Hole Cleaning During UBD in Horizontal and Inclined Wellbore

Abstract

Abstract This paper is an extended work of SPE 90038. Aerated non-Newtonian flow and inclined wellbore section were added to the previous model to study hole cleaning problem while drilling underbalanced well. This new mechanistic model for cuttings transport is developed by combining two-phase hydraulic equations, turbulent boundary layer theory and particle transport mechanism. It is shown that the model is useful for predicting minimum annular velocity and cuttings bed thickness in horizontal and inclined wellbore geometry. Effects of temperature, bottom hole pressure, liquid flow rate, gas injection rate, cuttings size and density, inclination angle and Rheological properties of drilling mud on hole cleaning are analyzed using this mechanistic model. Model is validated by available experimental data. Computer simulation indicates that cuttings bed thickness is very sensitive to the liquid phase flow rate. It dominates cuttings transport efficiency. However, during underbalanced drilling, increase of liquid phase fraction may not always be feasible while trying to keep low ECD. Meanwhile, injection of gas has positive effects on cuttings transportation depending on the flow patterns and drilling mud viscosity. Elevated temperature causes a significant increase of bed thickness and it is important to recognize this negative effect, especially when drilling HTHP wells. The effect of pressure on cuttings concentration is negative. Larger size and heavier cuttings make hole cleaning more difficult and require higher pump rate for low viscosity fluids. Increases of liquid phase density results in better hole cleaning. The range of hole angles from about 35 – 60 deg (from vertical) is the most difficult for cuttings transport. Frictional pressure losses in a deviated wellbore highly depend on cutting bed thickness. Simulation results are compared to available experiment data and show good agreement. In summary, this paper presents a model which is useful for practical hole cleaning during UBD.