Viscosity describes a substance’s resistance to flow. High-viscosity drilling mud is typically described as “thick,” while low-viscosity mud is characterized as “thin”. In the oilfield, the following terms are used to describe drilling fluid viscosity and rheological properties. Two viscosities that will be described in this section are Funnel Viscosity and Plastic Viscosity.
The funnel viscosity is timed in seconds of drilling mud flowing through the Marsh Funnel Viscosity. The Marsh funnel is easy-to-use equipment that is used to quickly check viscosity of the mud. The Marsh funnel is dimensioned so that the outflow of time of one quart of freshwater (946 cc) at a temperature of 70 F ± 5 F (21 C ± 3 C) in 26 ± 0.5 seconds.
Figure 1 – Marsh Funnel
For all drilling mud, especially oil-based mud, temperate has an effect on the viscosity of a base fluid. The base fluid will be less thick once the temperature increases. It means that the funnel viscosity will decrease. The funnel viscosity measures at only one rate of shear, but the temperature each time of measurement is not constant. This is the reason why the viscosity measured from the Marsh Funnel does not represent the true drilling mud viscosity. On the drilling rig, this measurement of the mud viscosity is still useful because it is a quick and simple test for observing trends of drilling mud. In order to use the funnel viscosity effectively, personnel must record the values frequently. Then looking at a trend of funnel viscosity, it will indicate if there is any issue with drilling mud. Please remember that only a single point of the funnel viscosity cannot tell you anything about a condition of drilling mud.
Plastic Viscosity (PV)
Plastic Viscosity (PV) is a resistance of fluid to flow. According to the Bingham plastic model, the PV is the slope of shear stress and shear rate. Typically, the viscometer is utilized to measure shear rates at 600, 300, 200, 100, 6, and 3 revolutions per minute (rpm).
Figure 2 – Viscometer
Inthefield,the plastic viscosity can be calculated by a simple calculation shown below.
Plastic Viscosity (PV) = Reading at 600 rpm – Reading at 300 rpm
The unit of PV is Centi Poise (CP).
For example, determine PV from these reading values from a viscometer.
Reading at 600 rpm = 56
Reading at 300 rpm = 35
Plastic Viscosity (PV) = 56 – 35 = 21 CP
Any increase in solid content in drilling mud such as barite, drill solid, lost circulation material, etc., will result in higher plastic viscosity. In order to lower the plastic viscosity, solid content must be removed and it can be achieved by using solid control equipment and/or diluting drilling mud with base fluid. Fluid temperature will increase while drilling deeper therefore plastic viscosity of drilling mud will decrease because the viscosity of the base fluid decreases.
Normally, the higher the mud weight, the higher plastic viscosity will be. However, if there is an increasing trend of plastic viscosity with constant mud weight, it means that there is an increase in drill solid content in a mud system. Moreover, if oil based mud is used, please keep in mind that water in oil based drilling fluid will act like a solid, and it will increase the plastic viscosity dramatically. It is very critical to ensure that amount of water in oil based mud is within the designed limit.
Several impacts of plastic viscosity on drilling operation are as follows;
Equivalent Circulating Density (ECD)
The more PV you have, the higher the ECD will be.
Surge and Swab Pressure
The PV has the same effect as ECD. If the PV increases, surge and swab pressure will also increase.
A chance for differential sticking will increase, especially in water based mud, when the plastic viscosity increases because of increases in solid content.
Rate of Penetration (ROP)
The ROP will be directly affected by the plastic viscosity. Thicker mud will have bigger hold down effect than thinner mud. Therefore, it causes in reduction in ROP.