Summary

This paper is an extended work of SPE 90038. An aerated non-Newtonian flow and an inclined wellbore section were added to the previous model to study the hole-cleaning problem while drilling an underbalanced well. This new mechanistic model for cuttings transport was 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. The 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 a low equivalent circulating density (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 high-pressure/high-temperature (HPHT) wells. The effect of pressure on cuttings concentration is negative. Larger size and heavier cuttings make hole cleaning more difficult and require higher pump rates for low-viscosity fluids. Increases of liquid phase density result in better hole cleaning. The range of hole angles from approximately 35° to 60° (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 that is useful for practical hole cleaning during underbalanced drilling (UBD).

Introduction

In drilling operation, a cuttings bed normally forms in horizontal or inclined sections when annular mud flow rate cannot prevent the cuttings from depositing. As bed thickness grows, mean annular velocity and wall shear stress increase until an equilibrium condition is reached. Further increase of flow rate erodes the cuttings bed and creates a new equilibrium. Previous study on cuttings transport with aerated mud in a horizontal wellbore at simulated downhole conditions is published in SPE90038. In that paper, both experimental and modeling studies were presented. Experiments were conducted in a unique, full-scale flow loop at simulated downhole conditions. A mechanistic model with aerated Newtonian fluids was developed for predicting cuttings concentration in the annulus. Only very few experimental studies of two-phase non-Newtonian flow are reported in the literature. Practical prediction methods are also fairly limited. Eisenberg and Weinberger (1979) derived a model for annular gas/non-Newtonian flow. A predictive model for stratified gas/non-Newtonian flow in a horizontal pipe has been given by Heywood and Charles (1979) and partially validated by Bishop and Deshphande (1983). Shu (1981) presented models for power-law fluids for several flow regimes. Dziubinski (1986) correlated a large amount of pressure drop for power-law fluid flow in a horizontal intermittent flow. In the study done by Kaminsky, he assumed the pressure drop of two-phase flow is dominated by the liquid contribution, which is the case expect for very low liquid holdup systems. Larsen (1990) conducted an experimental study to determine the critical flow velocity for cuttings transport in an inclined wellbore for single-phase drilling fluids. The effect of hole angle, mud rheology, annular velocity, cuttings size, mud weight, pipe eccentricity, and drillpipe rotation were investigated experimentally. Sharma et al. (2000) presented a steady-state model to study gas-liquid-solid mixtures flow in conduits. The liquid phase can be comprised of a mixture of Newtonian and non-Newtonian fluids. Gas can exist in a free state as well as dissolved in the liquid phase. The solid phase is assumed to be fully suspended in the multiphase flow mixture. Sunthankar (2002) conducted aerated non- Newtonian flow experiments in a full-scale flow loop. Water and CMC were used as the liquid phase and air as the gas phase. Flow patterns and pressure drop predictions were made and compared with his experimental results. Mendez (2002) did an experimental study of cuttings transport in horizontal wells with aerated fluids and drillpipe rotation. Polymeric fluid and water were used as the liquid phase and air as the gas phase in the test. Cuttings bed height in a horizontal section was recorded visually and a semiempirical model was presented.

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