This is the result of the ay geological processes that have influenced the sub-surface environment. It is also influenced by the fluid content and density contained within the sub-surface rocks.
In general the greater the depth the greater the formation pressure as a result of the increased load distributed through the grain to grain contact. So long as there is hydrostatic continuity, equilibrium will be maintained.
In drilling you may encounter abnormal formation pressures, these may cause problems that may result in costly drilling operation or may in the worst case scenario cause the shutting down of operations or loss of life.
There are 4 main mechanisms that account for abnormal pressures:
- Compaction effects
- Diagenetic effects
- Differential Density effects
- Fluid Migration effects
This occurs when particles are compressed closely together such that pore space is reduced. This results in increased pressure
This is the result of chemical alteration of the rocks by the geological process.
Differential Density Effects
This is as a result the fluid within a formation having a significantly different density relative to the surrounding normal fluid densities.
Fluid Migration Effects
This results in from the movement of fluids from one higher pressured formation moving into another lower pressured formation, usually from deeper to a shallower formation. This will usually result in the shallower formation having an abnormally higher formation pressure.