365 61 91 122 152 183 213 244

ANNULAR VELOCITY (ft./min.)

Figure 5-50. Cutting removal at low annular velocities (medium cutting, no rotation, 12—31/2 inch annulus, 12 lb/gal mud). Artificial cuttings Ve inch x 1/4 inch x 1/a inch (0.317 x 0.635 x 0.317 cm) Transport ratio = vc/va. (From Sifferman, etal.64 Copyright 1974 by SPE-AIME.)

ANNULAR VELOCITY (ft./min.)

Figure 5-50. Cutting removal at low annular velocities (medium cutting, no rotation, 12—31/2 inch annulus, 12 lb/gal mud). Artificial cuttings Ve inch x 1/4 inch x 1/a inch (0.317 x 0.635 x 0.317 cm) Transport ratio = vc/va. (From Sifferman, etal.64 Copyright 1974 by SPE-AIME.)

forces (see Figure 5 -52). In consequence, they turn on edge and migrate to the sides of the annulus, where they descend some distance before migrating back towards the center. The downward descent is caused partly by the low velocity prevailing at the walls, and partly by the edgeways orientation of the cutting.

In general, rotation of the drill pipe improves the transport ratio because it imparts a helical motion to the cuttings in the vicinity of the drill pipe (see Figure 5 -53), but the effect was shown by Sifferman et al64 to be rather small (see Figure 5- 54). Theoretically, turbulent flow should improve the transport ratio because the flatter profile eliminates the turning moment (Figure 5 55), but experimental evidence on this point is not consistent,64,05 06,07 possibly because of differences in experimental conditions, such as the size and shape of the cuttings. Recent tests in a model wellbore by Thomas et al67a showed that increasing rotary speed increases cutting transport at low-rising velocities, but the effect becomes negligible at high velocities. Eccentric rotation had little effect on cutting transport.

Optimum Annular Velocity

Although any velocity greater than the settling velocity of the largest cutting will theoretically lift all the cuttings to surface eventually, too low an annular

velocity will lead to an undesirably high concentration of cuttings in the annuius. Because of slip, the concentration of cuttings depends on the transport ratio as well as the volumetric flow rate and the rate of cuttings generation by the bit. Experience has shown that cutting concentrations more than about 5% by volume cause tight hole, or stuck pipe, when circulation is slopped for any reason.28,63 Figure 5-56 shows the theoretical upward cutting velocity necessary to maintain a cutting concentration of less than 5% for several hole geometries. Figure 5-57 shows the minimum annular velocities required to keep the cuttings concentration less then 4%, as calculated by Zamora,68 for the various muds shown in Figure 5-57. Zeidler68" developed semi-empirical equations for predicting the rising particle velocity necessary to maintain equilibrium between cutting generation and cutting transport. but his work was based on the behavior of drill cuttings in a clear resin

fluid. However, subsequent work by others648,673 has shown that Zeidler's equations are valid for drilling fluids under most conditions.

Since penetration rate decreases with increase in viscosity, it is preferable to maintain adequate hole cleaning by raising annular velocities rather than mud viscosities. However, annular velocities are limited by the following considerations:

1. The rate of increase of the transport ratio falls ofT increasingly at high annular velocities (as shown in Figure 5-50).

2. High circulation rates involve disproportionately high pumping costs because the pressure loss in the drill pipe increases with the square of the velocity in turbulent flow.

3. High annular velocities may cause hole erosion. However, high flow rates per se will not cause hole erosion in competent formations, even if

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