714 Subsurface Geologic Methods 



unexpected interruption in circulation, it will gel and prevent settling of 

 drill cuttings to the bottom of the well. The drilling fluid must absorb 

 heat generated in the drill pipe and bit by frictional contact on the walls 

 and bottom of the well. To prevent caving and to lubricate the drill pipe, 

 the fluid must deposit a thin sheath of clay on the walls of the well. The 

 clay, thus deposited, assists along with the density of the drilling fluid in 

 withholding high-pressure gas or water, and also seals unusually permeable 

 low-pressure formations through which the mud might be drained away 

 in sufficient quantity to cause loss of circulation. The drilling fluid must 

 have sufficient density to be capable of providing ample hydrostatic pres- 

 sure to prevent high-pressure gas, oil, or water from entering the well 

 and causing a destructive blow-out. 



To satisfy these requirements, the drilling fluid must have satisfactory 

 viscosity and density and must have well-developed colloidal properties. 

 It must be free of sand and dissolved substances that might cause rapid 

 flocculation, coagulation, or settling of clay particles. The density and 

 viscosity must permit separation of drill cuttings and entrained gas in the 

 facilities provided for such separation; and at the same time, the drilling 

 fluid must not be so viscous that undue pump pressure will be necessary to 

 force it through the system. 



The drilling fluid is usually a clay-laden fluid and is aptly termed 

 "drilling mud." Generally, drilling muds may be classified in two sys- 

 tems: water-base and oil-base. Water-base muds are used much more 

 extensively than the oil-base types as the latter are primarily restricted 

 to special-purpose drilling. 



Properties of Drilling Muds 



While drilling a well, one may encounter many subsurface forma- 

 tions and drilling conditions which affect the mud adversely or require 

 special mud properties for satisfactory handling. Since the basic system 

 of drilling muds is the water-clay type, it is a highly reactive type of 

 fluid, easily subject to contamination and development of objectionable 

 properties. In some cases fluid systems are especially compounded for the 

 particular purpose of eliminating one or more of the objectionable prop- 

 erties of water-clay muds. 



Among the properties of drilling fluids that are important in judging 

 their performance are viscosity, density, coUoidity, gel or shear strength, 

 sand, and salt content. The viscosity of the mud influences its fluidity 

 and its resistance to flow through the circulating system. The density is 

 significant in lifting drill cuttings and offsetting high formation pressures. 

 Colloidal properties of the drilling fluid are important in determining 

 ability to form a mud sheath on the wall of the well, lubricate the drill 

 pipe, prevent loss of fluid to the formation, and restrict admission of 

 formation fluids to the well. The gel or shear strength is also an expres- 

 sion of colloidity and measures the resistance offered to the settling of 



