Kostryba Ivan Vasilievich
- Associate Professor of the Department of Oil and Gas Equipment, PhD in tech. sciences, tel. +380662370309

Mosora Yuriy Romanovich
- Assistant of the Department of Oil and Gas Equipment,
tel. +380958601575, e-mail: yura_mosora@ukr.net

Due to the annual increase in the number of oil and gas wellsites, the labor intensity, cost and duration of work are increasing to keep them in good working order. Accordingly, the number of technical equipment necessary for the performance of these works, the costs of their acquisition and maintenance, are increasing. All this negatively affects the cost of production, so the need for significant changes in the methods, technologies and technical means used to develop, maintain and repair oil and gas wellsites is an urgent problem.

The development and repair of wellsites according to classical technologies with the damping of the reservoir is a long-term process, requires a complex set of equipment, considerable amount of time, labor and resources. Its main drawback is the high probability of clogging of the porous space of a reservoir with all the ensuing consequences. In addition, it is not always possible to obtain a positive result of the performed work. There are often cases when the productivity of a repaired wellsite through damping turned out to be less than before repair, even more often it takes a fairly long period of time, during which the wellsite returns to work in a stable mode. This is especially true for a large operational gas wellsite stock with an abnormally low reservoir pressure.
The above mentioned drawbacks of the wellsite repair technology with the previous damping are practically eliminated if a snubbing technology is introduced that allows repairing the wellsites under pressure without damping.
Attempts to repair wellsites in the conditions of drawdown on the reservoir were known almost 100 years ago. In 1920, G. Otis (USA) proposed an installation with a special tackle system for conducting round trip operations under pressure. However, the technical level of the means for implementing the snubbing technology at that time did not meet its requirements.
Snubbing technologies came into common use in the 60s-70s of the last century. This became possible due to the solution of such complex technical problems realized in modern snubbing installations.
Firstly, a round trip complex was created on the basis of power hydraulic cylinders (hydraulic lift), capable of ensuring the descent of the pipe string into the wellsite both under its own weight and forcibly. The traditional round trip complex, based on the principle of operation of the polyspast mechanism, can not provide forced descent.
Secondly, production of highly reliable blowout wellhead equipment, in particular ram and universal preventers, which are capable of guaranteeing fountain safety in the process of wellsite repair, has been mastered. Modern snubbing installations are equipped with two blocks of preventers: the operating, through which the pipe string under pressure is extended, and emergency ones, which are in the operational readiness mode. Modern designs of universal preventers are able to pass under pressure through the sealing element several thousands of couplings, tens of thousands of meters of smooth pipe. The variants of schemes of the wellhead equipment layout for the repair of wellsites under pressure are shown in Figure 1.
Thirdly, thanks to the development of such a direction as hydraulic machinery construction, it was possible to equip modern snubbing installations with a reliable and highly efficient control system, which guarantees a high level of safety of works under pressure, including fountain safety.
The control system of preventers is equipped with two power sources: from hydraulic pumps and hydropneumatic accumulators. The ram preventers have a manual fixing of the rams in the closed position, which is used for planned and unforeseen stops in the plant operation.
The control system, completeness and design of the equipment of the snubbing installation have wide and sufficient possibilities to maintain control over the wellsite in case of unforeseen situations: depressurization of the pipe column, pipe blowout devices; faulty spiders; failure of preventers; failure of the power unit of the installation, hydropneumatic accumulators; depressurization of hydraulic pipes to control the equipment of the installation; fire; oil spills, etc.
The availability of protection and interlocking systems made it possible to minimize the risks that occur when working under pressure.
Is the oil and gas industry of Ukraine ready for widespread introduction of snubbing technologies? This question is debatable, but I would like to draw attention to the following.
1. Presence of the legal and regulatory framework for the use of snubbing technologies. The current safety rules in the oil and gas industry of Ukraine provide for repair work in wellsites under pressure using coiled tubing units, as well as wire-line technique (lubricating wellhead equipment). Although it should be noted that this document does not categorically deny the work on full and current well repairs without damping at excess pressure at the wellhead (Section 6, paragraph 3.1), it only limits its use to coiled tubing installations, which in many respects are inferior to the snubbing and by themselves do not guarantee fountain security. In addition, at the time of implementation (since 01.08.2008) of the Rules, awareness in Ukraine about the practical implementation of snubbing technologies was low.
Nevertheless, the new version of the Safety Rules for the Oil and Gas Producing Industry of Ukraine, which is posted on the website of the State Service for Labor in Ukraine for discussion and is at the final stage of revision, contains a norm on the possibility of using snubbing technologies in Ukraine.
Also, there should be noted the need to develop appropriate technical regulations at the level of the SUC on a wide range of issues, requirements and norms related to the repair of wellsites using snubbing installations (preparatory work before pressure repairs, re-equipping wellhead equipment if necessary, organizational and technical measures for the fountain safety etc.).
2. Correspondence of the structures of the X-mas tree, which is equipped with a wellhead, with the requirements of snubbing technologies. The X-mas tree with which the head of gas and fountain oil wellsites are equipped in the Ukrainian oil fields, depending on the method of suspension of the tubing string, are classified according to two structural designs of the pipe heads, namely:
- with threaded-type hanger - hanging of the tubing string is carried out on the flanged adapter to the pipe head (Fig. 2);
- with a tubing hanger - the suspension of the tubing string is carried out on the body of the attachment point, which is hermetically placed in the casing of the pipe head (Fig. 3).
Both modifications of the X-mas tree are provided by the standard “GOST 13846 X-mas and injection trees. Typical schemes, basic parameters and technical requirements for the construction” and are also manufactured at the Konotop reinforcement plant. Moreover, the volumes of realization of the X-mas trees of both modifications by the said enterprise are approximately equal.
The X-mas tree is equipped with a pipe head (Figure 3) with a tubing hanger, adapted to the requirements of snubbing technologies. It makes it possible to erect the wellhead equipment of a snubbing installation without depressurizing the wellhead. In this case, preparatory work before the lifting of the tubing string is carried out according to the following generalized scheme:
- installation of a wellhead lubricator on a X-mas tree;
- sealing the tubing channel of the tubing string by installing, using cable technology, a plug (packer) in the lower part of the column;
- installation of the back valve in the tubing hanger of the pipe head of the X-mas tree;
- dismantling of the lubricator and the X-mas tree from the wellhead;
- installation of the sealing wellhead equipment of the snubbing unit on the pipe head of the X-mas tree.
X-mas trees (Figure 2) with threaded-type hanger of the tubing string are characterized by the simplicity of the design of the pipe head and at the same time a low level of fountain safety both during operation and the repair of wellsites.
It was designed, manufactured and used in the early stages of development of oil and gas technology and technique. And it is the presence of significant drawbacks in design, primarily associated with the requirements of the fountain safety, necessitated the development of an improved design of pipe heads with a tubing hanger of pipe columns.
In the oil and gas industries of Ukraine, the vast majority of flow wells are equipped with a X-mas trees with threaded-type hanger of the tubing string on the flanged adapter (Figure 2), which makes it impossible to use snubbing technology to repair wellsites without damping, as it requires dismantling the flanged adapter from the wellhead, leading to depressurization of the wellhead.
To implement the subbing technology for repair on such wellsites, it is necessary to re-equip the wellheads before repair - replace the flanged threaded-type hanger of the tubing string with the tubing one without damping the wellsite. Since this re-equipment of the wellhead must be carried out under pressure, it is possible to do this only with the use of a snubbing installation.
To date, foreign experience has been accumulated in the performance of such works. The generalized technical plan and the sequence of their implementation are shown in Figure 4.
As can be seen from the presented technological scheme (Figure 4), works on re-equipping the wellhead are complex, specific and are accompanied by a threat of open flowing. However, no doubt, they can be realized. To this end, it is necessary to develop and manufacture appropriate technical means (a sealing chamber, means for milling a flanged adapter to a pipe head with a threaded-type hanger of a tubing string and for retaining a tubing string after milling).
As for technical personnel for the work on the re-equipping of the wellhead, we should point out the presence in the oil and gas industry of highly qualified specialists of emergency rescue services having relevant experience in carrying out work under pressure and powers. These services include the Poltava militarized unit for the prevention and elimination of open flows and a similar company - LIKVO Specialized Emergency Rescue Service (Kharkov).
3. With regard to the availability of equipment for implementation of snubbing technologies, it should be noted that at present foreign companies have mastered a lot of snubbing installations with a wide range of technical parameters and various equipment. These companies include Balance Point Control B.V. (Netherlands); Halliburton, International Snubbing Services, National Oilwell Varco (USA) Snubco Pressure Control, Snubbertech (Canada).
Generalized technical parameters of snubbing installations of world manufacturers:
Lift load, kN 410-2270
Snub load, kN 230-1150
Stroke length of hydraulic cylinders, mm 1800-3500
Range of pipe diameters, mm 42-114
Wellhead pressure, MPa 14,21,35,70,105
Figure 5 shows a schematic diagram and a general view of a Snubbertech SIL-190-4 snubbing unit. The specified installation belongs to a private company KUB-GAZ LLC and is currently operated on the Ukrainian production fields.
Ukrainian machinery construction is fully capable of mastering production and providing the domestic market with equipment for implementation of snubbing technologies, subject to the availability of necessary design and technological documentation and financing for the development of production. The rapid and widespread introduction of snubbing technologies will have a tangible effect both in the oil and gas producing complex of Ukraine, and in the national economy in general and in the social sphere.
4. Psychological factor when working on snubbing installations. It is necessary to note the presence of a psychological factor that affects technical personnel who work on snubbing installations. Work on snubbing installations is accompanied by a constant danger of emergency release and loss of control over the wellsite.
To date, pressurized round trip operations with pipe columns are considered emergency ones. They are usually conducted in the process of liquidation of fluid kicks and open flows by technical personnel of specialized emergency rescue services.
Repair of wellsites under pressure with the use of snubbing installations is related to technological work, therefore it requires a high responsibility of the members of the working team for their actions. They should not only blindly follow the recommendations and requirements prescribed by the relevant regulatory documents, but also have knowledge of the processes that occur in the technical systems of the installation, be able to manage the relevant factors to ensure reliable equipment operation and wellsite control during the repair process under pressure.