The necessity to rapidly release the drillstring after a differential pressure sticking (DPS) stuck pipe event, in order to retrieve the drilling process minimizing risks and costs, is still representing a major issue in drilling operations. Usually, mud density reduction, the flow-in of mud chemicals and the application of large and impulsive forces to the drillstring are yet the basis to cope with the frictional resistance between the steel and the filter cake. More recently, these brute force approaches have been challenged, and the focus has shifted to a failure mechanism internal to the filter cake. Results show that the observed release mechanism is frequently a shear failure within the cake.
Furthermore, several experimental methods of assessing the differential sticking attitude of muds have been proposed. These involve sometime the measurement of the drilling parameters, such as overpulls, downhole torque, and surface torque on the drillstring, but most use direct measurements on muds and mud filter cakes.
After some years of development, a new laboratory equipment is now available in Eni for the in-depth study of stuck pipe events enabling: the simulation of mud-cake formation in downhole conditions; the replication of DPS phenomena; the generation of several kinds of actions to study drill string release.
The equipment, whose concept has been already presented in a previous paper, is ready to start a test campaign in the next few months in Eni's Bolgiano Laboratories.
This paper briefly illustrates the laboratory equipment and explains how the results that will be obtained can guide the development of a dedicated downhole tool to assist the drillers in case of stuck-pipe.
As reported in 10), 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.
