Nowadays the common approach to free a stuck pipe due to differential pressure sticking (DPS), in addition to mud density reduction and use of chemicals, is based on the application to the drill string of a force large enough to overcome the frictional resistance between the steel and the filter cake. More recently, this approach has been challenged, and focus has shifted to a failure mechanism internal to the filter cake. Despite published test data showing that sliding is possible, the release mechanism that has been consistently observed was frequently a shear failure within the cake. As reported in literature, several experimental methods of assessing the differential sticking affinity of muds have been proposed. While some involve measuring drilling parameters, such as overpulls, downhole torque, and surface torque on the drill string, most make direct measurements on muds and mud filter cakes.
The topic is not only academic and is of operational interest, so a laboratory equipment has been developed in order to study in depth this particular phenomenon by: simulating the mud-cake formation with the same bottom hole conditions during drilling phase; replicating the DPS phenomena; generating several kinds of actions to study the releasing condition of the drill string.
This paper describes in details the laboratory equipment, its features and capabilities in terms of fluids used (water-and oil-based muds), operating temperatures, operating pressures, applicable forces/torques.
Several studies have been performed to address the differential pressure sticking (DPS), a recurring problem during the drilling of oil wells with significant economic repercussions. As reported in 1), DPS happens when a part of the drill string, casing, or logging tool becomes embedded in the mud filter cake (an impermeable film of fine solids) and is held firm by mud pressure that exceeds the formation pressure, but unrestricted mud circulation is still possible. Significant mud overbalance, as well as an exposed permeable section, must also exist for differential sticking to occur.