Differential Pressure Sticking-Laboratory Studies of Friction Between Steel and Mud Filter Cake
Authors: Annis M.R. , Monaghan P.H. 
Published: December 2007

Abstract
The control of mud properties affords two practical means of mitigating pipe sticking caused by differential pressure:reducing mud weight and, therefore, differential pressure; andreducing the friction between the pipe and mud cake.
This paper describes investigation of the second of these-the friction between the pipe and the mud cake. Friction between a steel plate and a mud cake, held in contact by a differential pressure, was measured in the laboratory while maintaining a constant area of contact. Experiments were performed to determine how this friction varied with changes in mud composition and with changes in experimental conditions such as the differential pressure, time of contact of plate and mud cake, and filter-cake thickness. It was found that the apparent coefficient of friction, or the "sticking" coefficient, was not a constant; instead, it increased with increased time of contact between plate and mud cake, and with increased barite content of the mud. The sticking coefficient varied from about 0.05 to 0.2 after 20 minutes, and eventually reached values of 0.1 to 0.3 after two hours. Quebracho or ferrochrome lignosulfonate reduced the sticking coefficient at short set times but did not reduce the maximum value. Carboxymethylcellulose had no effect on the sticking coefficient. Emulsification of oil in the mud reduced the sticking coefficient. Some oils reduced the sticking coefficient to about one-third of its value in the oil-free base mud, while other oils reduced it only slightly. Addition of certain surfactants with the oils further reduced the sticking coefficient. Spotting a clean fluid over the stuck plate caused a reduction in sticking coefficient only if the differential pressure was reduced, either temporarily or permanently.

Introduction
Often during drilling operations the drill string becomes stuck and cannot be raised, lowered, or rotated. This condition can be brought about by a number of causes, such as sloughing of the hole wall, settling of large particles carried by the mud, accumulation of mud filter cake during long stoppage of circulation and, finally, sticking by pressure of the mud column holding the pipe against the filter cake on the hole wall. This paper is concerned with the last-mentioned phenomenon. Helmick and Longley in 1957 suggested that a pressure differential from the wellbore to a permeable formation covered with mud cake could hold the drill pipe against the borehole wall with great force. This situation occurs when a portion of the drill string rests against the wall of the borehole, imbedding itself in the filter cake. The area of the drill pipe in contact with filter cake is then sealed from the full hydrostatic pressure of the mud column. The pressure difference between the mud-column pressure and the formation pressure acts on the area of drill pipe in contact with the filter cake to hold the drill pipe against the wall of the borehole. Helmick and Longley also presented laboratory experiments which showed that the force required to move steel across a mud cake increased with increasing differential pressure and with the time the steel and mud cake had been in contact. Their data indicated that replacing the bulk mud with oil reduced the force required for movement. Field evidence was reported that spotting oil over the stuck interval sometimes freed the pipe. Outmans in 1958 presented a theoretical paper which described the sticking mechanism and explained the increase of sticking force with time with equations derived from consolidation theory. Since publication of these papers, there has been interest in the differential pressure sticking of drill strings, and several mud additives to reduce sticking or special equipment to free stuck pipe have been proposed. Haden and Welch have recently reported laboratory evidence showing that the composition of the filter cake influences the force necessary to move steel on the filter cake. There seems no doubt that differential pressure sticking is a real phenomenon and that its severity depends on the magnitude of the pressure differential across the mud cake, the area of contact and the friction between pipe and mud cake. The mud weight required to control a well is determined by the highest formation pressure in the well; hence, the magnitude of the differential pressure opposite normal or subnormal pressure formations cannot be reduced. The area of contact may be minimized in several ways (control of filter-cake thickness, use of stabilizers and spirally grooved drill collars), but there are practical limitations which prevent reduction of contact area from becoming a complete solution of the problem. However, the mud composition might be altered to reduce the friction between pipe and mud cake. This paper presents quantitative measurements of the friction between steel and mud filter cake and shows how the friction varies with mud composition for given experimental conditions.
JPT
P. 537^