bridged successively with smaller particles until eventually the smallest particles 

 approach the size of molecules. The colloidal properties of a drilling fluid largely 

 determine its ability to form thin impervious filter cakes and have low filter loss. 



High filter loss may often result in filter cakes becoming thick enough to 

 cause tight places in the hole and even sticking of drill pipe. High filter loss 

 with water-base muds often harms the productivity of wells by excessive in- 

 vasion of the formation, and sometimes causes an excessive dispersement of 

 cuttings and serious sloughing and caving of shale. In addition to minimizing 

 drilling problems, low filter loss aids in the recovery of cuttings and cores more 

 useful for geological examination and reduces the amount of cavings that 

 interfere with the evaluation of cutting samples. Low filter loss aids also in 

 obtaining satisfactory electric logs and reliable drill-stem tests. 



The permissible filter loss of a drilling fluid varies over a wide range and 

 depends upon the particular area and drilling situation. For example, clear 

 water is satisfactory for drilling to considerable depth in some areas, whereas 

 drilling fluids of low filter loss are ordinarily required for drilling deep wells, 

 particularly those in which incompetent shales must be penetrated. In general, 

 specification of the permissible filter loss must be based upon past drilling ex- 

 perience in the immediate area, including such overall factors as total drilling 

 cost, major drilling difficulties, productivity indices, and drilling-fluid costs. 



Filter loss is commonly determined in a small filter -press cell approximately 

 3 inches in diameter. The fluid is forced against a screen-supported filter paper 

 on which the filter cake is formed at 100 pounds per square inch. The thickness 

 of the filter cake and the amount of filtrate passing through the filter paper are 

 measured after 30 minutes. 



pH Values 



The pH of drilling fluids, i.e., the degree of alkalinity or acidity, is im- 

 portant in the control of several types of drilling fluids and in minimizing dis- 

 persion of cuttings. The pH is preferably maintained as low as practicable; 

 normally a pH of 9 to 10 is satisfactory for most treated native muds. The pH 

 is commonly from 10 to 11 for muds treated heavily with caustic soda and 

 organic thinners and above 12 for lime-treated and high-pH starch drilling fluids. 



Protection of Pay Zones 



Possibly the most critical property of a drilling fluid is that of protecting 

 the pay zone without impairing the productivity of the zone. The discovery of 

 valuable production from meticulous geologic and other exploration surveys 

 should not be prevented by plugging of the pay formation of the wildcat well 

 by drilling fluid. Formations differ widely in their tolerance to damage by 

 drilling fluids. Fortunately, the fact that many productive formations are not 



720 



