720 Subsurface Geologic Methods 



When it is known in advance that cement contamination will take 

 place, pretreatment of the fluid is advisable if a considerable amount of 

 cement is to be drilled. This pretreatment will reduce flocculation greatly 

 during cement drilling by precipitating most of the undesirable con- 

 stituents of the cement before serious damage has occurred. 



As mentioned in the section on cement contamination, any soluble 

 calcium as calcium phosphate or calcium carbonate and leave the sulfate 

 drilling, sections of lime or limy shale, anhydrite, gypsum, etc., are en- 

 countered. These . calcium salts are sufficiently soluble to cause trouble 

 from clay flocculation. 



When calcium contamination takes place, the fluid will have a fast 

 gel rate and a weak gel strength that prevents the proper settling of sand; 

 the viscosity will be abnormally low; and the water loss will usually be 

 moderately high. 



The primary aim in the chemical treatment for contamination by 

 soluble calcium salts is to precipitate the calcium as an insoluble com: 

 pound. This may be accomplished by use of disodium phosphate, soda 

 ash, or barium carbonate. Disodium phosphate and soda ash as chemical 

 precipitants for the calcium ions are similar in many respects. In the 

 treatment for calcium sulfate, for instance, these chemicals precipitate the 

 calcium salt will cause the flocculation of colloidal clays. In normal 

 ions behind as soluble sodium sulfate. Continued accumulation of such 

 soluble salts acts to increase the gel strength of the fluid. Barium carbon- 

 ate, on the other hand, is superior to soda ash or disodium phosphate for 

 the removal of anhydrite or gypsum. Complete precipitation of both the 

 calcium and the sulfate ions as well as the barium and the carbonate re- 

 sults, leaving no soluble salts in solution. 



When rock salt or salt water contacts a fresh-water drilling fluid, 

 there is initially an increase in apparent viscosity, gel strength, and water 

 loss. Later, as flocculation and dehydration of the colloids takes place, 

 the viscosity falls below normal, and the water loss rises in rough propor- 

 tion to the salt concentration. 



The treatment for salt contamination is not an easy one. In the 

 case of a salt-water flow, the first step in treatment is to increase the fluid 

 weight to the point required to stop the flow. 



For moderate salt concentration, disodium phosphate, soda ash, or 

 barium carbonate can be used to precipitate soluble calcium salts present 

 in the salt water and help reduce the gel strength of the fluid. The addition 

 of large quantities of tannin extracts can be used for reducing the high 

 gel strength produced by high salt concentrations. 



Where a water-clay system is being used and the salt contamination 

 does not exceed one percent, common practice is usually to continue to 

 use this type of fluid and to keep the physical properties in hand as much 

 as possible. The most extensive contamination of the fluid with salt comes 

 from the drilling of salt beds and domes. In these cases depths of several 



