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Evaluation of well geometry shows that 30 percent to 80 percent of casing pipe and well cement is spent to cover lost-circulation zones and rockslides. Though such zones are relatively weak (20-100 meters), casing strings used to cover them run for 500 to 4,000 meters or more. For this reason oil producers worldwide adopted zonal isolation technology with a telescopic arrangement of casing strings; the projects are designed with due consideration for covering all different-pressure zones and rockslides, which should ensure well drilling to the target depth with the minimum quantity of protective casing strings. Yet for difficult wells, particularly for exploration drilling, the technology is often out of options even at 4,000- to 5,000-meter depths – a well is brought up using small-diameter bits, which complicates drilling and fastening and severely limits the options for testing and subsequent production.
Traditionally, the technology used to address these issues is linked to the desire to increase the length of the uncased borehole section through constant regulation of drilling fluid density and its rheological properties. This approach leads to frequent oil-water-gas blowouts, circulation loss, rockslides, stuck drilling tools, and sometimes to the loss of the entire downhole. Casing quality of such segments also suffers, leading to behind-the-casing leaks and ultimately to negative consequences during well operation.
Also, some wells have to be abandoned or re-drilled – because there is currently no technology to isolate the unexpected trouble zones, that is, there is no protective casing strings for their isolation. Considering the above, designed in Tatarstan unconventional technology for local well casing by extensible-diameter dedicated casing pipes – without reduction of the well diameter and without cementing – has a particular significance for projects on complex and deep wells.
The researchers have provided the science behind the manufacturing of all equipment and hardware required for the local well casing. For serial production of extensible-diameter casing strings (dedicated cover segments), Perekryvatel company (a part of Sistema-Servis group of companies) installed a factory in Aznakaevo city, Tatarstan. For this project the company completed the entire set of works for design and manufacturing of the drawing-profiling mill (100-ton stretch force), designed a large range of non-standard equipment, improved the technological process of manufacturing the shaped tubes, developed the high-performance technology of induction heating of the pipes in annealing, and modernize the pipe shaping units. The result of this work is the complete solution to the problem of isolating the circulation loss zones, regardless of the intensity of the absorption and cavern porosity levels. Over 1,400 casing strings have been used instead of intermediate strings and tie-back casing.
A number of 3,000-4,000-meter deep wells (in Lithuania, Bratsk, Lena expeditions, Yamalneftegazgeologia, Orenburgneft) were doomed to abandonment and re-drilling as there was no technology to isolate the problem zones. All of them have been reworked using the equipment and technology of the local well casing and brought up to the target depth.
The high reliability of the local well casing technology and equipment for the fixing of local zones is proven by the fact that almost all the wells where the method was used were brought up to the target depth regardless of the number of tripping operations or drilling volume. For example, after fixing the thief zone of the Orenburgneft’s well No. 2602 with dedicated cover strings in the interval 3,294-3,321 meters, the company used rotary system to drill 1,167 meters within 1,008 hours, with 79 round-trip operations. There was no problem in the casing strings zone.
In February 2012 operator carried out a unique project on isolation of several problem zones using dedicated cover strings on well No. 2 at Muravlinskaya site in Saratov region. The well was drilled by “NewTech Services” company.
The well geometry is shown on Fig. 1. Main problems occurred on drilling from under the 245 mm diameter column in the segment 4,000-4,113 meters. In the segment 4,003-4,006 meters, mechanical speed grew to 17 meters per hour, water-gas blowouts started to appear with γ– 1.14 g/cu. cm. The segments 4,022-4,037 meters, 4,070-4,078 meters and 4,087-4,113 meters included the increase of the drilling speed, tool drops and circulation loss leading sometimes to a complete loss of circulation.
Reworking of the geological complication required 2,500 hours and 2,537 cubic meters of drilling muds, including 466 cubic meters of well clogging packs. During the liquidation of the absorption some 2,126 cubic meters of drilling fluid was lost.
After a large amount of isolation operations and the lack of a positive result, experts of “NewTech Services” and “Perekryvatel” decided to cover the problem zones  with a dedicated cover strings OLKS-216 S (local well casing hardware, diameter 216 mm, with welded connection of the shaped pipes).
Isolation operation was successful after revision of the segment for installation of the cover casing string in line with the geophysical survey (Fig. 2). Some 124 hours were spent on technology operations for installing 124.5 meters of local well casing in the segment 3,991.5-4,116 meters – without reducing the diameter of the hole and without cementing. This equals five percent of the total time spent on isolation of this segment using different types of plugging material.
It is clear that know-how and equipment for local well casing is a very reliable and efficient technology brought by Tatneft’s experts working in well installation field; using modern expertize, it cuts down consumption of energy and materials, in parallel reducing the target time of well construction.  The technology and hardware for local well casing with shaped pipes received over 45 patents in Russia and 59 – overseas. Developments in this field were awarded golden medals at the fourth International Salon of Industrial Property “Archimedes-2001” in Moscow and at IMPEX-17, the largest American exhibition of inventions that took place in May 2001 in Pittsburgh, Pennsylvania. In 2012 the invention was recognized with a silver medal of “Archimedes- 2012” for a modernized and more reliable hardware for installation of dedicated cover strings into the hole.
We hope that drilling and oil and gas companies wouldn’t keep on regardless of minor glitches, which tend to become serious problems with time. Simply provide project time for local well casing with extensible-diameter shaped tie-back casing.  
REFERENCES
1. Abdrakhmanov G.S. Well Casing by Expandable Pipes: Training Manual for Higher Education Institutions. Samara: ROSING Publishing House, 2003. 228 p.
2. Takhautdinov S.F., Ibragimov N.G., Abdrakhmanov G.S. et. al.  “International Recognition.” Neftegazovaya Vertical (analytical magazine), No. 12, 2003. p. 57-59.

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