Concept for Hole Cleaning in Deviated-Well Drilling Based on Top-Side Drillstring Movements

Summary In a deviated wellbore, hole cleaning problems are often present at depths where the inclination angle is between 30 to 60° from vertical. As a result, a cuttings bed is formed that is difficult to remove. When the drillstring is laying in the bed, rotation is an efficient way to remove cuttings. However in the curved section, the drillstring is often located on the high side of the wellbore because the normal components of the tension forces are larger than the gravitational forces. In this paper, we describe a concept in which lateral drillstring movement between the high side and the cuttings bed will induce jet flows that lift cuttings into the main stream for more effective hole cleaning. Completely proving this concept requires both experimental work and analysis using cuttings transport models. Such models and experiments require knowledge of what lateral motions can actually be induced by proper control of the hoisting system in a real well. The focus of this paper is therefore to outline a drillstring model that is able to tailor a concept for inducing a variety of lateral motions for any wellbore geometry and drillstring configuration. The model is a multidimensional-lumped-element model that describes the axial movement in a drillstring in deviated wellbores, including both static and dynamic Coulomb friction forces. Depending on which parts of the drillstring are stalled due to static friction, the drillstring movements follow different semianalytical submodels. The model is switching between computationally efficient submodels and is therefore suitable for a real-time implementation.