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T. H. Dunn 



The rotary-drilling method requires the use of a circulating fluid forced 

 down the drill pipe and through holes in the bit. After jetting against the bottom 

 of the hole at high velocity, the drilling fluid returns to the surface between the 

 drill pipe and the walls of the hole or between the drill pipe and the walls of 

 the casing. In the surface portion of the mud system, the drill cuttings are 

 removed by screening or gravity settling, and drilling fluid is recirculated. 



Drilling fluid performs a number of functions, the primary ones being to 

 remove cuttings from the hole; maintain sufficient hydrostatic pressure to pre- 

 vent gas, oil, and water from flowing into the well; cool and lubricate the bit; 

 prevent unconsolidated formations from caving into the hole; and suspend sand 

 and cuttings in the hole when circulation is stopped. 



In addition to the positive functions, drilling fluid has some negative re- 

 quirements: (1) it must not damage the producing capabilities of formations; 

 (2) it must not alter or flush cores and cuttings to the extent that estimates 

 cannot be made of the original fluid content; (3) it should not interfere with 

 electric-logging operations nor cause the curves to be altered in a manner that 

 will confuse the interpretation; (4) it must not be corrosive to equipment or 

 injurious and hazardous to persons; and (5) the cost must not be excessive. 



Such great strides have been made in the development of drilling fluids in 

 the last 30 years that now they can be designed to meet practically any desired 



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