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There are cost advantages to reaming while drilling. The pilot hole offers good directional control while the enlarged hole from reaming creates the right conditions for running casing or completion equipment which relates to savings in time and cost.
Types of reamers
An underreamer is device designed to run in conjunction with a drill bit. It has cutters that can be expanded or contracted by mechanical or hydraulic means and used to enlarge or ream a borehole below the casing. Baker Hughes and Statoil jointly developed the industry’s first on-command digital reamer as a single size prototype in 2007. The prototype was exclusively used in the Norwegian and UK sectors of the North Sea. The new remotely controlled hydraulic-electric reamer was developed using hydraulic blade activation, which was controllable from the surface.
Roller reamers significantly improve performance with chronic stick-slip or whirl-induced borehole which commonly limit drilling in hard formation and extended reach wells. Historically roller reamers are also used for conditioning the borehole.
An RFID reamer uses radio-frequency identification (RFID) technology to active the reamer. The electronically activated reamer provides increased flexibility for hole-enlargement-while-drilling (HEWD) operations. Weatherford’s RipTideTM RFID tags are inserted into the drillpipe ID at surface level and carried downhole in the drilling fluid. An electronic reader on the tool’s controller interprets instructions embedded in the tags to unlock the controller, thereby permitting the cutter blocks to extend fully from the reamer body. A surface-level pressure gauge confirms that the tool is in the open position. RFID features enables operators to activate and deactivate the tool at any time while drilling or tripping. Specific benefits RFID technology offers include:
- Prevents activation of the cutter blocks during jarring operations, significantly reducing the risk of getting the reamer stuck in the hole and incurring nonproductive time as a result;
- Helps protect casing and the tool’s cutting structure from damage while cleaning the hole at full circulation and rotation;
- Permits running of multiple drilling reamers in tandem, all of which can be independently controlled.
Near Bit Reamer (NBR) can be used to drill over sized holes. It is designed to enlarge the hole while drilling. They have been used in both conventional mud motor and rotary steerable directional assembles.
Fixed blade reamers
Bi-center reamer bit
A bi-center bit is composed of a pilot bit and a reamer. The reamer extends from one side of the bit so that when the pilot hole is drilled the hole is immediately reamed to a larger diameter. 
Expandable reamers were developed to improve efficiencies and reduce risk while drilling through problematic formations in deep water and other offshore wells, where wellbore stability is a major challenge. The first concentric expandable reamers were ball-activated. Once expanded, they could not be closed without stopping circulation. The second iteration was modified so that the reamer could be deactivated to allow circulation after reaming for better hole cleaning. The advantages of the concentric expandable reamer over its mechanical-arm predecessors included the attainment of a larger borehole size that could accommodate larger bottomhole assembly (BHA) components, flexibility in pilot bit selection, higher flow rates, and the ability to ream a previously drilled hole or to back ream a hole after drilling. The drawbacks to traditional expandable underreaming systems. Activation cannot be confirmed. Additionally, the placement of the reamer at 100 to 300 ft above the bit means that the equivalent portion of the hole, known as a rathole, cannot be enlarged without a dedicated trip that typically requires 1 to 2 days of rig time at a daily cost ranging from USD 1 million to USD 2 million.
Matching bit and reamer
Reamers and bits should be matched for cutter size and aggressiveness (depth of cut) to balance loads more closely while drilling through homogeneous formations. For example, if the drill bit out-drills the reamer, some inner formation stresses may have been released by the time the reamer starts drilling a new formation. In this situation, there is no WOB and the weight is transferred to the reamer, which can damage the cutters. Additionally, with no WOB, lateral and whirl vibrations can damage the lower BHA and quickly compromise wellbore stability. Matching the bit and reamer for aggressiveness can alleviate or eliminate these risks.
- Al-Essa Ahmed. Digital Reamer Enhances Drilling Efficiency, Economics, and Safety. Journal of Petroleum Engineering. http://www.spe.org/jpt/article/8736-technology-update-1-11/
- Sowers, S. F., Dupriest, F. E., Bailey, J. R., & Wang, L. 2009. Use of Roller Reamers Improves Drilling Performance in Wells Limited by Bit and Bottomhole Assembly Vibrations. Society of Petroleum Engineers. http://dx.doi.org/10.2118/119375-MS.
- "Weatherford RipTide Reamer." Weatherford. http://www.weatherford.com/doc/wft115705 Cite error: Invalid
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Noteworthy papers in OnePetro
Alcure, M., Placido, J. C. R., Rasheed, W., & Lage, A. C. V. M. 2013. Smart Reamer Calliper - Real Time Wellbore Diameter and Underreaming Verification. Offshore Technology Conference. http://dx.doi.org/doi:10.4043/24334-MS
Miller, G. C., & Childers, R. D. 2003. Innovative Concentric Reamer Enhances Deepwater Drilling Operations. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/84247-MS
Warren, T. M., Sinor, L. A., & Dykstra, M. W. 1995. Simultaneous Drilling and Reaming with Fixed Blade Reamers. Society of Petroleum Engineers. http://dx.doi.org/doi:10.2118/30474-MS
"How An RFID Drilling Reamer Works." Rigzone. http://www.rigzone.com/training/insight.asp?i_id=351