Shear Rate. Sec an eauilibrium viscosity of 11.0 cc

Instantaneous curves after pre-shearina to

Instantaneous curves after pre-shearina to an eauilibrium viscosity of 11.4 cc

after pre-shearina to an eauilibrium viscosity of 13.6 cp

Instantaneous curves n—


Dre-shearina to

Instantaneous curves an eauilibrium viscosity of 22.4 cd


Dre-shearina to an eauilibrium viscosity of 47.2 cc n—

Instantaneous curves

Instantaneous rurves after

Dre-shearina to an eauilibrium viscosity of 90.4 cd

Eauilibrium curve

Figure 5-12. Equilibrium and instantaneous curves for a 4.8% bentonite suspension. (From Cheng, et a!,12 Courtesy of the Institute of Chemical Engineers.)

Figure 5-14. Increase in gel strength of various mud types with time (Schematic).

which shows that the plot of tjS versus t should be a straight line, the slope of which gives k and the intercept at zero time gives 1 ¡S'k. Figure 515 shows plots of gel strength versus time for several bentonite suspensions, and Figure 5 16 shows tjS versus time for the same suspensions. Table 5-1 shows the ultimate gel strength and the gel rate calculated according to Equation 5 30.

There is little evidence in the literature to show whether or not Equation 5 30 applies to muds other than the bentonites tested by Garrison. Weintriti and Hughes1' measured the gel strengths of some field muds containing calcium sulfate and ferrochrome lignosulfonate in a rotary viscometer lor rest periods up to one day. Application of their data to Equation 5-30 shows apparent compliance—although there was considerable scatter of the points—for rest periods up to two hours, but major deviations thereafter.

The work of these investigators showed the inadequacy of the current method of evaluating gel strength after 10-second and 10-minute rest periods. For example, Figure 5 15 shows that the gel strength may increase very rapidly immediately after the cessation of shearing, so that the initial gel strength (as commonly determined) is very sensitive to time, and meaningful values are therefore hard to obtain. Figure 5-15 also shows that the 10-minute gel strength is not a reliable indication of the ultimate gel strength. For instance, Curves 1 and 4 show approximately the same 10-minute gel strengths, but Table 5 1 shows that the ultimate gel strength of the Curve 1 mud was 34 and that of the Curve 4 mud was 104.

One obvious source of scatter in gel strength determinations is variation in the rate of application of load. The importance of this factor was demonstrated by Lord and Menzies,16 who measured the gel strengths of a 10% bentonite suspension in a modified Fann rotary viscometer, at rates that varied from 0.5 to 100 rpm, and recorded the change in stress with time. Figures 5-17 shows the type of curves obtained. The maximum recorded stress was taken to be the gel strength, and the slope of the first part of the curve to be the rate of application of the load. Figure 5-18 shows that the observed gel strengths (designated Y) increased sharply with increase in stress, load rate (designated i). Similarly, in some tests using a pipe viscometer. Lord and Menzies observed that the breakdown pressure of a gelled mud increased with rate of application of pump pressure.

Table 5-1 Gel Rate Constants Calculated from Figure 5-15*


Suspension composition k pH

0 0

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