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Abnormal formation pressure

Drilling into overpressured strata can be hazardous because overpressured fluids escape rapidly, so careful preparation is made in areas of known overpressure.

Abnormal formation pressure

Abnormal formation pressure is defined as any pressure that devrates from the normal hydrostatic gradient.
Abnormal formation pressure - a subsurface condition in which the pore pressure of a geologic formation exceeds or is less than the expected, or normal, formation pressure.
When impermeable rocks such as shales are compacted rapidly, their pore fluids cannot always escape and must then support the total overlying rock column, leading to abnormally high formation pressures.
Excess pressure, called overpressure or geopressure, can cause a well to blowout or become uncontrollable during drilling.
Severe underpressure can cause the drillpipe to stick to the underpressured formation.
In order for these pressures to form and be preserved, a near-seal is required.
The seal is not necessarily impermeable, but may be a low permeabrlity formatron, such as shale, or a fault-related barrier.
Abnormally high pore pressure can occur in areas where burial of fluid-filled sediments is so rapid that pore fluids cannot escape, so the pressure of the pore fluids increases as overburden increases.
Drilling into overpressured strata can be hazardous because overpressured fluids escape rapidly, so careful preparation is made in areas of known overpressure.

Abnormally high formation pressures (AHFP) are found worldwide in formations ranging in age from the Pleistocene age (approximately 1 million years) to the Cambrian age (500 to 600 million years).
They may occur at depths as shallow as only a few 100 feet or exceeding 20,000 ft and may be present in shale/sand sequences and/or massive evaporite-carbonate sequences.

The causes of AHFP are related to a combination of geological, physical, geochemical and mechanical processes.

As defined, the magnitude of AHFP must be greater than the normal hydrostatic pressure for the location, and may be as high as the overburden pressure.
AHF gradients will thus be between the normal hydrostatic gradient (0,433 - 0.465 psi/ft) and the overburden gradient (generally 1,0 psi/ft).

Locally confined pore pressure gradients exceeding the overburden gradient by up to 40% are known in areas such as Pakistan, Iran, Papua New Guinea, and the CIS.
These super pressures can only exist because the internal strength of the rock overlying the super pressured zone assists the overburden load in containing the pressure.
The overlying rock can be considered to be in tension.

In the Himalayan foothills of Pakistan, formation pressure gradients of 1,3 psi/ft have been encountered.
In Iran, gradients of 1,0 psi/ft are common and in Papua New Guinea, a gradient of 1,04 psi/ft has been reported. 
In one area of Russia, local formation pressure in the range of 5870 - 7350 psi at 5250 feet were reported.
This equates to a formation pressure gradient of 1,12 to 1,4 psi/ft.
In the North Sea abnormal pressures occur with widely varying magnitudes in many geological formations.

The Tertiary sediments are mainly clays and may be overpressured for much of their thickness.
Pressure gradients of 0,52 psi/ft are common with locally occurring gradients of 0,8 psi/ft being encountered.
An expandible clay (gumbo) also occurs which is of volcanic origin and is still undergoing compaction.
The consequent decrease in clay density would normally indicate an abnormal pressure zone but this is not the case.
However, in some areas, mud weights of the order of 0,62 psi/ft or higher are required to keep the wellbore open because of the swelling nature of these clays.
This is almost equal to the low overburden gradients in these areas.

In the Mesozoic clays of the North Sea Central Graben, overpressures of 0,9 psi/ft have been recorded.
One reported case indicated a formation pressure gradient of 0,91 psi/ft in the Jurassic section.
In the Jurassic of the Viking Graben, abnormal formation pressure gradients of up to 0,69 psi/ft have been recorded.

In Triassic sediments, abnormally high formation pressures have been found in gas bearing zones of the Bunter Sandstone in the southern North Sea.
Also in the southern North Sea, overpressures are often found in Permian carbonates, evaporates and sandstones sandwiched between massive Zechsteins salts.