Formation Pressure or Pore pressure

Formation pressure (Pore pressure) is the pressure in the pores of a formation.As the sedimentation process goes on in a water filled environment ,more and more sediments are deposited over each other. Due to the weight of the overlying layers lower layers of sediments gets compact with increasing time and overlying weight.This compaction expels the fluid from the pores.But still some fluid is left in the pores.Due the weight of overlying formations this fluid will experience some pressure.This pressure is called Pore pressure.

If the pores are interconnected up to the surface then pore pressure at any depth will be equal to the pressure of a simple column of fluid.

Formation pressure is often represented in the form of pressure gradient.Pressure gradient is the change in pressure per unit depth.It is often expressed in Psi/ft.If the pressure gradient is known pressure at any depth can be calculated by:-

Pressure(psi)= Tvd(ft)*Pressure gradient(psi/ft)
Tvd= True vertical depth
Pressure gradient should be average pressure gradient for the complete depth.
Pore pressure profile

Generally the formation contains water mixed with salt and is known as Brine.Fresh water has a pressure gradient of 0.433 psi/ft when this water contains salt its average pressur gradient comes out to be 0.465 psi/ft.That's why 0.465 psi/ft is considered as Normal pressure gradient and any pressure above or below it is considered as abnormal pressure gradient.

Pore pressure profile prediction is important as it affects the casing design,drilling mud design.Inaccurate Pore pressure prediction also causes a lot of hole problems(Stuck pipe,Well control problem).

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