Wellbore Cleanout in Inclined and Horizontal Wellbores: The Effects of Flow Rate, Fluid Rheology, and Solids Density
- Soham Pandya (University of Oklahoma) | Ramadan Ahmed (University of Oklahoma) | Subhash Shah (University of Oklahoma)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 2020
- Document Type
- Journal Paper
- 48 - 68
- 2020.Society of Petroleum Engineers
- directional wells, cleanout, density, solids, well inclination
- 41 in the last 30 days
- 173 since 2007
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Coiled tubing (CT) is extensively used in wellbore-cleanout operations to remove solid particles such as drilled solids or residual proppant from hydraulic-fracturing treatments. The cleanout operation is often associated with nonproductive time and significantly adds to the operational cost. This study is aimed at developing a model to predict the required fluid-circulation rate and time to efficiently remove solids from the wellbore and optimize the cleanout operation.
To study the hole-cleaning mechanism, solids-bed-erosion experiments were conducted using a flow loop that has a 10.4-m-long annular test section. The effects of solid density, flow rate, inclination angle, and fluid type (Fluids 1, 2, and 3) on the cleanout operation were investigated by measuring bed erosion and hole-cleaning efficiency. During the experiment, a stable bed was initially formed in the annular test section. Then, the bed was eroded and the amount of solids removed was measured to determine cleanout efficiency. In addition, bed height was measured at different locations in the test section as a function of circulation time. The measurements resulted in the generation of a bed-erosion curve (plot of average bed height vs. circulation time) for each test. The erosion curves obtained from this study are consistent with previously reported measurements.
This study indicates that flow velocity, inclination, and fluid rheology are the primary factors in determining hole-cleaning efficiency. Bed erosion is highly prominent at high flow velocities, especially with low-viscosity fluids, because the increase in velocity results in strong turbulence and the lifting of bed particles. Alternatively, an increase in viscosity reduces particle settling and maintains particles in suspension once they enter the flow stream. This is attributed to a greater drag-force generation, which facilitates particle transport by hindering redeposition. However, an increase in viscosity limits the particle-lifting capacity of the fluids by affecting the near-bed velocity profile, which is critical for lifting bed particles. Therefore, it is recommended to use low-viscosity fluids in cleanout operations at high-inclination well sections, and vice versa. Solids density has a moderate (up to 30%) effect on cleanout efficiency compared with fluid velocity and rheology. For different solid densities at a constant flow rate, high-density solids make hole cleaning much more challenging, especially in nearly horizontal well sections. There is a critical angle of inclination between 64 and 69 at which the cleanout efficiencies of all fluids are approximately equal, irrespective of the flow rate. The outcomes of this investigation can be applied in the field to optimize cleanout operations in horizontal and highly deviated wells.
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