What is Pressure Gradient?

It can be defined as pressure per unit length. If the pressure is in psi and length is in feet then the unit of pressure gradient is psi/ft.

Following terms are used in common in context to pressure gradient:

• Hydrostatic pressure gradient
• Formation pressure gradient

If the hydrostatic pressure exerted by a drilling fluid column of density 10 ppg in wellbore at a depth of 1000 feet is 520 psi, then the hydrostatic pressure gradient is defined as the hydrostatic pressure per unit length, which is the ratio of hydrostatic pressure and the depth at which the hydrostatic pressure is considered, ie. 520 psi/1000 ft = 0.52 psi/ft.

Similarly, if the formation pressure at a depth of 1000 feet is 600 psi then the formation pressure gradient is defined as the formation pressure per unit length, which is the ratio of formation pressure and the depth at which the formation pressure is estimated, ie. 600 psi/ 1000 ft = 0.6 psi/ft.

If you remember the formula for calculation of pressure exerted by static fluid column,

Pressure exerted by fluid column (psi) = 0.052 x density of fluid (ppg) x height of fluid column (ft)

Now, Pressure gradient = pressure exerted by fluid column/ depth of fluid column

If we look closely at above two solutions. once can see that: 

Thus, we can conclude that:

Pressure = Pressure Gradient x Length for which pressure gradient is estimated