Hydrostatic pressure is defined as the pressure exerted by a column of fluid. The pressure is a function of the average fluid density and the vertical height or depth of the fluid column.
Mathematically, hydrostatic pressure is expressed as:
HP = g x ρf x D (1.1)
where:
HP = hydrostatic pressure
g = gravitational acceleration
ρf = average fluid density
D = true vertical depth or height of the column
In field operations, the fluid density is usually expressed in pounds per gallon (ppg), psi per foot, pounds per cubic foot (ppf) or as specific gravity (SG).
In the Imperial system of units, when fluid density is expressed in ppg (pounds/gallon) and depth in feet, the hydrostatic pressure is expressed in psi (lb/in2):
HP (psi) = 0.052 x ρf (ppg) x D (ft) (1.2)
For the purposes of interpretation, all wellbore pressures, such as formation pressure, fracture pressure, fluid density and overburden pressure, are measured in terms of hydrostatic pressure.
When planning or drilling a well it is often more convenient to refer to hydrostatic pressures in terms of a pressure gradient. A pressure gradient is the rate of increase in pressure per unit vertical depth i.e., psi per foot (psi/ft). It should be noted that fluid densities, measured in ppg or SG, are also gradients.
Hydrostatic pressures can easily be converted to equivalent mud weights and pressure gradients.Hydrostatic pressure gradient is given by:
HG = HP / D … (psi/ft) (1.3)
It is usual to convert wellbore pressures to gradients relative to a fixed datum, such as seabed, mean sea level or ground level. The resulting figure (pressure gradient) allows direct comparison of pore pressures, fracture pressures, overburden pressures, mud weights and Equivalent Circulating Density (ECD) on the same basis. In addition the use of pressure gradients accentuates variations in pressure regimes in a given area when values are plotted or tabulated.
When pressure gradients are used to express magnitudes of wellbore pressure, it is usual to record these as Equivalent Mud Weight (EMW) in ppg.
