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Cuttings-Transport Problems and Solutions in Coiled-Tubing Drilling

In spite of the many technological advances that have accompanied the growth of coiled-tubing (CT) drilling, one significant challenge remains-effective cuttings transport, particularly in deviated wells. This paper presents a summary of cuttings-transport problems and current solutions. It is shown that, in many circumstances, hole cleaning is more efficient if a low-viscosity fluid is pumped in turbulent flow rather than a high-viscosity fluid in laminar flow. Case studies are presented that illustrate both cuttings-transport problems and routine applications without cuttings-transport difficulties. The proposed hole-cleaning models are used to interpret these data and to suggest possible alternative approaches. Two novel approaches to understanding hole cleaning are introduced. First, for laminar flow, the distance that a particle will travel (downstream) before it falls across the annulus clearance is calculated with Stokes' law and the local viscosity while flowing. This analysis may easily be applied to optimize mud selection and wiper trips. Applying this model to high low-shear-rate-viscosity (LSRV) gels shows that they may perform well inside casing but are expected to do a poor job of hole cleaning in a narrow, open-hole, horizontal annulus without rotation. Second, for turbulent flow in horizontal wells, the concept of using annular velocity (AV) as a measure of hole cleaning is shown to be insufficient. A more complete term, annular velocity/root diameter (ν A R D ), is introduced and should be used to compare cuttings transport in turbulent flow in horizontal wells of different cross-sectional areas.