Marsh Funnel viscosity, in the context of drilling fluids, refers to the time it takes for a specific volume of the fluid to flow through a standardized Marsh funnel. It’s not a true measure of viscosity in the scientific sense, but rather a quick and simple way to get a qualitative indication of the fluid’s consistency.
This image below is a mash funnel used to measure viscosity of oil based mud drilling fluid on the rig.
A mash funnel
Here’s a breakdown of funnel viscosity for drilling fluids:
Measurement:
- A Marsh funnel, a conical-shaped container with a calibrated orifice at the bottom, is used.
- The funnel is filled with a specific volume of drilling fluid (usually one quart or 946 ml).
- The time it takes for the fluid to completely flow out through the orifice is measured in seconds.
Interpretation:
- Lower funnel viscosity times indicate thinner fluids that flow more easily, while higher times indicate thicker fluids with greater resistance to flow.
- Typical funnel viscosity times for drilling fluids range from 25 to 70 seconds. Values outside this range might require further investigation into the fluid’s properties or potential issues.
- It is used to see the trend over time. If there is any abnormal change, it may indicate something influx into the well. This trend is an early indicator for personnel on the rig to early identify something wrong before the problem gets worse.
Limitations:
- Funnel viscosity doesn’t provide true viscosity because it only measures the flow at a single shear rate, while drilling fluids exhibit shear-thinning behavior, meaning their viscosity changes depending on the applied shear force.
- Temperature also affects funnel viscosity, with higher temperatures leading to lower times. Therefore, measurements should be done at consistent temperatures for accurate comparisons.
Overall, funnel viscosity is a valuable tool for:
- Monitoring drilling fluid consistency in the field.
- Identifying potential changes in the fluid’s properties that might affect drilling performance.
- Making quick comparisons between different drilling fluids.
References
Andy Philips, 2012. So You Want to be a Mud Engineer: An Introduction to Drilling Fluids Technology. Edition. CreateSpace Independent Publishing Platform.
Bourgoyne, A.T. (1986) Applied drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Mitchell, R.F., Miska, S.Z. and Aadnoy, B.S. (2012) Fundamentals of drilling engineering. Richardson, TX: Society of Petroleum Engineers.
Ryen Caenn, 2011. Composition and Properties of Drilling and Completion Fluids, Sixth Edition. 6 Edition. Gulf Professional Publishing.
