3.2 Marsh Funnel Viscosity
The marsh funnel viscosity however is the time measured in seconds it
takes for one quart of the drilling mud sample to flow out of the funnel
into the cup.
\(Viscosity,\mu=\frac{Force,\ F}{Area,\ A\ \times Time,\ s}\) (8)
The gel strength signifies the amount of pump pressure that would be
required to circulate the mud after a shutdown period.
For Newtonian fluids such as water,
\begin{equation}
Shear-stress,\ \tau\ \nonumber \\
\end{equation}\(=\ Absolute\ Viscosity,\ \mu\ \times\ Shear-rate,\ \gamma\) (9)
While for Non Newtonian fluids there are two model equations namely:
The Bingham plastic model:
\begin{equation}
Shear-stress,\ \tau\ \nonumber \\
\end{equation}\(=\ Yield\ stress+(Plastic\ Viscosity,\ \mu\ \times\ Shear-rate\ \gamma)\)(10)
The Power law model:
\(Shear-stress,\ \tau\ =k^{\prime}\times\left(\ Shear-rate\ \gamma\right)\)n
(11)
The temperature of the sample was measured using a digital thermometer.
Room temperature was 24OC, the original temperature of
the mud sample was 22.4OC. When the target temperature
of 30OC was achieved, the sample was taken off the
hotplate and transferred to the viscometer. The sample was mixed on stir
for 10 seconds, the temperature was noted and then the knob was rotated
to 600rpm. When the dial on the scale settled, a reading was taking and
the temperature noted again. This process was repeated for all speeds
until 6rpm. A 10 second and 10 minute gel test was then carried out to
show the amount of gelation that would occur in the event of ceased
circulation and static mud. The mud was mixed for 10 seconds in stir
mode after which the knob was rotated to gel mode and the power shut of
simultaneously. After the sleeve stopped rotating, counted 10 seconds
but positioned in a position to read the dial at about the 7th second so
the once the power is turned on, maximum dial deflection before the gel
broke can be noted. This exact same process was repeated for the
10-minute gel test.
Table 4: OFITE Viscometer viscosity reading for mud sample one