however, when the density must be carried at high values to control formation 

 fluids or caving, maximum use of this property should be utilized to attain satis- 

 factory recovery of cuttings. 



The amount and specific gravity of the suspended solids usually determine 

 the density of drilling fluid. Many wells are drilled with densities not over 9 

 pounds per gallon, whereas in other instances, densities as high as 20 pounds 

 per gallon are needed. Ground barite is commonly used to increase the density 

 of drilling fluids. 



Viscosity 



Viscosity is important in many of the drilling-fluid functions and character- 

 istics. Viscosity, as applied to drilling fluid, may be regarded basically as 

 meaning the resistance that the drilling fluid offers to flow on being pumped. 

 The removal of cuttings and sloughing shale fragments requires a fluid of 

 certain minimum viscosity for a given upward velocity of mud circulation. 

 Viscosity affects the slip of cuttings; the greater the viscosity the lower will be 

 the slip. Water, rather than more viscous drilling fluids, can be used for carry- 

 ing cuttings to the surface; however if water is used, it must be circulated at a 

 greater velocity than more viscous media. If the rate of circulation is too low 

 for a drilling fluid of a given viscosity, the cuttings are commonly reground 

 to so small a size that they are useless for geological examination. 



Most drilling fluids are plastic fluids rather than true fluids. The most 

 important difference between a true fluid and a plastic fluid in predicting flow 

 characteristics is that a plastic fluid does not have a constant viscosity in the 

 region of streamline flow; i.e., the apparent viscosity decreases steadily as the 

 rate of shear increases. In the quiescent state, a plastic fluid is an elastic solid 

 that will resist permanent deformation by any force less than its initial shearing 

 stress or yield value. In the turbulent-flow region, both plastic fluids and true 

 fluids react the same way. Drilling fluid is usually in turbulent flow in the drill 

 string and in at least part of the annulus immediately above the bit. 



The viscosity of a drilling fluid depends upon the amount and character 

 of the suspended solids; the greater the percentage of suspended solids, the 

 greater the viscosity. Plastic clays, particularly bentonite, develop much higher 

 viscosities than do noncolloidal substances such as sand and most clays and 

 shales. 



Viscosity of drilling fluid ordinarily is determined in the field by means 

 of a Marsh funnel, which is a cone-shaped funnel that is filled with the fluid 

 to be tested. The time necessary to discharge a measured volume of the fluid 

 is an indication of its relative viscosity. Rotational viscometers, the Stormer 

 and others, are used both in the laboratory and in the field. In these viscosi- 

 meters a spindle is rotated in a test cup, and the force necessary to drive the 



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