Thus, there would have been a 46% error in the rate of shear, and a 62% error in the effective viscosity.

The shear rates in the drill pipe usually approximate to those prevailing in the rotary viscometer between 600 and 300 rpm, and the shear rates in the annulus usually approximate to those between 100 and 6 rpm in the viscometer. If the rate of shear in the well thus calculated falls below the shear rate range in the viscometer at which n was determined, then another calculation should be made using the value of n indicated by a lower shear rate range. Similarly, if the calculated rate falls above the viscometer range, then a higher range should be used to determine n. Of course, many muds will exhibit linear or near linear plots, in which case it is possible to use the value of « given by the 600 to 300 rpm range in the viscometer without undue error.

For maximum accuracy the pressure loss in the various sections of the annulus should be calculated separately according to their annular width. The total pressure loss in the well is given by the sum of the pressure losses in the drill pipe, through the bit nozzles, and in the annulus. The pressure loss through the bit nozzles is given by:60

where C is the nozzle constant, which may be taken as 0.95, and A is the total area of the nozzles.

The total annular pressure gradient, static plus hydraulic, is commonly expressed in terms of the equivalent circulating density, ecd:


= 81.6 sec"1, instead of 151 sec

Thus in the hydraulic calculation given above, the predicted pressure gradient was

—j— = 9.7 dynes/cnr/cm and the mud density was 1.32 g cm\

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