Page 24 - Drilling Technology in Nontechnical Language

Chapter 1 – DRILLING GEOLOGY 15

Fluids (liquids and gases) exert pressure in all directions against a

vessel containing the fluid and against anything submerged in the fluid.

This principle is used in hydraulics, where a fluid is used to transmit a
force from one place (a hydraulic pump) to another (a hydraulic jack used
to lift a car).
In a vertical column of fluid, gravity causes the pressure inside the fluid

to change with depth. The pressure in the fluid at a particular depth has to

support the weight of the fluid above that depth. This can be explained one

step at a time.
Imagine a liquid (a salty water) that weighs 0.0417 lb for each cubic
inch (in ) of volume. This liquid is stored in a 12" deep square tube that
3
measures 1" on each side.
Now the volume inside this tube is 12 in (12" × 1" × 1"). If the weight
3
of 1 in is 0.0417 lb, then the weight of the 12 in inside the tube is found
3
3
as follows:
12 in × 0.0417 lb/in = 0.5 lb
3
3
Pressure is calculated by dividing a load (in pounds) by the area
supporting that load (in square inches, in ). In this case, the area at the
2
bottom of the tube is 1 in , so the pressure at the bottom of the tube is
2
found as follows:

0.5 lb ÷ 1 in = 0.5 pounds per square inch (psi)
2

Half way down the tube, there is only one-half of the weight of fluid

(0.25 lb) sitting on the same area (1 in ), so the pressure is one-half of
2
what it is at the bottom of the tube. If the depth halves, the pressure
halves. Conversely, if the depth doubles to 2 ft, the pressure doubles to 1
psi. For each foot of depth increased, the pressure in pounds per square
inch increases by one-half. This allows a very easy way to calculate the
pressure at any depth. For this fluid, it is simply the depth in feet multiplied

by ½ psi. The pressure gradient is thus 0.5 psi per foot (psi/ft).
Fresh water has a pressure gradient of 0.433 psi/ft, and seawater
(though it varies depending on the amount and types of salt dissolved in
the water) is around 0.465 psi/ft. In a well that is 10,000 ft deep and full of
seawater with a pressure gradient of 0.465 psi/ft, the pressure on bottom of
that well would be 4,650 psi.

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