before a protective filter cake has formed. Likewise, filtrates from drilling fluids 

 cannot harm formations they are not permitted to enter. Filtrate penetration 

 can be minimized by maintaining differential pressures as low as possible and 

 by using drilling fluids having low filter losses. 



Certain water-base drilling fluids termed inhibited muds have been develop- 

 ed for some formations that are specifically sensitive to water filtrates because 

 of the swelling of interstitial clays. When the formations are sensitive to filtrates 

 from water-base drilling fluids for several reasons other than swelling of clay, 

 the use of oil-base drilling fluid or oil-base emulsions that filter only oil may be 

 necessary. 



Air or gas is advantageous especially as a drilling fluid in drilling sensitive 

 formations since gaseous drilling fluids cause minimum damage to such forma- 

 tions. 



TYPES OF The drilling fluids in use today fall into three 



DRILLING FLUIDS general classes: water-base, oil-base, and gas- 



base (fig. 35-1). Drilling fluids intermediate 

 between water- or oil-base and gas-base are of the gas-in-water or gas-in-oil 

 type, which are generally known as gas-cut drilling fluids; or they are of the 

 water- or oil-in-gas type, about which little has been published. Drilling fluids 

 intermediate between water-base and oil-base are of the water-in-oil and oil-in- 

 water types which generally are known as oil-base emulsions and water-base 

 emulsions, respectively. 



Water-base drilling fluids comprise the major portion of all drilling fluids 

 in use today. Treated native muds predominate in this class of drilling fluids. 



Oil-base drilling fluids consist of oil in which are dispersed materials for 

 raising viscosity and gel strength and lowering filter loss. 



Gas-base drilling fluids consist of a gas, such as air and methane, in which 

 are suspended the material being drilled and eroded or sloughing from exposed 

 formations. 



KINDS OF Numerous types of drilling fluids are avail- 



DRILLING FLUIDS able within the major classifications of water- 



base and oil-base mud. The diversity of drill- 

 ing-mud systems results from the varied requirements of drilling fluid — require- 

 ments that depend upon such factors as depth of well, type of formation, and 

 local structural conditions. For example, drilling through limestone entails the 

 problem of loss of fluid in cavernous voids but no sloughing or caving of the 

 hole, whereas drilling through some shale types entails problems of sloughing and 

 caving but no problem of loss of fluid. In these two instances, the requirements 



722 



