What is Hydrostatic Pressure?

While drilling a well, drilling fluid (mud) is pumped through drill string, it comes out through bit nozzles, enters the annulus, travels up the annulus and finally reaches the surface and comes back to mud pit through flow line. Figure below is illustrative of what i have stated.

Thus in the process of drilling the annulus is always kept full of mud.

Hydro static pressure can be defined as the pressure exerted by static column of drilling fluid due to the local force of gravity. If the density of the fluid and the height of fluid column is given, then the hydrostatic pressure exerted by the fluid column at any point can be calculated utilizing the formula below:

Hydro Static Pressure (psi) = 0.052 x Mud Weight (ppg) x True Vertical Depth (feet)

One can notice in the above formula that pressure is a function of True Vertical Depth (TVD). Thus, no matter what the Measured Depth (MD) is, the pressure will be a function of TVD only. 

As illustrated in example below, if both the wellbore is filled with mud of same density, say 10 ppg, the hydrostatic pressure in both the cases (A) and (B) will be same because the TVD is same in both the cases, ie. 1000 feet.

Case (A):
Hydrostatic Pressure = 0.052 x 10 x 1000 = 520 psi

Case (B):
Hydrostatic Pressure = 0.052 x 10 x 1000 = 520 psi

The 0.052 is needed as the conversion factor to psi unit of Hydrostatic Pressure.

To convert these units to SI units, one can use:

1 ppg = ≈ 119.8264273 kg/m3
1 ft = 0.3048 metres
1 psi = 0.0689475729 bar
1 bar = 10^5 pascals

IADC Lexicon defines hydrostatic pressure as:

"The pressure that exists at any point in the well bore due to the weight of the vertical column of fluid above that point."

Source: API RP 59, Recommended Practice for Well Control Operations, Second Edition, May 2006.