spindle at selected rates of rotation is measured and converted into centipoises 

 by means of suitable calibrations. 



Plastic-Flow Properties 



The fact that most drilling fluids are plastic fluids instead of true fluids has 

 led to much research in an attempt to analyze correctly the complex flow 

 behavior of plastic flow and to develop appropriate rheological measurements. 

 It is now generally recognized that Bingham's law of plastic flow can be utilized 

 in describing the hydrodynamic behavior of drilling fluids in the nonturbulent 

 flow range; namely, the flow behavior is characterized by two constants, plastic 

 viscosity and yield value. Sometimes the term rigidity is used for plastic vis- 

 cosity, or the term mobility for its reciprocal. 



The plastic viscosity of a substance obeying Bingham's equation is defined 

 as the constant ratio of a given change in the shearing stress to the correspond- 

 ing change in the rate of shear when the body is undergoing permanent deforma- 

 tion or flow. This same definition applies also to the viscosity of a true or 

 Newtonian fluid. The yield value of a plastic substance may be defined as the 

 difference between the shearing stress and the product of the plastic viscosity 

 and rate of shear. 



Rotational-type viscometers capable of being operated over a wide range of 

 shear rates are commonly used to measure the two plastic-flow constants. Methods 

 have been developed of estimating from the flow-constant measurements the 

 effects of changes of drilling-fluid properties on various drilling items; for 

 example, circulating rates and pressures, pump-horsepower requirements, and 

 cuttings-carrying capacity. It has been shown that the cuttings-carrying capacity 

 of a plastic drilling fluid of a given density is dependent upon the size and shape 

 of the cuttings, the plastic-flow constants, and the flow state of the drilling 

 fluid, i.e., laminar or turbulent flow. Where the drilling fluid is in laminar flow, 

 an increase in either plastic viscosity or yield value results in increased ability 

 to lift cuttings, whereas in turbulent flow an increase in the plastic viscosity, will 

 increase the ability of the fluid to lift cuttings. 



Selection of optimum flow constants in a particular drilling situation is 

 possibly one of the most important means of improving the recovery of cuttings. 

 Despite considerable study of this subject to date, it has not been possible to 

 develop precise generalized recommendations to cover all drilling conditions — 

 for instance, channeling of mud past enlarged spots in the hole and the fall of 

 cuttings through a thixotropic mud in which flow has been suspended. 



In practice, a drilling fluid having a high yield value, a low plastic viscosity, 

 and some initial gel strength will usually serve to carry cuttings from the bit to 

 the surface most effectively. This drilling fluid will tend to be in laminar flow 

 in the annulus between the drill pipe and wall of the hole and in plug or laminar 

 flow in hole enlargements. 



718 



