STANDARD HANDBOOK OF PETROLEUM & NATURAL GAS ...

652 Drilling and Well Completions
Testing of Drilling Fluids
Proper control of the properties of drilling mud is very important for their
preparation and maintenance. Although oil-base muds are substantially different
from water-base muds, several basic tests (such as specific weight, API funnel
viscosity, API filtration, and retort analysis) are run in the same way. The test
interpretations, however, are somewhat different. In addition, oil-base muds have
several unique properties, such as temperature sensitivity, emulsion stability,
aniline point, and oil coating-water wettability that require other tests. Therefore,
testing of water and oil-base muds will be considered separately.
Water-Base Muds
Specific Weight of Mud. Often shortened to mud weight, this may be expressed
as pounds per gallon (lb/gal), pounds per cubic foot (lb/ft3), specific gravity
(S,,,), or pressure gradient (psi/ft) (see Table 4-44). Any instrument of sufficient
accuracy within fO.l lb/gal or k0.5 lb/ft3 may be used. The mud balance is the
instrument most commonly used [23]. The weight of a mud cup attached to one
end of the beam is balanced on the other end by a fixed counterweight and a
rider free to move along a graduated scale.
Viscosity. Mud viscosity is a measure of the mud’s resistance to flow. The
primary function of proper viscosity is to enable the mud to transport cuttings
to the surface. Viscosity must be so high enough that the weighting material
will remain suspended, but low enough to permit sand and cuttings to settle
out and entrained gas to escape at the surface. Also, excessive viscosity creates
high pump pressure and magnifies the swabbing or surging effect during
tripping operations.
Gel Strength. This is a measure of the interparticle forces and indicates the
gelling that will occur when circulation is stopped. This property prevents the
cuttings from settling in the hole and sticking to the drill stem. High pump
pressure is required to “break” circulation in a high gel mud. The following instruments
are used to measure the viscosity and/or gel strength of drilling muds:
Marsh Funnel. The funnel is dimensioned so that, by following standard
procedures, the outflow time of 1 qt (946 ml) of freshwater at a temperature
of 70f5”F is 26f0.5 seconds [23]. A graduated cup or 1-qt bottle is used as
a receiver.
Direct /ndicating Viscometer. This is a rotational type instrument powered by
an electric motor or by a hand crank. Mud is contained in the annular space
between two cylinders. The outer cylinder or rotor sleeve is driven at a constant
rotational velocity; its rotation in the mud produces a torque on the inner
cylinder or bob. A torsion spring restrains the movement. A dial attached to
the bob indicates its displacement. Instrument constants have been so adjusted
that plastic viscosity, apparent viscosity, and yield point are obtained by using
readings from rotor sleeve speeds of 300 and 600 rpm.
Plastic viscosity (PV) is centipoises equals the 600 rpm reading minus the
300 rpm reading. Yield point (YP) in pounds per 100 ft2 equals the 300-rpm
reading minus plastic viscosity. Apparent viscosity in centipoises equals the 600-rpm
reading, divided by two. The interpretations of PV and YP measurements are
presented in Figure 4-107.