Colloid Chemistry of Clay Drilling Fluids

Abstract

It is only within the past 10 years that serious attempt appears to havebeen made to improve rotary drilling fluids by the application of theprinciples of colloid chemistry, although the use of chemicals to control therheological properties of clay slips has long been known in the ceramicindustry. The main difficulty apparently lay in the lack of a complete analysisof all of the functions of a drilling fluid in the drilling of the well, together with the lack of adequate consideration of how colloid science couldbe applied to secure these ends, and particularly how apparent antitheticalproperties of the fluid could be compromised. Thus one analysis1 of the manyrequirements of a drilling fluid proved conclusively that the viscosity shouldbe low, in order that cuttings and entrained gas might be eliminatedefficiently in the settling ditch, while the loss in carrying capacityaccompanying low viscosity could be regained by increasing either the densityof the mud or the upward velocity of the mud stream in the annular space of thewell, and the danger of sticking of the drill stem due to rapid settling incase of enforced suspension of drilling operations could be eliminated byenhancing the thixotropic properties of the fluid.The desired properties of the drilling fluid having thus been established, itwas a natural step to treat the fluid with viscosity-reducing chemicals, suchas salts of weak organic acids- for example, sodium tannate or gallate-and toadjust the pH to the alkaline side to such a degree of thixotropy as would give low viscosityand low yield point under the conditions of flow prevailing in the settlingditch, but rapid gelation and high yield point when quiescent in the annularspace during shutdowns. In fact, flow in the settling ditch should be justshort of turbulent while flow in the well, together with the flow of mud pastthe cuttings, is practically always turbulent. The use of chemicals other thanthose mentioned, such as Calgon (sodium hexametaphosphate) and certain otherphosphates together with sodium tannate, has now become standard practice andit is the consensus that this method of treatment has proved to be an importantstep in the development of the art of drilling oil wells.It is safe to assume that the application of colloid chemical principles canalso help in solving other outstanding problems of drilling fluids, such as thedevelopment of cheaper and more efficient viscosity reducers; the developmentof chemicals that do not lose their effectiveness in a short time duringdrilling operations; the finding of chemicals to preserve the plasteringproperties of muds when concentrated brines are encountered or in drillingthrough salt beds; improvement in the plastering properties of poor muds byfurther deflocculation, and, conversely, the prevention of excessivedeflocculation of certain marine shales sometimes encountered; the preparationof muds suitable for the successful drilling of heaving shale formations; thefinding of chemicals to treat the mud to prevent or to diminish lostcirculation;T.P. 1201