722 Subsurface Geologic Methods 



effective as thinners. This requires constant additions of large amounts 

 of the complex phosphates to maintain a satisfactory viscosity. 



Increased fluid temperatures also increase the chemical reactive prop- 

 erties of the fluid. The reaction rates of cement, gypsum, and salt with 

 the fluid thus become more pronounced at the higher temperatures. 



Lost Circulation 



One trouble encountered frequently in drilling is the loss of fluid in 

 porous low-pressure sand or gravel beds. It is usually possible in this 

 case to regain circulation by the use of various mechanical plastering 

 agents. Among those most commonly used are shredded cellophane, 

 sugar-cane fibers, and mica flakes. These materials are added to the fluid 

 in the pit and are pumped down opposite the zone to which the fluid 

 is being lost. Such materials must be removed from the fluid after circula- 

 tion has been regained. 



Types of Drilling Fluids 



As stated before, the water-clay type is the basic, most widely used 

 fluid system. This usage is the natural result of the availability and norm- 

 ally satisfactory functioning of water as the fluid vehicle. Other fluid 

 systems, however, have been developed to overcome drilling conditions 

 which water-clay fluids have difiiculty in handling. These special systems 

 are usually more costly to build and maintain than the water-clay type, 

 but their use for the situation for which they are best fitted is often the 

 difference between completing or abandoning a hole. Some of these 

 special-purpose fluids and some of the conditions for which they are useful 

 are 



(1) Water-starch-high pH fluid. 



Primarily used for salt contamination over 1 percent and 

 less than 15 or 20 percent, for drilling gypsum, anhydrite, 

 heaving shale, and for drilling into the production zone. 



(2) Salt-water-starch fluid. 



Used primarily where excessive salt contamination results 

 in saturation of the fluid. May also be used for drilling-in 

 or completing wells in formations subject to contamination 

 by water or for wells with low formation pressures. 



(3) Sodium-silicate fluid. 



Used for drilling heaving-shale formations or high-pressure 

 salt-water flows. 



(4) Oil-base fluid. 



Used primarily for drilling-in or recompleting wells in 

 formations subject to contamination by water or for wells 

 with low formation pressures, 



(5) Oil-emulsion fluid. 



Used where low fluid-loss, very thin cake, and good lubrica- 



