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Hydraulic parameters affecting cuttings transport for horizontal coiled tubing drilling

Kelesidis Vasilis, Mpandelis G. E.

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Summary

A niche market for the exploration and development of oil and gas fields is drilling horizontal wells withcontinuous tubing, the coiled tubing. Among the many challenges that require engineering solutions is a fluidmechanics challenge, the effective transport of the solid particles from downhole to the surface. This is achievedwith fluid circulation through the annulus and the most difficult section is the horizontal section of the well.Fluids used are normally non-Newtonian and analysis indicates that the fluids should have the right rheologicalproperties and should flow at the required flow rates.Liquids with appropriate rheological properties and flowing at sufficient flow rates carry the solids, whichnormally move on the bottom side of the annulus. If conditions are not right, the solids form a bed, which canbecome stationary thus making progress in drilling difficult and requiring frequent and expensive cleaningactivities.In this paper we present first results of the ongoing research project aiming at shedding additional light into theparameters affecting cuttings transport in horizontal wells. Experiments were performed with water and aqueoussolutions of Carboxy-Methyl-Cellulose (CMC) in a recirculating flow loop with a non rotating concentricannulus. The characteristics of the flow patterns and the particle - liquid interactions at the various flow ratesthat have been visually observed are presented and described in detail and the issues that they have arisen fromthis research are discussed. Particles were glass beads with diameter of 2mm and density of 2,59g/cm3 and withsolids concentration of 0,4 to 0,6% w/v.The most significant parameter for no bed formation is flow rate. At low liquids rates particles form a movingbed which can be eroded if the flow rate is increased. If conditions are right, accumulations of solids (“blobs”)are formed, about 20cm long and covering almost half the height of the annulus. Once a critical height isreached and the velocity of the liquid above the bed is sufficient, it is eroded and the whole ‘blob’ is moved, byerosion, further along the annulus. The erosion velocity is higher than the velocity predicted using Kelvin –Helmholtz instability theory. At higher flow rates, but not sufficiently high for full solid suspension, the solidsdo not deposit on the wall but flow in streaks near the bottom wall of the annulus. A plausible explanation forthe solids staying in the low speed region is that they are held there by the turbulent sweeps known to exist nearthe wall in turbulent flow.No solids are observed in most of the annulus flow area for all flow patterns studied, thus pointing out to the factthat the solid - diffusion equation, normally used to describe the phenomenon in slurry transportation, should notbe used, at least for the conditions of this research

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