A Comparison of Marsh-funnel and Stormer Viscosities of Drilling Muds

Experiments on twenty-six drilling muds were carried out in an effort toestablish a correlation between the apparent viscosity values for these muds asobtained with the Marsh funnel and the Stormer viscometer. This required acalibration of both instruments in which the density of the calibrating liquidswas taken into account. Corresponding to the Marsh-funnel viscosity values thatwere obtained at 1500 c.c. in, 1 qt. out, equivalent Stormer viscosity valueswere obtained at a Stormer cylinder test speed of approximately 1100 r.p.m.Further, a linear relation between the logarithms of the Stormer cylinder testspeed and the resulting apparent viscosity was found to exist for the mudstested for 200 to 1000 r.p.m. The significance of the slope of this linearrelationship is discussed as it applies to the field use of drilling muds. Itwas concluded that Marsh-funnel viscosity measurements are expressible inabsolute units that may be correlated with Stormer viscosity measurements, provided these are made in the manner described in the paper. Introduction The Marsh funnel is used almost universally to determine the viscosity ofdrilling muds in the field. The Stormer viscometer is commonly used todetermine the viscosity of drilling muds in the laboratory. With the Marshfunnel, the time required for one quart of mud to flow out of the funneloriginally filled with 1500 c.c. is often taken as the measurement ofviscosity. With the Stormer instrument, drilling mud is tested at a cylinderspeed of 600 r.p.m. and by means of calibration data the apparent viscosity isexpressed in absolute units. Neither of these instruments is an absoluteviscometer, and apparent viscosities obtained with either are influenced by thearbitrarily standardized conditions under which the measurements are made. It is disconcerting to the man in the field to be told that laboratorymeasurements of the viscosities of a group of muds differ widely, when he hasalready observed that they have identical viscosities when tested in the Marshfunnel. Drilling muds are not perfect fluids, therefore it is not surprising tofind a lack of correlation between the apparent viscosity of a mud asdetermined with the Marsh funnel and that determined with the Stormerviscometer. The work described here was undertaken to develop a correlation betweenMarsh-funnel and Stormer viscosity measurements. To a considerable degree thishas been accomplished. This study has resulted not only in a betterunderstanding of the two viscosity instruments and the data obtainable fromthem, but has given considerable information regarding the viscositycharacteristics of drilling muds. T.P. 1373