offer some of the advantages of air or gas in increased drilling rates and, in 

 addition, have the advantage of being applicable in drilling water zones and in 

 being easily and rapidly convertible to heavier muds when necessary. Corrosion 

 is often a severe problem in the use of aerated muds. 



DRILLING CONDITIONS A particularly important problem relating to 



AFFECTING drilling and drilling fluids is the loss of hole 



DRILLING FLUIDS fluid into formational voids. This loss of 



drilling fluid is termed lost circulation or lost 

 Lost" Circulation returns and differs from filter loss in that the 



complete drilling fluid enters the formation. 

 Lost circulation occurs in two general types of formations: (1) formations 

 naturally capable of taking drilling fluid because of intrinsic fractures, channels, 

 vugs, or intergranular type porosity, and (2) incompetent formations in which 

 fractures are induced by hydrostatic pressure of the drilling-fluid column. De- 

 spite precautionary measures to avoid imposing unnecessarily high pressures on 

 the penetrated formations, lost circulation still occurs in many instances, and it 

 is necessary then to add materials of relatively large particle size to seal the voids 

 in the wall of the hole. Among the materials most commonly used to stop loss 

 of returns are fibrous materials (leather fibers, sugar-cane fibers, asbestos 

 fibers), granular materials (ground walnut shells, expanded perlite), and flaked 

 materials (cellophane flakes, mica flakes). 



Drilling-String Mechanics 



Poor drilling-string mechanics can result in increased drilling fluid main- 

 tenance costs and in decreased and unreliable cuttings recovery. When high 

 weight is maintained on the bits by imposing weight beyond the buckling strength 

 of the drilling string, excessive grinding action results between the drilling 

 string in compression and the walls of the hole. This action not only tends to 

 grind up cuttings and cavings to form suspended solids in the drilling mud but 

 also enlarges the bore hole, thus reducing the efficiency of the drilling mud to 

 lift cuttings and cavings to the surface. This condition can be remedied by using 

 a sufficient number of adequately sized drill collars and keeping the rotary 

 speed below the critical point above which instability results in the drilling 

 string. 



Annular Velocity of Drilling Fluid 



Maintaining the velocity of the drilling fluid in the annulus as high as 

 possible will improve materially the removal of cuttings and cavings from the 

 bore hole. All cuttings and cavings will be subjected to a certain amount of 



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