718 Subsurface Geologic Methods 



Sand Content 



The sand content of a drilling fluid may be extremely detrimental. 

 Sand in the drilling fluid is abrasive and may result in rapid wear of pump 

 liners, drill pipe, casing, and other metallic equipment with which it 

 comes in contact. Sand-laden fluid also has poor wall-building prop- 

 erties, and develops a thick filter cake. If present in sufiicient amount, it 

 may freeze the drill pipe to the walls of the well and, perhaps, cause a 

 twist-off. 



All material coarser than 200 mesh may be regarded as sand, and 

 good practice requires that the sand content of a drilling fluid be main- 

 tained below five percent. To determine whether or not the fluid meets 

 this requirement, occasional tests for sand content should be made. Several 

 test methods are employed, including elutriation, centrifuging, dilution 

 and gravity settling, and sieve analysis. 



Chemical Treatment of Drilling Fluids 



Chemical treatment of drilling fluids has been widely accepted be- 

 cause it saves money and improves fluid quality. Savings in costs often 

 more than justify chemical treatment. More important, when viscosity, 

 weight, and filter loss of the drilling fluid are controlled to suit the con- 

 ditions, wells are drilled faster, depreciation of equipment is lessened, and 

 insurance against blowouts and stuck drill pipe is provided. 



Chemical treatment of drilling fluids is generally for the purpose of 

 decreasing the viscosity and gel strengths, and reducing the filter rate. 

 Treatment for the reduction of viscosity and gel strengths is commonly 

 called "thinning," and chemicals used for this purpose are often termed 

 "thinners." These chemicals are deflocculating agents which cause a higher 

 degree of dispersion of the colloidal particles of the fluid. More complete 

 dispersion of the particles, in turn, produces a thinner filter cake, lower 

 filter loss, and lower viscosity and gels. 



Chemicals should be selected on the basis of safety, simplicity, and 

 economy. When chemicals are tested in any fluid, the effect of repeated 

 treatment with the same chemical should be considered. Some chemicals 

 that work well at first may later hinder treatment when it is urgently 

 needed. 



During the drilling of oil wells, the quality of the fluid in circulation 

 changes constantly. Drilling fluids may be adversely affected by: 



(1) Cement, anhydrite, gypsum, etc. 



(2) Salt water 



(3) Excess bentonitic shale 



(4) Gas 



(5) Heat 



When tests show poor fluid conditions, particularly when viscosity 

 or filter loss are too high, small quantities of chemicals, properly applied, 

 will often quickly restore satisfactory fluid quality. 



There are two classes of compounds that are particularly effective 



