Reverse circulation has always been viewed with some level of suspicion and reverse circulation with coiled tubing has often been avoided at all costs. Yet, results of over 1600 CT operations involving reverse circulating in Alaska's oil fields, with just a handful of failures, has proven reverse circulation to be a reliable procedure for a specific range of solids removal that is extremely effective in removing sand fill from wells with very large tubulars and in high deviations. Results from North Sea, Colombia, California and the US GoM are also included. This paper presents the basics of reverse circulation designed specifically for coiled tubing; including necessary surface equipment, BHA nozzle design, pump requirements, fluid rates, and procedures to reduce the potential for problems. The procedures and job overviews contained here were contributed during an ICOTA panel discussion on reverse circulating, held September 18, 2003, in Houston, TX.
Reverse circulation utilizing coiled tubing provides many opportunities to improve interventions, lower cost and reduce operation time. As with any process, there are both risks and rewards. Reverse circulating is an accepted method of fluid handling in certain operations such as drilling1, cementing2–5, pickling, free protection and fracturing,6 however, the accepted operations commonly utilize jointed tubing. Even some experienced coiled tubing users are not aware that reverse circulating through coil is a common process and sufficiently widespread to have gained exposure in most parts of the industry. The results in this paper cover approximately 1600 jobs in Alaska, over 28 wells in the North Sea, 36 jobs in Colombia (deepest is over 16,500 ft), 4 jobs in California, and several in the US GoM. Of these wells, problems with reversing with limited to 3 collapses and 2 pinhole leaks. No loss of well control was experienced and no coil was stuck in any of the wells that were reversed.
Interestingly, when comparing coil reverse circulating with forward circulation cleanouts, risk of sticking the coil in the well is virtually eliminated when using reverse circulating.
Increasingly, coiled tubing services are called upon to deliver well work in more demanding areas. CT has carved out a large niche of common use for well cleanouts, well kick-offs, and fluid placement. Traditionally, the mode of use for these operations involving CT has been forward circulation, i.e. down the coil and up the coil/production tubing or coil/casing annulus. In many cases of large casing, heavier fill, deep wells, low pressure wells and increasing well deviation, normal circulation does not provide enough circulation rate to effectively lift particles in the annulus. In many of these cases, the smaller diameter coil available for a platform with crane lifting limits, access weight restrictions, local availability or other limits, is not capable of delivering the necessary threshold cleanout rate or the cost of horsepower to overcome friction is too high. In these cases, reverse circulation opens up potential for CT application, provided that the risks associated with reversing through coil can be managed.
One specific example from the North Sea highlights the capability of reverse circulating with CT in a well where conventional cleanout, if even possible, would have been extremely high risk. In a 60° deviation, very low pressure well with 9–5/8" casing and a 3–1/2" tail pipe, fill was covering a depleted zone and a potential lower pay zone. Reverse circulating with 1–3/4" coil removed the debris and uncovered the zone, allowing perforating and a return to production.