Stick-slip and Torsional Friction Factors in Inclined Wellbores

Abstract

Abstract. Stick slip is usually considered a phenomenon of bit-rock interaction, but is also often observed in the field with the bit off bottom. In this paper we present a distributed model of a drill string with an alongstring Coulomb stiction to investigate the effect of borehole inclination and borehole friction on the incidence of stick-slip. This model is validated with high frequency surface and downhole data and then used to estimate static and dynamic friction. A derivation of the torsional drill string model is shown and includes the along-string Coulomb stiction of the borehole acting on the string and the 'velocity weakening' between static and dynamic friction. The relative effects of these two frictions is investigated and the resulting drillstring behavior is presented. To isolate the effect of the along-string friction from the bit-rock interaction, field data from rotational start-ups after a connection (with bit off bottom) is considered. This high frequency surface and downhole data is then used to validate the surface and downhole behavior predicted by the model. The model is shown to have a good match with the surface and downhole behavior of two deviated wellbores for depths ranging from 1500 to 3000 meters. In particular, the model replicates the amplitude and period of the oscillations, in both the topside torque and the downhole RPM, as caused by the along-string stick slip. It is further shown that by using the surface behavior of the drill-string during rotational startup, an estimate of the static and dynamic friction factors along the wellbore can be obtained, even during stick-slip oscillations, if axial tension in the drillstring is considered. This presents a possible method to estimate friction factors in the field when off-bottom stick slip is encountered, and points in the direction of avoiding stick slip through the design of an appropriate torsional start-up procedure without the need of an explicit friction test.

Nonlinear dynamic analysis of liquid-solid coupling and rubbing on drill string

“…The potential resources of NGH in north continental slope of south China sea are up to 185108 t. The NGH resources is amount to 6 t of oil equivalent and about half of China's total onshore and offshore oil and gas resources. Thus, it has a good future of development [5][6][7].…”

Estimating friction factors while drilling

“…Off-bottom stick slip is a well known phenomenon from the field, and when mentioned in literature is hypothesized to be caused by a negative difference between static and kinetic along-string Coulomb-type friction [8,15,21]. This cause is distinct [4] from stick-slip caused by the regenerative effect in the bit-rock interaction [6,11,18].…”

Effect of shock subs on self-excited vibrations in drilling systems

“…Although the rate-weakening of side forces and the regenerative bitrock interaction are distinct phenomena, these can both lead to stick slip (Aarsnes and Shor, 2018b). We will in this paper focus on the regenerative effect in the bit-rock interaction as a cause of vibrations.…”