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filter loss, and therefore only filter loss needs to be specified. Actually, although cake thickness is related to filter loss, the specific relationship varies from mud to mud, because the value of Q„IQ( in Equation 6-6 depends on the concentration of solids in the mud and on the amount of water retained in the cake. The filter loss decreases with increase in the concentration of solids, but the cake volume increases, as shown in Figure 6-4: If an operator adds extra clay to a mud to reduce filter loss, he may believe that he is also reducing cake thickness, but he is actually increasing it.

The amount of water retained in the cakes of muds with different clay bases depends on the swelling properties of the clay minerals involved. Bentonite, for example, has strong swelling properties, and bentonitic cakes therefore

PERMEABILITY OF CAKE, md Cake Volume cm3

PERMEABILITY OF CAKE, md Cake Volume cm3

Figure 6-4. Variation of filtrate volume, cake volume, and permeability with concentration of solids in a suspension of Altwarmbüchen clay. (Data from von Engelhardt and Schinedwolf.)3

Table 6-2

Correlation Between the Amount of Water Adsorbed by Dry Clay and Water Retained in the Filter Cake

Table 6-2

Correlation Between the Amount of Water Adsorbed by Dry Clay and Water Retained in the Filter Cake

Type of clay

Swelling test % solids in swollen clay

0 0

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