Page 27 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 21

This book uses a mixture of oilfield and SI units, with the emphasis
on oilfield units, since they are still the most widely used. Most people
working in the industry will have had some exposure to psi, barrels, feet,
etc. SI answers are provided for some of the worked examples.

1.1.6 Hydrostatic pressure calculations
Understanding hydrostatic pressure is fundamental to well control. An
ability to calculate hydrostatic pressure at any point in the wellbore is an
essential skill.
Mass: Mass is the term for a quantity of matter. The oilfield unit of
measurement is the pound, and the SI unit of measurement is the gram
or kilogram.
Density: Density is an expression giving the mass of gas, fluid, or solid
matter in a given volume, i.e., mass per unit volume. For example, in oil-
field units, density is normally reported as pounds (mass) per US gallon
(volume), and abbreviated as ppg. In SI units, density is normally
3
recorded in kilograms per m , or kilograms per liter (Kg/L).
Liquid density can also be expressed relative to fresh water. Fresh
water density will vary with temperature, and there are various
figures quoted. However, for consistency, 8.33 lbs/gallon is used for fresh
water. This is the value used in nearly all oil industry text books.
Since the mass of 1 (US) gallon of fresh water is known (8.33 ppg),
that value is used to calculate the mass of any fluid relative to the mass of
fresh water. This is termed specific gravity (SG):
ppg
SG 5 (1.3)
8:33
Example: What is the SG of a 10 ppg brine?
10ppg
5 1:2004SG:
8:33ppg

The equation can be rearranged to find the mass of the fluid (ppg) if
the relative density (SG) is known.

ppg 5 8:33 3 SG
In SI units, fluid mass, and SG are effectively the same, since 1 L of
fresh water (at 4 C) has a mass of 1 Kg.

Temperature and pressure effects on density: Fresh water density will vary

with temperature. Fresh water has a relative density of 1 SG at 4 C.