16

*From Havenaar.27 Copyright by SPE-AIME 1956.

filtration rates beneath the bit, unlike filtration rates through the borehole walls, are influenced by the permeability of the formation (see Figure 6- 20). Likewise, microbit drilling tests by Lawhon et al28 showed no correlation with API filter loss.

Evaluation of Downhole Filtration Rates

The lack of correlation between the API filter loss and the dynamic filtration rate shown in Figure 6-17 casts doubts upon the validity of the API lest as a criterion for downhole filtration rates. Experimental work by Krueger29 substantiated these doubts. Krueger added increasing amounts of various common filtration-reducing agents to samples of a standard clay mud, and measured changes in equilibrium dynamic filtration rates as compared to the API filter loss. In the dynamic tests, the mud was circulated past the faces of sandstone cores mounted on a cylindrical cell containing a concentric mandrel. Filtration pressure was 500 psi (36 kg/cm2), the temperature was 170°F (77°C), and fluid velocity was 110 ft/min (33m/min). The results showed that there was a different relationship between dynamic filtration rate and API filter loss for each agent ( see Figure 6-21). Furthermore, the API loss decreased continuously with each addition of starch, CMC, and polyacrylate, but the dynamic rates decreased to a minimum, and then increased. In contrast, the API filter loss decreased very little with additions of lignosulfonate and quebracho, but the dynamic rates were almost as low as those obtained with starch, and much lower than those obtained with CMC and polyacrylate. Much the same rate relationships were obtained when dynamic rates were measured after the deposition of a static cake (Figure 6-22), and when

Figure 6-20. Effect of rock permeability on dynamic filtration rate below the bit. (From Horner, etal.2* Copyright 1957by SPE-AIME.)

the equilibrium thickness of the cake depends only on the erodability of the cake. For example, if the concentration of clay in a suspension is increased, the API filter loss decreases, but the dynamic filtration remains the same.

Outmans4 has suggested that differences in viscosity may also be responsible for the poor correlation, because viscosity influences the shear stress (t in Equation 6-14) exerted by the mud stream on the surface of the cake. However, neither Prokop23 nor Horner et al25 found any significant relationship between viscosity and dynamic filtration rate.

The danger of relying on the API filter loss as a criterion for downhole dynamic filtration rates is obvious. A treating agent that was recommended on the basis of API test results might give higher rates downhole than another agent that gave a higher API filter loss. Worse still, an agent that reduccd the API loss might increase the downhole filtration rate.

Wyant et al29b compared dynamic and static high temperature, and high pres sure filtration rates of a 13 lb/gal invert oil emulsion mud containing increasing amounts of various filtration control agents. The dynamic rates were tested in the ceil shown in Figure 6-24. Two such cells replaced the dynamic cells in the

Mud Out

Pressure Valve

Control

Pressure Valve

Control

Mud Out rj

Filtrate Measurement

Figure 6-24. Dynamic filtration cell and filtrate collection system.

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