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In petroleum engineering is very important everything that can be studied in relation to the design and construction of an oil well, for this we must refine every detail of engineering, when we drill an oil well is essential that the drilling fluid can have all the properties according to the drilling does not stop and thus do not occur unproductive times.
One of the properties of the drilling fluid that is very important to take care of and monitor is the viscosity, the viscosity values must be supervised to realize that they are within the normal values that are reflected in the drilling program.
As the depth of the well becomes greater, is when certain properties of the fluid must adapt to the new values of pressure and temperature downhole, among these properties is the viscosity, and in order to monitor to know if the values of the drilling fluid viscosity are within the values contemplated in the drilling program we must know the actual values of viscosity of the drilling fluid.
To calculate the viscosity of the drilling fluid on site, the mud engineer must take a sample of the drilling fluid from the active mud tanks, which is then taken to the laboratory of the company providing the drilling fluid analysis service.
The Marsh funnel is the instrument used to measure the viscosity of the drilling fluid.
The sample volume to be collected to pass through the Marsh funnel is 1/4 gallon, or 0.94 liters of drilling fluid, so it is even advisable to collect if possible 1 liter of drilling fluid.
When passing 1/4 gallon of drilling fluid through the Marsh funnel, depending on the configuration and additives mixed in the drilling fluid, and also depending on the downhole temperature conditions, the time that can be obtained in passing 1/4 gallon of drilling fluid through the Marsh funnel is between 34 to 50 seconds, however given the geological conditions, downhole pressure and temperature, it must be taken into account that the engineers in the design of the drilling program will program a specific viscosity value depending on the depth at which it is being drilled. .
An example might be: Suppose you are drilling at a depth of 9500 feet, the mud engineer is asked to take a mud sample and take an actual measurement of Marsh viscosity. Then the engineer takes the Marsh viscosity and gives it 40 seconds, and when they look in the program at that depth the Marsh viscosity has to be at 45 seconds, that means that at that depth the viscosity must have higher viscosity, which is why you have to add chemical additives that can increase the viscosity of the drilling mud from 40 seconds to 45 seconds.
Conclusion
The Marsh funnel is not the only way in which the viscosity of the drilling fluid can be calculated, for example we can also calculate the viscosity with other techniques that maximize the rheological behavior of non-Newtonian fluids, since the drilling fluid, being a non-Newtonian fluid, its viscosity can be calculated with a rheometer, also called in the oil industry as fann viscometer.
Can the Marsh funnel viscosity measurement be adjusted?
The answer is yes, there is an equation that adjusts the Marsh funnel measure to a viscosity called effective viscosity, which is adjusted to fluid density values, the equation is as follows:
where μ = effective viscosity in centipoise.
ρ = density in g/cm3.
t = quarter funnel time in seconds.
For example, a mud with a funnel time of 38 seconds and a density of 1.3 g/cm³ has an effective viscosity of approximately 16.9 centipoise.
Although viscosity can be calculated in ways other than the Marsh funnel, the importance of the Marsh funnel is that we can estimate viscosity values quickly and easily.
References
H. N. Marsh (1931)Trans AIME vol 92 pp 234-251 "Properties and Treatment of Rotary Mud"
13B-1 Recommended Standard Procedure for Field Testing Water-Based Drilling Fluids, Dallas: American Petroleum Institute, 1990
A. W. McCray & F. W. Cole (1979) Oil Well Drilling Technology University of Oklahoma Press ISBN 0-8061-0423-6