Miscellaneous Subsurface Methods 715 



drill cuttings when circulation stops. Sand content is undesirable in a 

 drilling fluid because of the abrasion of the sand on metal surfaces during 

 circulation. Salt content may have an adverse effect on the colloidal 

 properties of a drilling fluid and render it unsuitable for use under certain 

 conditions. 



Viscosity 



The viscosity of a drilling fluid depends upon the amount and char- 

 acter of the suspended solids. In general, the greater the percentage of 

 suspended solids, the greater will be the viscosity; and plastic clays de- 

 velop higher viscosities than noncolloidal substances. Bentonite has 

 well-developed colloidal and thixotropic properties. When the clay used 

 in a drilling operation is deficient in colloidal material and lacks thixo- 

 tropic properties, controlled amounts of finely ground bentonite may be 

 added to the drilling fluid to improve its quality. Viscosities of drilling 

 fluids may fluctuate over a wide range from but a few centipoises to more 

 than 300 centipoises. 



Several instruments are available for measuring drilling fluid vis- 

 cosity; some are primarily for laboratory use, others have been designed 

 for field use. The most commonly employed instruments for measuring 

 relative viscosities of drilling fluids on location are the Marsh funnel and 

 the Stormer viscosimeter. The Marsh funnel is a cone-shaped funnel with 

 an accurately bored discharge tube at the bottom. The funnel is filled 

 with the fluid to be tested and the time in seconds necessary to discharge 

 a measured volume of the fluid is an indication of its relative viscosity. 



The Stormer viscosimeter is adapted for determining drilling fluid 

 viscosity and gel strength. The instrument consists principally of a spindle 

 which is rotated in a test cup by a pair of gears driven by a weight. The 

 fluid in the test cup is agitated and then the weight is adjusted until the 

 spindle is made to revolve at a rate of 600 r.p.m. By means of a calibra- 

 tion chart furnished with each instrument, the weight required to achieve 

 this rate may be converted into centipoises. 



Density 



The amount and the specific gravity of the suspended solids deter- 

 mine the density of the drilling fluid. Many wells are drilled with fluid 

 densities not over 9 lb. per gal. or 67 lb. per cu. ft. In other cases where 

 abnormal pressures are encountered, fluid densities as high as 20 lb. per 

 gal. or 150 lb. per cu. ft. may be prepared that are not too viscous to be 

 handled by the pumps. Fluids weighing more than about 11.5 lb. per 

 gal. or 86 lb. per cu. ft., if prepared exclusively with clay and water, are 

 Uable to be too viscous to be readily handled by the pumps, and gas and 

 sand will not readily separate out. If heavier fluids are needed, finely 

 ground heavy minerals may be used to contribute high density without a 

 corresponding increase in viscosity. The most commonly used weighting 

 material, barite, has a specific gravity of 4.3 and, by adding it in a finely 



