Modeling the effects of drillstring eccentricity, pipe rotation and annular blockage on cuttings transport in deviated wells

Abstract

Transportation of drilled cuttings from the bit to the surface is one of the primary functions of a drilling fluid. This transport is facilitated primarily by pump rate and the rheology and density of the drilling fluid, which all contribute to cuttings lift. There are, however, other important factors that have a significant effect on cuttings transport, as shown by a large amount of academic and industry research. These include rotation and the eccentricity of the drillstring. These two parameters are interrelated when it comes to hole cleaning: the effect of pipe rotation on cuttings evacuation changes with varying eccentricity. This adds a level of complexity to the hole cleaning challenge, both from a modeling perspective as well as the practical management of cuttings evacuation from wells in the field.A novel cuttings transport model that includes essential effects such as pipe rotation, eccentricity and annulus blockage is presented in this study. Velocity profiles for the well annulus subdivided into small grids are calculated for any given fluid, which can be a Newtonian or a non-Newtonian fluid such as described by the Bingham Plastic, Power Law or Yield Power Law rheological models. These local velocities are compared against the concept of the critical velocity for cuttings transport. Then, the locations of the settled cuttings and the magnitude of annular blockage are calculated numerically. Continuity and momentum equations are solved for the blocked annulus to estimate the new local velocities. By doing so, a realistic representation of a wellbore's pressure and velocity profiles is obtained. The work aims to make hole cleaning modeling more comprehensive and representative in order to prevent hole cleaning-related non-productive time (NPT) or invisible lost time (ILT) events in the field. These include stuck pipe and lost circulation events, low rates of penetration, high torque and drag, failure to land the casing at the desired depth, and poor cement jobs leading in turn to poor zonal isolation.