Downhole torque measurements have become an inherent part of MWD (measurement while drilling) systems. It allows drilling engineers to estimate the downhole friction and detect the presence of obstructions, for instance, cuttings bed caused by insufficient cuttings transport. The latter is a big concern when drilling horizontal and extended-reach wells.
This paper describes a method of well friction analysis as a function of adjustable drilling parameters and parameters that describe moving cuttings dune. Experiments are performed in a pipe with an internal diameter of 0.04 m and 5° inclination from the horizon, which represents a wellbore. An inside rotating pipe with an outer diameter of 0.025 m simulates a drillpipe, with a possibility of rotating speed from 0 to 1000 RPM (revolutions per minute). Glass pellets are used to simulate and display cuttings behavior in the wellbore. An electric motor is used to provide rotation of the drillpipe, supplied with an encoder to measure the electric current associated with the torque. The measurements are used to calculate the torque loss over the studied pipe length. A high-speed camera installed outside the outer pipe allows capturing images of the height of the particle bed as well as the pipe eccentricity, which increases with distance from the origin (rotating mechanism) due to the pipe’s weight and inclination.
The combined effect of these factors on the kinetic friction between the outer and inner pipes is investigated in this paper. Ultimately, the friction factor is calculated by equating measured motor torque to the theoretical torque.
The main objective of the study is a development of a methodology to express the friction factor in terms of adjustable drilling parameters in an environment, complicated with the presence of moving cuttings bed and varying pipe eccentricity. This study can supplement other research that is aimed to improve understanding of the intricate phenomenon of the wellbore friction.