Gel Strength 



The gel strengths of drilling fluids are of particular importance. Gel 

 strength refers to the minimum shearing stress that will produce permanent de- 

 formation of the plastic drilling fluid after a given period of quiescence. Gel 

 strength increases with increased time of quiescence of the drilling fluid until 

 a maximum is reached. It is principally the gel strength of the drilling fluid 

 that holds cuttings and weighting material in suspension when circulation is 

 stopped. 



In general, gel strength should be low enough to allow sand and shale 

 cuttings to settle out in the ditches and mud pits, permit entrained gas to escape 

 at the surface, minimize swabbing when pipe is pulled from the hole, and permit 

 starting of circulation without the use of high pump pressure. The gel strength 

 usually should be high enough to retard the settling of weighting material in 

 the pits and to prevent the formation of a settled fill at the bottom of the hole 

 when circulation of drilling fluid is suspended. Except in special cases, 10- 

 minute Stormer gel strengths of 10 to 20 grams are suitable. The subsequent 

 increase of gel strength with increased time of quiescence should be as small as 

 practicable because development of high gel strength promotes the formation 

 of stagnant gelled zones of drilling fluid in hole enlargements. Cuttings and cav- 

 ings tend to collect in these zones, and later unload into the flowing drilling- 

 fluid stream. When this condition exists, cuttings identification from formations 

 then being penetrated is difficult. This condition also can cause severe drilling 

 difficulties. 



A Stormer viscometer, which is the rotational type mentioned previously, 

 commonly is used to determine gel strengths. The fluid to be tested is stirred 

 thoroughly and poured into the test cup. The initial gel strength is the minimum 

 weight in grams required to cause rotation of the spindle 14 revolution. The 

 10-minute gel strength is the minimum weight in grams required to cause ro- 

 tation of the spindle after the thoroughly agitated fluid has remained quiescent 

 for 10 minutes. 



Filter Loss 



The filter loss and wall-building properties of drilling fluids are recognized 

 as being of major importance in the proper drilling of wells. As the drilling fluid 

 is circulated over the walls of the hole, there is a tendency for the liquid phase 

 of the mud to be filtered into the surrounding formation and to leave the solid 

 matter to be deposited on the face of the formation in the form of a coating or 

 filter cake. The formation of a filter cake is essentially a bridging of the ex- 

 posed pore openings in the wall of the well. Relatively coarse mud particles 

 may be required to start the bridging of the pores. If the filter loss is to be 

 reduced effectively, the space between the relatively large particles must be 



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