Quantitative Evaluation of Critical Conditions Required for Effective Hole Cleaning in Coiled-Tubing Drilling of Horizontal Wells

The problem of solids cleanout in horizontal wellbores was studied experimentally. The special case of drilling-fluid circulation with no inner-pipe rotation was considered. This case is similar to coiled tubing (CT) drilling in which frequent hole cleanout must be performed. Sand-sized cuttings (ranging from 260 to 1240 mm) were used. Critical velocity and wall shear stress required for starting bed erosion were measured. Water and viscous-polymer base fluids with three different polymer concentrations were used. Results have shown that water always starts cuttings movement at lower flow rates than polymer solutions. Fluids with higher polymer concentration (and higher viscosity) required higher flow rates to start eroding the bed. Critical wall shear stress was also determined from pressure-loss measurements. Analyzing the data revealed that water starts cuttings removal at lower pressure loss than more-viscous fluids. Higher-viscosity fluids always showed higher pressure loss at the start of bed erosion. For the range of cuttings size studied, results show that an intermediate cuttings size was slightly easier to remove. However, the impact of cuttings size was far less than that of fluid rheology. Overall cuttings size was found to have a small impact on hole cleaning. Dimensionless analysis of parameters relevant to the process of cuttings movement was performed. It was shown that dimensionless wall shear stress (in the forms of Shields' stress and also ratio of shear velocity to settling velocity) at the onset of bed erosion correlated well with particle Reynolds number. On the basis of this finding, two correlations were developed to predict critical wall shear stress. A procedure was developed to calculate critical flow rate as well. Friction-factor data for the flow through the annulus with a stationary cuttings bed are also reported.