# Shear Rate RPM

Table 1

Location

Shear Rate, sec

Drill Pipe Drill Collars

700 - 3000

Bit Nozzles

Annulus

Mud Pits

Plastic Viscosity (PV) - Drilling muds are usually composed of a continuous fluid phase in which solids are dispersed. Plastic viscosity is that part of the resistance to flow caused by mechanical friction. The friction is caused by:

• Solids concentration,
• Size and shape of solids,
• Viscosity of the fluid phase.

For practical field applications, plastic viscosity is regarded as a guide to solids control. Plastic viscosity increases if the volume percent of solids increases, or if the volume percent remains constant, and the size of the particle decreases. Decreasing particle size increases surface area, which increases frictional drag. Plastic viscosity can be decreased by decreasing solids concentration or by decreasing surface area. Plastic viscosity is decreased by reducing the solids concentration by dilution or by mechanical separation. As the viscosity of water decreases with temperature, the plastic viscosity decreases proportionally. Therefore, controlling PV of a mud in practical terms involves controlling size, concentration and shape of the solids and minimizing the viscosity of the liquid phase - such as avoiding viscosifying polymers and salts unless absolutely needed.

The value of plastic viscosity is obtained by subtracting the 300 RPM reading from the 600 rpm reading (Figure 9).

PV of a mud is the theoretical minimum viscosity a mud can have because it is the effective viscosity as shear rate approaches infinity. The highest shear rate occurs as the mud passes through the bit nozzles; therefore, PV will approximate the mud's viscosity at the nozzles. This is illustrated in Figure 10, where the effective viscosity of the mud approaches the value of plastic viscosity at high shear rates.

Yield Point (YP) - The yield point is the initial resistance to flow caused by electrochemical forces between the particles. This electrochemical force is due to charges on the surface of the particles dispersed in the fluid phase. Yield point is a measure of these forces under flow conditions and is dependent upon:

• The surface properties of the mud solids,
• The volume concentration of the solids and

Figure 10 Comparison of Effective Viscosity Newtonian vs. Non-Newtonian

Figure 10 Comparison of Effective Viscosity Newtonian vs. Non-Newtonian

Shear Rate, y

• Ionic environment of the liquid surrounding the solids.

### High viscosity resulting from high yield point is caused by:

• Introduction of soluble contaminant (ions) such as: salt, cement, anhydrite or gypsum, which interact with the negative charges on the clay particles,
• Breaking of the clay particles through mechanical grinding action creating new surface area of the particles. These new charged surfaces (positive and negative) pull particles together as a flocs.
• Introduction of inert solids (barite) into the system, increasing the yield point. This is the result of the particles being forced closer together. Because the distance between the particles is now decreased, the attraction between particles is greatly increased.
• Drilling hydratable shales or clays which introduces new, active solids into the system, increasing attractive forces by bringing the particles closer together and by increasing the total number of charges, and
• Insufficient deflocculant treatment.

Yield point can be controlled by proper chemical treatment. As the attractive forces are reduced by chemical treatment, the yield point will decrease. The yield point can be lowered by the following methods:

• Charges on the positive edges of particles can be neutralized by adsorption of large negative ions on the edge of the clay particles. These residual charges are satisfied by chemicals such as: tannins, lignins, complex phosphates, lignosulfonates, etc. The attractive forces

that previously existed are satisfied by the chemicals, and the negative charge of the clay particles predominates, so that the solids now repel each other.

• In the case of contamination from calcium or magnesium, the ions causing the attractive force are removed as insoluble precipitants, thus decreasing the attractive forces and YP of the mud.
• Water dilution can lower the yield point, but unless the solids concentration is very high, it is relatively ineffective.

Yield point (YP) is calculated from VG measurements as follows:

The limitation of the Bingham plastic model is that most drilling fluids, being pseudoplastic, exhibit an actual yield stress which is considerably less than calculated Bingham yield point. This error exists because the Bingham plastic parameters are calculated using a VG meter at 600 RPM (1022 sec-1) and 300 RPM (511 sec-1); whereas, typical annular shear rates are much less (Table 1).

Gel Strength - Gel strengths, 10-second and 10-minute, measured on the VG meter, indicate strength of attractive forces (gelation) in a drilling fluid under static conditions. Excessive gelation is caused by high solids concentration leading to flocculation.

Signs of rheological trouble in a mud system often are reflected by a mud's gel strength development with time. When there is a wide range between the initial and 10-minute gel readings they are called "progressive gels". This is not a desirable situation. If initial and 10-minute gels are both high, with no appreciable difference in the two, these are "high-flat gels", also undesirable. The magnitude of gelation with time is a key factor in the performance of the drilling fluid. Gelation should not be allowed to become much higher than is necessary to perform the function of suspension of cuttings and weight material. For suspension "low-flat gels" are desired - as indicated in Figure 11.

### Excessive gel strengths can cause:

• Swabbing, when pipe is pulled,
• Surging, when pipe is lowered,
• Difficulty in getting logging tools to bottom,
• Retaining of entrapped air or gas in the mud, and
• Retaining of sand and cuttings while drilling.

Gel strengths and yield point are both a measure of the attractive forces in a mud system. A decrease in one usually results in a decrease in the other; therefore, similar chemical treatments are used to modify them both. The 10-second gel reading more closely approximates the true yield stress in most drilling fluid systems. Water dilution can be effective in lowering gel strengths, especially when solids are high in the mud.

Apparent Viscosity (ma) - Apparent viscosity, measured by the VG meter, is the viscosity that a drilling fluid has at 600 RPM (1022 sec-1). It is a reflection of the plastic viscosity and yield point combined. An increase in either or both will cause a rise in apparent viscosity (and probably in funnel viscosity). This is sometimes called single point viscosity. The equation for apparent viscosity is:

CD 20

10 0

Figure 11 Types of Gels

High-flat Gels

(undesirable)

Figure 11 Types of Gels

High-flat Gels

(undesirable)

CD 20

Low-Gels

10 20

(desirable)

30 40 50 60 70 80 Time (minutes)

Low-Gels

10 20

(desirable)

30 40 50 60 70 80 Time (minutes)

As shown in Figure 12, various muds may have the same ma at 1,022 sec-1, but the effective viscosities at other shear rates may vary widely.

Effective Viscosity (me) - The effective viscosity from a VG meter is the viscosity of the drilling fluid at that particular RPM. It is calculated by the equation below.

NOTE: One unit on the dial and one RPM

1.067 lb/100 ft2 5.11 dynes/cm2 1.703 sec-1

Funnel Viscosity - The funnel viscosity is measured with the Marsh funnel and is a timed rate of flow in seconds per quart. It is basically a quick reference check that is made routinely on a mud system; however, there is no shear rate/shear stress relationship in the funnel viscosity test. Thus, it cannot be related to any other viscosity nor can it give a clue as to why the viscosity may be high or low.

Figure 12 Viscosity vs. Shear Rate For Various P/PV Ratios

Figure 12 Viscosity vs. Shear Rate For Various P/PV Ratios

+2 0

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### Responses

• KAROLINA
What is correlation between plastic viscosity and solids control?
8 years ago
• j
What is shape ratio (SAR) of drilling fluids?
7 years ago
• Dennis Wurfel
Why are 25 seconds subtracted from the marsh funnel effective viscosity equation?
6 years ago
• roy
How to calculate shear rate from rpm?
4 years ago
• hanna-mari
What if effective viscosity increases during drilling?
3 years ago
• Arvid
How to convert shear rate to rpm?
2 years ago
• cosimo
How can get shear rate from % torque and RPM?
2 years ago
• hanna
Why should mud static shear stress be measured?
1 year ago
• aida
How to calculate viscosity from shear stress and shear rate?
9 months ago
• sofia
How to get shear stress and shear rate in drilling mud rheology?
7 months ago
• Amanuel
How to compute for shear rate from revolution rpm?
2 months ago