For those working at wellsites, a basic knowledge of “fluid” properties is required, especially those properties that distinguish fluids from solids. Fluids can be either a gas or a liquid, where gases are highly compressible and its volume being dependent upon pressure and temperature. Liquids,
on the other hand, are only slightly compressible, and their volume being only slightly dependent upon temperature.
We shall be dealing with only liquids in this text. Since drilling muds are commonly referred to as drilling fluids, the term “fluid” will be used throughout the text. The effects of temperature and pressure on a volume of drilling fluid will be ignored.
A cube of water measuring 1 foot along each edge weighs 62.4 lbs. The density or “specific weight” is then 62.4 lb/ft3. Specific weight divided by the gravitational constant is known as “mass density” or just density. This same cube of water exerts a hydrostatic pressure of 62.4 lbs distributed evenly over its bottom surface of 1 ft2 or 0.433 psi (62.4lbs ¸ 144 in2).
Hydrostatic pressure of a column of fluid is thus determined by:
Hp = (Dv - Fl) x MD x g
where: Hp = hydrostatic pressure.
Dv = vertical depth.
Fl = flowline depth.
MD = fluid density.
g = gravitational constant.
Note that this is dependent upon vertical depth and fluid density. In oilfield units the fluid density will be the “mud density”, with a conversion factor 0.0519. The conversion factor is derived from:
There are 7.48 gallons in 1 cu/ft and 144 sq inches in 1 sq/ft
because: lb/gal x 7.48 gal/ft3 x 1/144 ft2/in2 = psi/ft
and: 7.48/144 = psi/ft/lb/gal
therefore: 0.0519 = psi/ft/lb/gal
A drilling fluid of 8.34 lb/gal exerts a pressure of;
8.34 x 0.0519 = 0.4328 psi/ft
In SI units the conversion factor is 0.0098, therefore:
Hp (kPa) = MD (kg/m3) x Dv(m) x 0.0098
