Page 29 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 23

This formula can be rearranged to calculate the force from a given
pressure and a unit area:
Force 5 P 3 A (1.5)

When using oilfield units, pressure is expressed as the pounds of force
applied against a one square inch area, i.e., pounds per square inch, and is
commonly abbreviated as psi.
Pressure in SI units is expressed in pascals (Pa). A pascal is a pressure of
1 N (0.102 kg) per square meter. Since this unit is impractically small for
2
most oil industry applications, the kilopascal (kPa) equal to 1000 N/m is
more commonly used (1 psi 5 6.895 kPa).
a
In some locations, bar is used for pressure measurement. One bar is equal
to 100 kPa, and is slightly less than atmospheric pressure (0.987 atm).

1.1.6.1 Hydrostatic pressure
Hydrostatic pressure is the total fluid pressure created by the weight of a
column of fluid (liquid or gas) acting at any given point in the well. The
derivation of the word comes from “hydro,” Greek for water and static,
meaning not moving. To calculate hydrostatic pressure, well depth, and
the density of the fluid in the well must be known.
Fluid density is converted to a pressure increase per unit depth, or fluid
gradient.
When using oilfield units, fluid weight in ppg is converted to a gradient
(psi/ft) by multiplying the fluid weight by a conversion factor of 0.052.
It is worth understanding the derivation of this number, since it is
used so frequently. It is also one worth committing to memory.
One cubic foot contains 7.48 US gallons. A cubic foot filled with a
fluid having a density of 1 ppg would weigh 7.48 lbs, and the pressure at
the bottom of the container will be:

WeightðforceÞ 7:48lbs 2
5 5 7:48lbs=ft
Area 1ft 2

a
Although the bar is a metric unit and used for pressure measurement in some locations,
it is not approved as part of the International (SI) System of Units, probably because it
was developed in Norway and not France.