BAKER HUGHES - Drilling Fluids Reference Manual

PRESSURE PREDICTION AND CONTROL
ABNORMAL FORMATION PRESSURES
Formation pressures in excess of normal formation pressures for an area are referred to as
abnormal pressures. The fundamental difference between normally and abnormally pressured
rocks is that in abnormally pressured zones the pore fluids no longer communicate 100%
efficiently with the water table (surface communication). Some mechanism is providing a seal
or cap to interfere with the fluid column and preventing it from achieving normal hydrostatic
equilibrium. Abnormal pressure gradients in excess of l.0 psi/ft have been recorded, but they
generally do not exceed 0.91 psi/ft. Some of the causes given for abnormal formation pressures
are:
• Compaction disequilibrium is a common cause of abnormal pressure. This is especially true in
rapid filling (Tertiary) sedimentary basins. Rapid sedimentation rates accompanied by thick, lowpermeability
shale sections and growth faults entrap pore fluids. This process restricts the ability
of the fluids in the sediments to escape as in a normal compaction situation. When entrapped pore
fluids are coupled with overburden pressures, then abnormal pressures are generated. Examples
are seen in the Gulf of Mexico, the North Sea and the Niger Delta.
• Tectonic activities such as salt and shale intrusions, faulting, folding, etc., may contribute to
abnormal pressures (see Figure 12-1). In the figure below the sand has been elevated while
retaining its pore pressure and is thus abnormally pressured at its uplifted depth.
Abnormally pressured sand
Figure 12 - 1 Tectonic Activities
• Structural features such as dipping lenses and anticlines having normal pressures in the deeper
part of the zone can transmit pressure to the higher section of the zone, resulting in abnormal
pressure (see Figure 12-2). Also uplifting of “rafts” in salt domes can cause severe overpressures
due to the effectiveness of the seal of the surrounding salt.
• Hydrocarbon generation and migration are amongst the most significant factors in the generation
of over-pressures. The hydrocarbon that produces the greatest over-pressures is gas. Gas expands
much more than oil or water, and whenever it is trapped, it reduces the hydrostatic control.
Adequate drainage is required to prevent pressure increases. The movement of hydrocarbons into
permeable zones capped by impermeable zones often generates overpressures.
BAKER HUGHES DRILLING FLUIDS
REFERENCE MANUAL
REVISION 2006 12-2