ABSTRACT:

Removing various types of unwanted fill from well bores is one of the major applications of coiled tubing (CT) today and has been for several decades. The fill can be removed by circulating a cleaning fluid down through the CT and taking returns from the well bore via the completion and CT annulus - conventional circulation. Alternatively, the cleaning fluid may be pumped down the CT and completion annulus with the returns being delivered up the inside of the coiled tubing - reverse circulation. Reverse circulation offers some advantages over conventional circulation, especially in larger completions where annular flow rates may be too low to transport the solids material to surface. The smaller internal diameter of the CT allows a higher return fluid velocity to be maintained. Reverse circulation also has attractions for the offshore environment, as smaller diameter CT can be employed to achieve the cleanout when compared to a conventional cleanout solution. The smaller diameter CT means that the coiled tubing reel's overall weight will be lighter, allowing rigs and platforms with restricted crane capacities to lift the load onto the deck from a supply vessel. The challenges associated with reverse circulation are related to: the selection of the fluid system; the speed at which penetration should be made into the fill material to avoid burying the CT and becoming stuck, especially in deviated wells; how to cope with a leak in the coiled tubing at surface as check valves are not be used; collapse pressure being applied to the coiled tubing; losses to any open formation; and internal erosion of the CT. This paper will present some new full-size laboratory studies that have been conducted to assess the relationships between particle size, cleaning fluid, deviation and rate of penetration when reverse circulating. The other concerns surrounding these types of operations will be addressed by reviewing some practices, tools and techniques that have become available in the last few years, together with job design considerations

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