Down Hole Pressure Variation Due to Axial Stick Slip Motion Effect on Drill String

Abstract

In order to predict downhole surge and swab pressure variation due to movement of the drill pipe, the Stribeck friction model has been outlined to analysis the friction induced stick slip motion in inclined bore holes. This model describes the axial stick slip motion of the drill string through four regimes of contact including sticking, boundary lubrication, partial fluid lubrication and full lubrication. The results from the single degree of freedom (dof) model are compared with the Coulomb friction model. The results indicate significant increase in peak velocity of the lower part of the drill string and Bottom Hole Assembly (BHA) compared to the velocity of the drillstring at surface, i.e., during drill pipe connection using a MODU (Mobile Offshore Drilling Unit). Simulations also show that the wave form of the downhole drill string motion will be different compared to the surface motion due to the stick phase caused by variation of contact force between the drillstring and the borehole in the curved section of the bore hole. In long high deviated boreholes, the movement of the BHA and the corresponding surge and swab pressure during drill pipe connection will be low or negligible due to high drag force and elasticity of the drill string.