1. Hydrostatic Pressure
This is the pressure due to the unit weight and vertical height of a static column of fluid. It is expressed mathematically as:
(a) In Field Units
PH = 0.052*r*D
PH = Hydrostatic pressure, psi
r =Density, pounds per gallon(ppg)
D = Well depth , ft.
(b) In Metre Units
PH= rD/1 0
where
PH = Hydrostatic pressure, kg/cm2
r = Density, pounds per gallon, gm/cc
D = Well depth, meters
2. Overburden Pressure
This is the pressure exerted by the total weight of solids and fluids in the
formation. It can be defined mathematically as :
(a) In Metric units
sob = rb * D/10 (5)
rb = average bulk density for interval, gm/cc
D = depth of sediment, metres
(b) In Field Units
sob = 0.433* rb * D (6)
rb = Specific weight, gm/cc
D = Depth, ft.
rb = rg*(1-f) + rfl *f
The bulk density can be obtained from the combined Density, Resistivity and Gamma ray logs.
f = (rg – rb )/( rg – rfl)
One of the most important aspect of well planning is the estimation of bulk
density for the various formations drilled. This can be computed from seismic data, shale density or sonic logs.
The Overburden gradient is defined as sob/D ( psi/ft)
Bulk density is a function of depth and as the overburden increases with depth, porosity decreases due to compaction effect thus increasing the bulk density
