Casing exits (the process of exiting from the original or main bore casing and creating a secondary wellbore) typically use a whipstock assembly that directs a mill toward the side of the existing casing where an exit point is then milled. When creating multilaterals, however, conventional whipstock assemblies must be retrieved from the hole to regain main bore production. This process requires extra trips into the well, adding time and risk to the operation. Furthermore, in removing the whipstock, a bent joint becomes necessary to guide the lateral liner into the lateral hole. That joint remains at the bottom of the lateral, creating additional torque and drag during lateral liner run-in. With extended-reach multilaterals, the resulting increase in torque and drag can greatly increase the challenges in reaching intended depths with the lateral liner assembly.
Reducing time, cost
Among the newer advances in reentry technology is the StarBurst Multilateral System that uses a hollow whipstock and packer assembly for window milling prior to lateral drilling and liner running and hanging. The hollow design of the whipstock tool allows it to be left in place, eliminating the need for one or more trips back into the well before the liner is run, a process that can take many hours or even days. Eliminating these whipstock retrieval trips also ensures operators can install the liner quickly after the hole has been drilled, minimizing the possibility of the liner sticking prematurely.
In addition to time and trip reductions, the hollow whipstock provides a reliable deflection point to guide the lateral liner through the casing exit and down the hole, eliminating the need for a bent joint and thus reducing problematic torque and drag. The technology can be used alone or in conjunction with torque and drag reduction tools such as the LoTAD Mechanical Friction-Reduction System.
With the hollow whipstock and packer installed and the casing exit created, the main borehole is completely isolated, while allowing access to the lateral for drilling, liner running, and completion endeavors such as fracing. Production from the main bore can be reinitiated by perforating the liner and whipstock face in a single trip using a perforating gun run in on either tubing or wireline. The shallow angle of the hollow whipstock face, 2 degrees versus the conventional 3 degrees or more, provides a longer window, which facilitates a gentler bend at the window exit, reducing the possibility of excessive bending loads on the drillstring that can cause tool failures, stall-outs, and excessive friction.
The latest generation of the multilateral system takes only a single trip to install and mill the window for 9 5/ 8 -in. casing. The system was designed primarily for multilateral applications, but it also has been used to perform sidetracks in extended-reach wells (ERWs) that proved problematic for conventional whipstock assemblies. The technology has been deployed successfully in more than 60 operations globally, with about 75% of installations in offshore environments. The hollow whipstock system was used to create two separate multilaterals in a well in the Gullfaks field offshore Norway, originally drilled in 1989. Due to concerns about formation damage, the main bore could not be exposed to drilling fluids, so a timer plug was installed and programmed to open after the hollow whipstock had been deployed at a measured depth (MD) of 1,372 m (4,500 ft). The lateral produced for six months, after which the hollow whipstock was perforated and the main bore returned to production.
A second casing exit sidetrack was later created in the 7-in. lateral casing using another multilateral system. For this smaller lateral creation, a 6-in. thru-tubing drilling system was used to drill the lateral 1,097 m (3,600 ft) MD in the opposite direction of the first lateral, after which a 5-in. liner was run and cemented in place. The second lateral produced with better-than-expected results. Eventually this second hollow whipstock was also perforated, allowing production from all three zones going forward.
Combining technologies
The combination of advanced whipstock technologies and mechanical friction reduction tools can be particularly effective in improving milling efficiency and reducing risk in long ERWs where excessive torque and drag can make casing exits difficult. The combination can reduce torque and drag while milling, moving casing and tools, and applying the necessary weight to set tools on the bottom.
This approach proved effective in an extended-reach land well in Russia that had been drilled to a total measured depth of more than 7,925 m (26,000 ft). In this case, a shallow angle whipstock, similar in dimensions to the hollow whipstock design, and a hydraulic anchor were deployed to sidetrack and drill an 8½-in. lateral from 7,895 m (25,902 ft) MD.
Due to the extreme depth of the well, 160 sub-based friction reduction tools were initially placed in the drill-string at intervals from 570 m to 4,690 m (1,870 ft to 15,388 ft) to ensure sufficient slack-off weight to set and then shear off the mill from the whipstock. The whipstock /packer assembly was installed at almost 7,925 m (26,000 ft), and a casing exit window, 5.5 m (18 ft) in length, was then milled. In the second planned milling run, 15 m (49 ft) of rathole was drilled, after which an 8½-in. hole was drilled with 150 friction reduction tools on the drill-string at initial depths of 3,735 m to 7,850 m (12,255 ft to 25,756 ft), achieving the world's deepest horizontal casing exit and lateral.
Nearly half of the hollow whipstock systems deployed to date have been installed in the North Sea. Additional operations have taken place in Asia, primarily the South China Sea, and Alaska.
Although these systems have primarily been deployed offshore or in high-expense locations, there is potential for further application of the systems in older brownfield wells, where laterals can be drilled off existing main bores. Doing so will capture additional behind-pipe reserves to supplement the marginal production of the older, main bore reservoirs.
Reference: Portions of this article appeared in SPE-SAS 1258 Inaccessible Drilling Targets and Casing Exits Made Possible by the Alleviation of Excessive Torque and Drag, John McCormick and Cliff Hogg, Weatherford International Ltd., SPE
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