Yield Point lbs/100 sqft
This parameter is also obtained from the viscometer. The yield point (YP), as mentioned earlier, is a measure of the electro-chemical attractive forces within the mud under flowing conditions. These forces are the result of positive and negative charges located near or on the particle’s surfaces.
With this in mind, the yield point is then a function of the surface properties of the mud solids, the volume concentration of the solids, and the concentration and type of ions within the fluid phase.
The yield point is the shear stress at zero shear rate, and is measured in the field by either;
YP = 300 rpm reading - PV
or YP = (2 x 300 rpm reading) - 600 rpm reading
This gives a Bingham yield point, which is generally higher than the actual or true yield.
As stated earlier, at low shear rates, the Bingham model does not give particularly good readings.
High viscosity, resulting from a high yield point is caused by:
1. Introduction of soluble contaminants such as salt, cement, anhydrite, or gypsum, which neutralize negative charges of the clay particles, resulting in flocculation.
2. The breaking of clay particles by the grinding action of the bit and pipe, which creates “broken bond valences” on the edges of the particles, causing the particles to pull together.
3. Introduction of inert solids causes the particles to be closer together into disorganized form or flocks.
4. Drilling of hydratable clays introduces active solids into the system, increasing the attractive forces by increasing the number of charges and by bringing the particles closer together.
5. Both insufficient or over-treatment of the mud with chemicals will increase the attractive forces of the mud.
Treatment for increased yield point may be controlled by chemical action, but reduction of the yield point will also decrease the apparent viscosity.
Yield point may be lowered by the following:
1. Broken bond valences may be neutralized by adsorption of certain negative ions at the edge of the clay particles. These residual valences are almost totally satisfied by chemicals such as tannins, lignins, lignosulfonates and complex phosphates. The attractive forces are satisfied by chemicals, and the clay's natural negative charge remains, so that the particles repel each other.
2. If calcium or magnesium contamination occurs, the ion is removed as an insoluble precipitate, thus decreasing the attractive forces and hence the yield point.
3. Water can be used if the solid content is very high, but it is generally ineffective and may alter other properties drastically (i.e., mud density).
As mentioned earlier, the chemicals that are added to deflocculate the mud and act as “thinners” are commonly lignosulfonates and tannins. These also have a secondary function of acting as filtration agents.
This parameter is also obtained from the viscometer. The yield point (YP), as mentioned earlier, is a measure of the electro-chemical attractive forces within the mud under flowing conditions. These forces are the result of positive and negative charges located near or on the particle’s surfaces.
With this in mind, the yield point is then a function of the surface properties of the mud solids, the volume concentration of the solids, and the concentration and type of ions within the fluid phase.
The yield point is the shear stress at zero shear rate, and is measured in the field by either;
YP = 300 rpm reading - PV
or YP = (2 x 300 rpm reading) - 600 rpm reading
This gives a Bingham yield point, which is generally higher than the actual or true yield.
As stated earlier, at low shear rates, the Bingham model does not give particularly good readings.
High viscosity, resulting from a high yield point is caused by:
1. Introduction of soluble contaminants such as salt, cement, anhydrite, or gypsum, which neutralize negative charges of the clay particles, resulting in flocculation.
2. The breaking of clay particles by the grinding action of the bit and pipe, which creates “broken bond valences” on the edges of the particles, causing the particles to pull together.
3. Introduction of inert solids causes the particles to be closer together into disorganized form or flocks.
4. Drilling of hydratable clays introduces active solids into the system, increasing the attractive forces by increasing the number of charges and by bringing the particles closer together.
5. Both insufficient or over-treatment of the mud with chemicals will increase the attractive forces of the mud.
Treatment for increased yield point may be controlled by chemical action, but reduction of the yield point will also decrease the apparent viscosity.
Yield point may be lowered by the following:
1. Broken bond valences may be neutralized by adsorption of certain negative ions at the edge of the clay particles. These residual valences are almost totally satisfied by chemicals such as tannins, lignins, lignosulfonates and complex phosphates. The attractive forces are satisfied by chemicals, and the clay's natural negative charge remains, so that the particles repel each other.
2. If calcium or magnesium contamination occurs, the ion is removed as an insoluble precipitate, thus decreasing the attractive forces and hence the yield point.
3. Water can be used if the solid content is very high, but it is generally ineffective and may alter other properties drastically (i.e., mud density).
As mentioned earlier, the chemicals that are added to deflocculate the mud and act as “thinners” are commonly lignosulfonates and tannins. These also have a secondary function of acting as filtration agents.
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