Mechanistic modeling of wellbore cleanout in horizontal and inclined wells

Abstract

The accumulation of rock cuttings, proppant, and other solid debris in the wellbore due to inadequatecleanout remarkably impedes field operations. This article presents a new hole cleaning model, whichcalculates the Critical Transport Velocity (CTV) in conventional and fibrous fluids. The study is aimed toestablish an accurate mechanistic model for optimizing wellbore cleanout in horizontal and inclined wells. The new CTV model is established to predict the initiation of bed particle movement during cleanoutoperations. The model is formulated considering the impact of fiber using a special drag coefficient (i.e. fiberdrag coefficient), which represents the mechanical and hydrodynamic actions of suspended fiber particlesand their network. The dominant forces acting on a single bed particle are considered to develop the model.Furthermore, to enhance the precision of the model, recently developed hydraulic correlations are employedto compute the average bed shear stress, which is required to determine the CTV. In horizontal and highlydeviated wells, the wellbore geometry is often eccentric, resulting in the formation of flow stagnant zonesthat are difficult to clean. The bed shear stress in these zones is sensitive to the bed thickness. The existingwellbore cleanout models do not account for the variation in bed shear stress. Thus, their accuracy islimited when stagnant zones are formed. The new model addresses this problem by incorporating hydrauliccorrelations to account for bed shear stress variation with bed height. The accuracy of the new model is validated with published measurements and compared with theprecision of an existing model. The use of fiber drag and bed shear stress correlations has improved modelaccuracy and aided in capturing the contribution of fiber in improving wellbore cleanout. As a result, forfibrous and conventional fluids, the predictions of the new model have demonstrated good agreement withexperimental measurements and provided better predictions than the existing model. Model predictionsshow a noticeable reduction in fluid circulation rate due to the addition of a small quantity of fiber (0.04%w/w) in the fluid. In addition, results show that the existing model over predicts the cleaning performanceof both conventional and fibrous fluids.