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Summary 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 µm) 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.

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Fully developed turbulent flow of drag reducing fluids through a horizontal flow loop with concentric annular geometry was investigated using the particle image velocimetry (PIV) technique. Experiments were conducted at solvent Reynolds numbers ranged from 38,700 to 56,400. Axial mean velocity profile was found to be following the universal wall law close to the wall (i.e., y+ < 10), but it deviated from log law results with an increased slope in the logarithmic zone (i.e., y+ > 30). The study was also focused on turbulence statistics such as near wall Reynolds stress distribution, axial and radial velocity fluctuations, vorticity and turbulent kinetic energy budget.

A CFD simulation was conducted to analyze the near wall turbulence characteristics of a drag reducing (DR) polymer fluid (0.12% V/V) flow through concentric annulus. The continuity and momentum equations were solved by using a commercial CFD package (CFX 14) with the Shear-Stress-Transport (SST) model option. The simulation results were compared to the experimental data obtained by using high resolution Particle Image Velocimetry (PIV) analyses of drag reducing polymer fluid flow in a horizontal concentric annulus. A fully developed turbulent flow of water through a horizontal flow loop (ID = 9.5 cm) with concentric annular geometry (inner to outer pipe radius ratio = 0.4) was used for comparison purpose. The flow rates ranged from 3.92 to 5.95 kg/s. Drag reducing PHPA solutions behaved as a power law fluid with the rheological model (μ = Kγn−1) for the shear rate of 1/s to 600/s. Bulk and near wall velocity profile obtained from simulation showed good agreements with the experimental data. Drag reducing polymer reduce the Reynolds stresses level due to weaker and fewer turbulent eddies formation near the wall. Results of the simulation study also showed that if the flow rates of power law fluid increased from 3.92 to 5.95 kg/s, the drag reduction in the annuli is increased from 10% to 20% compared to water case indicating the strong damping to turbulent kinetic energy in the flow. The CFD analyses using SST model is computationally inexpensive and, therefore, can be conveniently used for investigating the flow characteristics of drag reducing polymer fluids in concentric annulus.