Miscellaneous Subsurface Methods 717 



device for applying pressure to the fluid enclosed within it. The fluid is 

 forced through a screen-supported filter paper on which the filter cake is 

 formed. The thickness of the filter cake and the filtration rate of the base 

 fluid through it are considered a measurement of the effectiveness of the 

 quantity and type of colloidal materials in the fluid. 



Gel Strength 



Whereas viscosity is a measure of the resistance of fluids to infini- 

 tesimal uniform shear, the gel strength of plastic fluids, such as drilling 

 fluids, is a measure of the minimum shearing stress necessary to produce 

 slipwise movement. Such a gel strength is zero for true fluids, no matter 

 how viscous, but it is appreciable for suspensions such as drilling fluids. 

 Gel strength in drilling fluids is responsible not only in part for the ability 

 of the fluid to transport cuttings upward, but is wholly responsible for the 

 ability of the fluid to hold cuttings and weight material in suspension 

 when circulation is suspended. Non-colloidal particles will eventually 

 sink in a true fluid having zero gel strength, regardless of its viscosity. 



Many fluids exhibit an increase of gel strength upon standing qui- 

 escent for some time and can be restored to fluidity upon subsequent 

 agitation. Such fluids are termed thixotropic and are tested for initial 

 gel strength immediately upon cessation of agitation. A measure of the 

 thixotropy of a fluid is given by the difference in gel strengths observed 

 immediately after agitation and after some period of quiescence. An 

 adequate gel strength is a valuable characteristic when shutdowns occur, 

 as even fairly large cuttings do not sink very far before the fluid gels 

 sufficiently to support them. 



With the Stormer viscosimeter described previously, gel strength de- 

 terminations may be made over a wide working range. This wide working 

 range is essential in the close of control of the gel-strength property, which 

 is required in meeting certain drilling conditions and also in evaluating 

 treating chemicals designed to reduce the gel strength of drilling fluids. 



The fluid to be tested is thoroughly agitated and immediately poured 

 into the test cup. The initial gel-strength measurement is made by deter- 

 mining the minimum weight required to effect a movement of approxi- 

 mately one-quarter revolution of the spindle after the brake is slowly re- 

 leased. The total weight in grams required to obtain this rotation is 

 reported as the initial gel strength. 



A second gel-strength determination is made after the fluid in the 

 cup has been allowed to remain quiescent for ten minutes following the 

 initial gel-strength observation. Additional driving weights are added to 

 the line and the brake is slowly released. If no rotation of the spindle 

 occurs, the brake is set again, more weight is added to the line, and the 

 brake is released again. This process is repeated until the minimum weight 

 required to cause rotation is found. This weight is reported as the ten- 

 minute gel-strength reading. 



