Page 147 - Origin and Prediction of Abnormal Formation Pressures

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

The concept of the process of rock compaction is a fundamental part of many
prediction methods (Serebryakov and Chilingar, 1994). In basins with well-compacted
rocks, the influence of temperature on the formation pressure is an important factor.
Tectonically caused abnormal pressures (overcompaction) caused by squeezing of water
from shales into associated sandstones are discussed in Chapter 8.
Some of the most reliable quantitative overpressure detection and evaluation tech-
niques are based on geophysical wireline logging methods. These methods, however,
are usually after-the-fact techniques, i.e., the wellbore is drilled prior to logging. Many
short logging runs are frequently necessary, if the logging-while-drilling is not used.
Logs that can detect abnormally high formation pressures have been discussed in detail
by Fertl (1976, pp. 177-230).
Wireline formation testers, which record several pressures over the entire length of
the uncased borehole, are being used. Measurements from these testers can be used to
'calibrate' other drilling and/or log-derived pressures. Cased-hole wireline formation
testers are also available to calibrate logging data.
Applications of good surface seismic data allow: (1) determination of interval
velocity; (2) the study of lithologic and stratigraphic variations in geologic sections;
(3) estimation of geologic age and/or average geothermal gradients; (4) the study of
the effects of lateral folding pressure on a regional scale; (5) detection of hydrocarbon
presence, especially gas, at shallow depths (i.e., the bright spot technique); (6) detection
of natural and/or artificially caused hydrocarbon seepages from the ocean floor; (7)
investigation of the ocean floor and sub-bottom properties (buried ancient glacial
channels, sediment stability, and mud lumps) for proper planning of offshore drilling
and production operations; and (8) detection of the presence and top of abnormally
pressured formations and determination of the magnitude of pressure.
Drilling parameters and logging-while-drilling methods provide instantaneous infor-
mation. Drilling mud parameters and shale cutting analysis are an excellent source of
information as these data can be compiled during drilling. The latter data, however, are
delayed by the time required for circulation and sample return (see Table 5-1).
Interpretation of recorded data is not always straightforward. Accuracy depends on
geologic factors, borehole environment, sample selection at certain depth increments,
and plotting techniques (Table 5-2).
Digital recording systems can yield borehole seismic recordings at the wellsite.
Behavior of the borehole wall in response to sound energy from a surface source can
be detected by a three-component geophone tool and recorded. Table 5-3 lists several
interpretive product lines related to borehole seismic technology.
An approach that does not rely on the concept of rock compaction is the methodology
of detection of abnormally pressured zones based on the concentration of radioactive
isotope 4~
With the exception of the SP-curve data, all parameters which are recorded in
shales are plotted versus depth. Trendlines are then established for normal compaction.
Interpretation of the logs depends on the magnitude of the departure from the normal
trend, due to the divergence of formation pressure from the normal hydrostatic pressure
at a specific depth. Application of these methods, however, is not always simple and
straightforward.