Cuttings Transport Problems and Solutions in Coiled Tubing Drilling

Abstract

Abstract Coiled tubing (CT) drilling has grown tremendously in the last few years. In spite of the many technological advances that have accompanied this growth, one significant remaining challenge is effective cuttings transport, particularly in deviated wells. This paper presents a summary of cuttings transport problems and current solutions (including fluid selection, flow rates, and operational techniques such as short trips and pumping viscous slugs). 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. Several detailed case studies are presented that illustrate both severe cuttings transport problems and routine applications without cuttings transport difficulties. The proposed hole cleaning models are used to interpret these data and suggest possible alternative approaches. Hole cleaning relying on viscous fluids in laminar flow has proved to be inefficient because of the inability to rotate the string to agitate bedded cuttings. Alternatively, a high flow rate for turbulent hole cleaning is more effective, but difficult to achieve because of high friction pressures in the coiled tubing. Therefore, a cuttings bed is almost always present in CT (or any slide) drilling. This has a major operational impact resulting from required remedial activities, increased risk of stuck pipe and the inability to attain the desired reach. 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 using Stokes' law and the local viscosity while flowing. This analysis may be easily 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 openhole horizontal annul us 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 called AVRD is introduced, which is the product of the AV and the square root of the hydraulic diameter. This term should be used to compare cuttings transport in turbulent flow in horizontal wells of different cross sectional areas. P. 7