. Sometimes, a well can be lost due to a blowout if an
AHFP is unexpectedly encountered.