Shale shakers Suction line

NORMAL FORMATION PRESSURE.

The initial sediments are laid down with about 60 - 70%v water. This material is gradually buried deeper and deeper under the slow process of deposition.
At a burial depth of approximately 1000 m (±3,000 ft) grain to grain contact occurs for the majority of the minerals and the material is regarded as a rock. At this stage it contains about 26 - 30%v water. In the mudstone/claystone /shale formations the water is held as layers between the clay platelets by forces other than purely physical confinement (bound water).
As the pressure increases from more overlying sediments, and the temperature rises with burial, more water is stripped from between the layers of clay particles. At around 2,000m (±7,000 ft) sufficient energy has built up in the system to have fully dislodged one layer of bound water from the clay structure. Further layers are removed by approximately 5,000 m (±16,500 ft), 10,000 m (±33,000 ft) and 20,000 m (±65,000 ft).
Mixing with generated hydrocarbons, the released water slowly migrates through the very low permeability environment of the clays until it finds a more permeable formation, such as a sand. The excess fluid bleeds through this zone back towards the surface. With geological time the pressure in the shale is relieved back to the equivalent of a column of water to that depth. The pressure in the pore spaces is therefore equal to the hydrostatic column of the water. The remainder of the overburden stress is supported by the rock matrix. This is termed a ‘normally’ pressured formation.
The water escaping from the compacting clay formations leaves behind some of the originally dissolved salts. With increasing depth the pore water has an increasing salinity resulting in a higher fluid density. Thus the pressure exerted increases with depth. This can be determined by integrating the resistivity log.
As a rule of thumb: A ‘normal’ pressure gradient of 10·5 kPa/m (0·465 psi/ft) can be used to depths down to 3,000m.