Differential Pressure Sticking-Laboratory Studies of Friction Between Steel and Mud Filter Cake

Abstract

The control of mud properties affords two practical means of mitigating pipe sticking caused by differential pressure: (1) reducing mud weight and, therefore, differential pressure; and (2) reducing 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.