Miscellaneous Subsurface Methods 721 



thousand feet of salt may be penetrated and the drilling fluid soon becomes 

 saturated. Before entering such salt beds, a change to a salt inert-type 

 fluid is necessary. 



With certain saline types of drilling fluids, the filter loss and gel 

 strength cannot be maintained by the use of bentonite. In this case it 

 is necessary to use special salt-resisting clays which will furnish suspend- 

 ing properties. These clays generally do not possess good wall-building 

 characteristics, and colloidal additives like pregelatinized starch are used 

 to provide these properties. 



Various organic colloids and particularly the starches have been 

 found useful in controlling the wall-building properties of salt-water fluids. 

 These organic colloids are subject to bacterial attack, which reduces their 

 effectiveness and produces objectionable odors in the fluid. Chemical 

 preservatives, high salt content, or high pH of the fluid must then be re- 

 sorted to for protection against the decomposition. 



The highly colloidal, heaving shales are similar in many respects to 

 clay colloids of the type used in drilling fluids. Therefore, any chemical 

 treatment designed to prevent the swelling of those shales is likely to be 

 detrimental to the fluid. The presence of such "bentonitic" shale is evi- 

 denced by a sharp increase in viscosity and, usually, an appreciable re- 

 duction in water loss of the fluid. In most cases it is possible to control 

 bentonitic shale by conditioning the drilling fluid to minimize the colloidal 

 tendencies of the clay. Special drilling fluids which prevent hydration of 

 the bentonitic shale by maintaining an excess of calcium ions in the fluid 

 have been successfully used. These fluids are amenable to weighting to 

 high densities and to treatments to obtain low filter losses. 



Gas cutting is largely a mechanical rather than a chemical problem 

 in drilling-fluid control. The hydrostatic head of the fluid column must 

 be slightly greater than the formation pressure at every point in the hole 

 in order to prevent the flow of gas into the fluid system. Gas in the fluid 

 indicates that the hydrostatic head of the fluid is too low to meet this 

 requirement, and the obvious solution is to increase the fluid weight by 

 the addition of commercial weighting materials. 



Temperature Effects on Drilling Fluids 



In very deep wells, or in drilling near shallow salt domes, abnormally 

 high temperatures are encountered. Such elevated temperatures may have 

 a detrimental effect upon the drilling fluid. The viscosity of most liquids 

 decreases with temperature increase. The rate of fluid filtration through 

 a filter cake is inversely proportional to the viscosity of the filtrate. Thus 

 the wall cake will tend to be thicker in holes of high bottom-hole tempera- 

 tures. 



One effect of increased fluid temperatures is to revert the complex 

 phosphates to the orthophosphate form, in which condition they are in- 



