Pore pressure is defined as the pressure acting on the fluids in the pore spaces of the rock. This is the scientific meaning of what is generally referred to as formation (pore) pressure. Depending on the magnitude of pore pressure, it can be described as being either normal, abnormal or subnormal. A definition of each follows.
NORMAL PORE PRESSURE
Normal pore pressure is equal to the hydrostatic pressure of a column of formation fluid extending from the surface to the subsurface formation being considered In other words, if the formation was opened up and allowed to fill a column whose length is equal to the depth of the formation then the pressure at the bottom of the column will be equal to the formation pressure and the pressure at surface is equal to zero.
Normal pore pressure is not a constant. The magnitude of normal pore pressure varies with the concentration of dissolved salts, type of fluid, gases present and temperature gradient. For example, as the concentration of dissolved salts increases the magnitude of normal pore pressure increases.
ABNORMAL PORE PRESSURE
Abnormal pore pressure is defined as any pore pressure that is greater than the hydrostatic pressure of the formation water occupying the pore space. Abnormal pressure is sometimes called overpressure or geopressure. Abnormal pressure can be thought of as being made up of a normal hydrostatic component plus an extra amount of pressure. This excess pressure is the reason why surface control equipment (e.g. BOPs) are required when drilling oil and gas wells.
Abnormal pore pressure can occur at any depth ranging from only a few hundred feet to depths exceeding 25,000 ft. The cause of abnormal pore pressure is attributed to a combination of various geological, geochemical, geothermal and mechanical changes.
SUBNORMAL PORE PRESSURE
Subnormal pore pressure is defined as any formation pressure that is less than the corresponding fluid hydrostatic pressure at a given depth. Subnormal pore pressures are encountered less frequently than abnormal pore pressures and are often developed long after the formation is deposited. Subnormal pressures may have natural causes related to the stratigraphic, tectonic and geochemical history of an area, or may have been caused artificially by the production of reservoir fluids. The Rough field in
the Southern North Sea is an example of a depleted reservoir with a subnormal pressure.
