determination of productive zones depends upon correct interpretation of well 

 cuttings. The use of refined oil is not as serious since fluorescence will differen- 

 tiate between most crudes and refined oil. 



Oil-base emulsion fluids have most of the properties of oil-base drilling fluid 

 since oil is the continuous phase and predominates in the filtrate. One of these 

 emulsion fluids is prepared by addition of a dry concentrate to the proper ratio 

 of oil and water (Lummus, 1954). In field practice a ratio of 40 to 60 percent 

 water and the remainder oil is used normally. A nonfluorescing oil and an 

 emulsifier in this mud minimize difficulties in geological interpretations. Oil- 

 base emulsion fluids are used primarily for drilling in or recompleting wells in 

 formations whose productivity may be harmed by water-base-type drilling fluids. 

 Special interpretations of electric logs usually are required when these drilling 

 fluids are used. 



Oil Base 



Commercial oil-base fluids are various base oils in which materials for 

 raising viscosity, gel strength, and lowering filter loss are dispersed. Dry con- 

 centrations have been developed for conveniently preparing oil-base fluid from 

 a wide variety of oils. The main use for oil-base fluids has been for drilling in 

 formations that may be harmed by water-base fluids, for coring operations to 

 recover native state cores for connate water determinations, and for reservoir 

 studies. Special interpretation of electric logs is required due to the high re- 

 sistance of the oil-base fluids. An advance in electric logging in oil-base fluids 

 has resulted from the development of the induction log. 



Crude oil alone is used frequently as a drilling fluid for completing wells 

 in tight sands and for workover jobs. 



Gas Base 



Air or natural gas is being used as drilling fluid to an increasing extent in 

 some areas. Use is limited to areas where weighted fluids are not required and 

 where formations contain little or no water. The main advantages of gas drilling 

 are high drilling rates and minimum damage to producing formations. Exces- 

 sive moisture causes balling of the bit cuttings around the drill stem and fre- 

 quent sticking of the tools; this inability to cope with water-bearing strata is a 

 severe limitation. 



Suitable cores and cuttings usually are obtained in gas drilling both for 

 geological and reservoir study. Electric logging is complicated in this method of 

 drilling. 



Combinations of air- and water-base muds, commonly referred to as aerated 

 muds, have been utilized as drilling fluids. The water-base muds used in aerated 

 mud drilling can range from clear water to high pH viscous muds. Aerated muds 



726 



