Hydrostatic pressure is pressure that exert by density of fluid column. The relationship of hydrostatic pressure is shown in the equation below.
HP (Hydrostatic Pressure) = density x g (gravity acceleration) x h (True Vertical Depth, TVD)
In oilfield term, the formula above is modified so that people can use it easily. The formulas are as follows:
HP = Constant x MW x TVD
HP = 0.052 x MW (ppg) x TVD (ft) ** Most frequent used in the oilfield **
HP = 0.007 x MW (pcf) x TVD (ft)
HP = 0.00981 x MW (kg/m3) x TVD (m)
According to the equation, Hydrostatic Pressure is not a function of hole geometry. Only mud weight and True Vertical Depth (TVD) affect on Hydrostatic Pressure. For example (a picture below); well A and well B have the same vertical depth. With the same mud density in hole, the bottom hole pressure due to hydrostatic pressure is the same. The only different between Well A and Well B is mud volume.
This concept is basic and very important for many aspects such as well control, balance cementing, u-tube, etc.
This is one of well control series. To be continue 
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December 24th, 2009 at 4:25 am
[...] Well Control is hydrostatic pressure provided by drilling fluid more than formation pressure but less than fracture gradient while [...]
January 5th, 2010 at 6:47 am
[...] so formation pressure at 8000’ TVD = 8000 ft x 0.465 psi/ft = 3720 psi. Click here to learn how to calculate hydrostatic pressure in [...]
January 5th, 2010 at 7:47 pm
We do Horizontal Directional Boring with hole diameters upward to 54″ through which we pull a 36″ HDPE pipe.
The depths are geneally only 20-40 feet deep and the lenghts are upwards to 2000 lineal feet.
We are constantly in discussions that we need to be deeper to help prevent frac outs and potential groung heave issues with the Engineers reoresenting our customers.
I know that there are many variables that impct these issues, but it would be most helpful to be able to identify where the ground pressures vs hyrostatic indicate potential problems in soil conditions of sand, clay, or rock.
Can you help?
Thanks.
January 17th, 2010 at 1:45 am
[...] drill pipe side: BHP = 0 psi (Drill pipe Pressure) + 0.052×9.8×8,500 (Hydrostatic Pressure at drill pipe side) = 4,331 [...]