Modeling Cuttings Transport and Annular Pack-Off Using Local Fluid Velocities with the Effects of Drillstring Rotation and Eccentricity

Abstract

Abstract One of the primary functions of the drilling fluid is to transport cuttings from the bit to the surface. This transport is mainly a function of fluid properties (rheology and density), pump rate, wellbore trajectory and geometry, drillstring rotation and eccentricity. Because these interrelated parameters all have a significant effect on cuttings transport, it is important that hole cleaning modeling is comprehensive and inclusive to yield useful, reliable results. Inaccurate models that either do not consider significant effects or oversimplify them can lead to inadequate hole cleaning advice, resulting in non-productive and invisible lost time. A novel cuttings transport model that includes relevant effects such as pipe rotation, eccentricity and the annular blockage is presented. Local velocity profiles are calculated for a given fluid and compared against the local critical velocity for cuttings transport. Then, the location and the magnitude of annulus blockage are numerically assessed. After an annulus becomes partially blocked with cuttings deposited in a cuttings bed, continuity and momentum equations are solved for the blocked annulus to estimate the new local velocities. The annulus is divided into small sections and the pressure profile is calculated with the progression of time. By doing so, a realistic representation of a wellbore's pressure and velocity profiles is obtained. The results from this study are in very good agreement with large-scale flow loop experiments and field observations, e.g. showing that axial flow alone is not enough for effective hole cleaning in high deviation and horizontal wells. The simulations show that in such wells the cuttings tend to deposit even at elevated axial flow rates without pipe rotation in a fully eccentric annulus, due to a significantly reduced flow rate where the drillstring is close to the borehole or previous casing string. The results also provided a way to predict "pack-off" events, when the string can become stuck, the equivalent circulating density can spike, and lost circulation can occur. The non-uniform settling of the cuttings along the wellbore will cause non-uniform distribution of pressure losses, with local pressure spikes when cuttings blockage becomes substantial. A pack-off sensitivity analysis is presented in this work. The overall aim of this work is to provide more comprehensive cuttings transport modeling for the industry to conduct safe and improved hole cleaning operations.