specifications by the use of materials available in nearly all oil fields. Improved 

 drilling fluids, together with great advances in the mechanics of drilling, have 

 enabled penetration of the earth's crust to a depth of 4 miles. The necessity of 

 drilling at increasingly greater depths for oil and gas has been accompanied 

 by progressively greater drilling costs. These mounting costs render it more 

 necessary than ever that commercial oil or gas zones are not overlooked and 

 that reliable geological information be obtained even in dry holes to avoid 

 unnecessary additional drilling. The reliability of information furnished by 

 cuttings, cores, and electric logs is dependent upon the properties of the drilling 

 fluid and drilling conditions. 



The various functions and requirements demanded of drilling fluids assume 

 a different order of importance under widely varying drilling conditions; thus, 

 it is difficult to generalize as to the most important functions or characteristics 

 of these fluids. Compromises must always be made to effect some balance 

 between contradictory requirements. It will be to the geologist's advantage to 

 recognize these limitations and to cooperate actively with the drilling engineers 

 in devising a drilling-fluid program that best satisfies all demands. 



PROPERTIES AFFECTING The density of a drilling fluid is important in 



PERFORMANCE OF confining formational fluids to their respec- 



DRILLING FLUIDS tive zones. The difference between the pres- 



sure exerted by the column of drilling fluid 

 Density and the pressure of the confined fluid in the 



formation determines the safety factor in 

 controlling the ingress of fluids into the hole. This property of the fluid is of 

 extreme importance in preventing blowouts in most deep-drilling operations. 

 Density is also important in preventing unconsolidated formations from caving 

 into the hole. In some instances, density must be controlled carefully to prevent 

 the drilling fluid from becoming heavy enough to induce fractures and to escape 

 into the formation. The costly drilling problems of caving and loss of drilling 

 fluid also interfere with the continuous recovery of cuttings needed by the 

 geologist for making logs of the formations being penetrated. 



The density of a drilling fluid has a pronounced effect upon the ability of 

 the fluid to recover cuttings from a well. If all drilling-fluid properties and 

 drilling conditions are held constant, the rate of slip of cuttings in the drilling 

 fluid is a direct function of the density of the cuttings minus the density of the 

 drilling fluid. As the density of a drilling fluid approaches 20 pounds per gallon, 

 cutting slip becomes negligble and cutting recovery becomes independent of the 

 yield value and plastic viscosity properties of the drilling fluid. It is usually 

 impractical to increase the density of drilling fluids to improve cutting recovery ; 



716 



