It is ironic that of all the different types of tests
that can be performed on drilling fluids, the one test most commonly performed
provides us with the least amount of information. The marsh funnel viscosity
test undoubtedly is the hands-down favorite among drilling contractors from any
drilling industry that utilizes either mineral (bentonite) or polymer-based
drilling fluids or slurry, but funnel viscosity only tells us the thickness of
a drilling fluid. Determining the funnel viscosity of a drilling fluid does not
accurately tell us how well the fluid will keep a hole open, convey cuttings to
the surface, or perform any other function expected of a drilling fluid. It is
important to understand the limitations of the funnel viscosity test, and know
when to delve deeper into drilling fluids testing and rheology in order to
determine whether a drilling fluid is capable of performing the functions
required to complete a drilling project effectively.
Looking up the word “viscosity” in Webster’s New World Dictionary, you will
find that viscosity is “the internal friction of a fluid which makes it resist
flowing past a solid surface or other layers of the fluid.” This definition
should get the attention of anyone using drilling fluids for rotary mud
drilling, because this statement indicates that viscosity can work against us
in our efforts to perform the functions expected of a drilling fluid.
Higher-viscosity (thicker) drilling fluids will require more pressure at the
drill bit or reamer to move the column of drilling fluid out of the hole,
especially when drill cuttings are thrown into the mix, further increasing the
viscosity of the returns. Additional pressure at the drilling head or reamer
increases the chances of compromising the structural integrity of the hole,
which can result in formation damage, loss circulation and/or frac-outs
(especially in horizontal directional drilling applications). Higher-viscosity
drilling fluids are more difficult to keep clean, regardless of whether one is
utilizing mechanical solids-removal equipment or relying on gravity to settle
drill cuttings, resulting in higher drill solids and poor-quality drilling
fluids.
Bentonite drilling fluid is non-Newtonian, which means that the viscosity
changes with shear. Viscosity in Newtonian fluids, such as water and oil,
changes with temperature (the hotter the fluid, the thinner it gets), whereas
non-Newtonian bentonite drilling fluids get thinner with shear and should pump
easily, yet transform into a gel-like substance at rest and suspend drill
cuttings. Bentonite drilling fluid also can be described as thyrotrophic,
according to Principles of Drilling Fluid Control, because of its “ability to
build up a rigid or semi-rigid gel structure if allowed to stand at rest, yet
can be returned to a fluid state by mechanical agitation.” The most effective
and efficient drilling fluid is one that can perform all of the functions of a drilling
fluid (gel strength, suspend drill cuttings, stabilize the borehole, etc.) at
the lowest possible viscosity.
The routine of checking funnel viscosity gives us a relative indication of how
much bentonite or polymer is in a drilling fluid, and is a first step in
matching drilling fluids to soil conditions. When recycling/re-circulating
drilling fluid, it is important to test the funnel viscosity frequently in
order to maintain the drilling fluid properties needed to complete a drilling
project. As drilling fluid is re-circulated along with an increasing amount of
retained of drill cuttings, these drill cuttings continually get ground down
until they are close to the particle size of clay (0 to 2 microns). A drilling
fluid contaminated with ultra-fine drill cuttings, or contaminants such as
lime, can exhibit a high funnel viscosity, yet have poor fluid-loss properties
that hamper the ability to maintain an open hole; low gel strength, which can
lead to hole plugging and/or stuck drill pipe; and high densities that increase
the chances of formation damage and loss circulation. A sand-content test will
indicate the amount of drill cuttings 74 microns and larger in a drilling
fluid, but a mud balance is needed in order to identify high concentrations of
ultra-fine solids in a drilling fluid.
Mud weights above nine pounds per gallon indicate high levels of ultra-fine
solids, and the only remedy is to discard and replace partial volumes of the
solids-laden fluid with new (clean) drilling fluid to lower the solids
concentration/density.
Although testing the funnel viscosity of a drilling fluid yields little
information on the performance or quality of the fluid, it is a vital step in
matching a drilling fluid to soil conditions, as well as maintaining a drilling
fluid throughout the drilling process. It is important to understand that
additional testing is needed to develop a complete picture of what is happening
with a drilling fluid, and determine what is needed to make the necessary corrections
to get the job done.
ND