Shale shakers Suction line

ABNORMAL PRESSURE

If at any time the shale, or the permeable formation, is isolated then the excess pore fluid cannot bleed off. Then the increasing overburden will be born by the incompressible pore fluid. Thus it will be overpressured by the added mass of the formations from the depth where it was isolated to its current depth.
Rapid deposition, or regional tectonic movement, can create additional stress on the rock. This can accelerate the release of bound water beyond the bleed off rate of the low permeability shale.
An uplifted, isolated, formation will retain the original internal pressure. Leaks along a fault, or through a bad cement job, can connect a lower, higher pressured formation with a higher zone. These formations will have a higher than normal pressure gradient.
Any one of the above phenomena raises the pressure of the fluid in the pore spaces higher than normal. This is termed an ‘abnormally’ pressured formation. Multiple overpressure situations are sometimes termed ‘supernormally’ pressured formations.

SUBNORMAL PRESSURE

The opposite effect can occur when two faults drop a centre block, graben faulting. A normally pressured zone is suddenly (in geological terms) at a depth greater than its equivalent column of water pressure.
A reservoir, produced for some time, may be depleted to below the normal hydrostatic pressure gradient.
16. WELLCONTROL
Being able to control subsurface pressures is critical for the safety of the whole drilling operation.
PREVENTION
Except when under-balanced drilling, the pressure exerted by the drilling fluid must be higher than the bottom hole formation pressure.
Where abnormal pressures are present the drilling fluid density must be routinely and frequently checked. Additions of water (or oil) at all locations on the rig must be controlled and monitored.
EFFECTS OF KICK TYPES
SALT WATER
Dilution of additives and flocculation of bentonite are the major problems. In IOES the water must be rapidly emulsified to prevent water wetting the weighting material which will then drop out.
OIL
Entrained oil acts as a solid in water-based drilling fluids. In IOES it dilutes additive concentration and density.
GAS
In IOES gas can be soluble in the hydrocarbon phase whilst under pressure. This complicates annular pressure response during well killing as the gas comes out of solution and rapidly expands higher in the well.
CURE
Raise the density of the fluid to control the pore pressure and circulate out the influx. Any detrimental impact on the drilling fluid should be treated as soon as possible.