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.
Reservoir pore fluid pressure that is not similar to normal saltwater gradient pressure. The term is usually associated with higher than normal pressure, increased complexity for the well designer and an increased risk of well control problems. Pressure gradients in excess of around 10 pounds per gallon equivalent fluid density (0.52 psi/foot of depth) are considered abnormal. Gradients below normal are commonly called subnormal.
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