136

mud rate of flow: 500 g.p.m. annular velocity for gauge hole: 218 ft/min

8 3/4" GAUGE HOLE

Figure 5-45. Typical hole enlargement in shale section. (From Hopkin.sz Copyright 1967 by SPE-AIME.)

ror.5ia Finally, there is no practical way to account for the influence of thixo-tropy on the viscosity of the mud as it rises in the annulus. The high shear rates in the drill pipe and bit nozzles reduce the structural component of the viscosity to a very low value. The shear rates in the annulus are far lower, but change in each section, depending on the drill collar, the drill pipe, and the casing diameters, and on the degree of hole enlargement. The viscosity adjusts to each shear rate, but takes time to do so, and may never reach the equilibrium value except in long sections of gauge or of cased hole.

To summarize, accurate pressure losses in the drill pipe and bit can be reliably predicted, but pressure losses in the annulus are much more questionable. However, quite accurate pressure losses for the whole circuit can be predicted because the annular loss forms such a small percentage of the total. The results of the field tests of Fontenot and Clark-" support these conclusions. Figure 5-46 compares the predicted pressure losses (curves) for a water base mud in a well in Utah with the values measured by subsurface pressure gauges (points). Note that the agreement between the predicted and measured standpipe pressures is quite good, regardless of whether the calculations were based on constant mud properties (i.e.. determined at 11 5 F

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