and lay on this side for some distance up the hole as long as the drilling string 

 is in compression. The force with which the drilling string bears against the 

 wall of the hole is directly proportional to the angle the hole is inclined to the 

 vertical as well as to the effective weight per foot of the drilling string in com- 

 pression in the drilling fluid. The greater the bearing force of the drilling string 

 against the walls of the hole, the more the cuttings are pulverized to complicate 

 their proper identification. Drilling a hole as vertical as possible is important in 

 avoiding drilling problems and assures the recovery of large-size cuttings for 

 logging purposes. 



SURFACE EQUIPMENT Screening equipment is used to assure posi- 



FOR HANDLING tive removal of cuttings from drilling fluids. 



DRILLING FLUIDS This removal reduces the increase in viscosity 



and gel strength of the drilling fluid resulting 

 Screens from dispersing cuttings into the mud when 



they are recirculated. Proper drilling-fluid 

 screening equipment also is essential to secure for geological examination 

 cuttings that are free from recycled material. Screens of the vibrating type 

 commonly called shale shakers are most satisfactory because a finer mesh wire 

 cloth can be used on them and thickey drilling fluids can be screened. Screens 

 of the rotary type necessitate larger mesh wire cloth and function properly only 

 on less viscous drilling fluids. 



Desanders, Centrifuges, and Hydraulic Cyclones (liquid type) 



Sand is undesirable in drilling fluids because of its abrasive properties. 

 Excessive sand and other fine material also interferes with cutting analysis. 

 Desanders, centrifuges and hydraulic cyclones can be used when screens prove 

 inadequate for removing sufficient sand and other finely divided material from 

 drilling fluids. Cyclones and centrifuges are being used to an increasing extent 

 to recover barite from drilling fluids and to control excessive viscosity and gela- 

 tion properties by removal of some of the clay fraction from clay-base drilling 

 fluids. Significant savings in drilling costs are achieved in many areas through 

 use of centrifuges and hydraulic cyclones. 



Degassers 



Drilling fluids of high density often become gas-cut; that is, all the gas 

 absorbed in drilling is not released at the surface, and as drilling progresses 

 the fluid may become so lightened that it is dangerous. Recycled gas in the 

 drilling fluid may also affect or complicate the detection of shows from pay 

 formations as determined by mud logging. Various types of degassing equip- 



730 



