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ANNULAR VELOCITY (ft./min.)

Figure 5-54. Effect of rotary speed on cutting transport (medium cutting, 8x4 inch annulus, 12 lb/gal mud). Small cutting—1/16 inch x 1/8 inch x 1/3 inch (0.159 x 0.317 x 0.317 cm). Large cuttings—1/4 inch x 1/2 inch x 1/8 inch (0.635 x 1.27 x 0.317 cm). (From Sifferman, etalCopyright 1979 by SPE-AIME.)

the flow is turbulent. The shear stresses exerted by the fluid on the hole walls are in the order of pounds per hundred square feet, whereas the shear strengths of competent rocks and shales is in the order of thousands of pounds per square inch (see Table 9-3). Thus, in some geologic regions, holes can be drilled with water in highly turbulent flow without causing significant hole enlargement. Unfortunately, in most places where oilwells are drilled the formations are either fractured by tectonic movements, or gravely weakened by physico-chemical reactions with the mud. Under such circumstances, hole enlargement will increase with increase in annular velocity.69

It follows from the above considerations that the optimum annular velocity and mud viscosity depend on whether or not a gauge hole can be maintained. A gauge hole can be kept clean either by water or by minimum viscosity muds, but if the hole enlarges significantly, the fluid velocity in the enlarged sections decreases, so there may be a wide variation in velocity between the gauge and enlarged sections (see Figure 5 -45). Increasing the volumetric flow rate to obtain velocities sufficient to clean the enlarged sections may result in excessive velocities in the gauge sections. Under these conditions the

Figure 5-55. Discs transported in turbu lent flow {center pipe stationary)

{From

Williams and Bruce.65 Copyright 1951 by

SPE-AIME.)

rheological properties of the mud must be adjusted to increase the transport ratio.

Optimum Rheotogical Properties for Hole Cleaning

On general principles, a mud with predominantly structural viscosity—as indicated by a high ratio of yield point to plastic viscosity, or a low flow-behavior index, n—is desirable for hole cleaning purposes. Such a mud will be a a shear-thinning mud, so that the effective viscosity will increase in the enlarged sections, where fluid velocities are low, and decrease in gauge hole sections, where fluid velocities are high.

A mud with a high structural viscosity component would be expected to lift cuttings more efficiently than a Newtonian or near Newtonian fluid, but expert-

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