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"Snake-method" of coiled tubing stuck pipe elimination (step-by-step instructions)

"What we know is limited, but what we don't know has no limits"

Lucius Apuleius


Table of contents:

·       Terms and concepts used.

·       Preface.

·       The problem.

·       Traditional methods of coiled tubing stuck pipe elimination.

·       Well background, preconditions for complications.

·       Equipment and tools necessary for further operations.

·       "Snake method". Step-by-step instructions.

·        Results.


Terms and concepts used:

Complication - violation of technological process despite compliance with technical design and common safety rules caused by subsurface conditions. In the event of complications, operation may continue, but special measures are required.

Well - a circular cross-sectional mine opening drilled from the surface or from underground workings without human access to the bottomhole at any angle to the horizon, the diameter of which is much smaller than its depth.

CT (coiled tubing)- is a long metal pipe, usually 1 to 3.25 inches (25 to 83 mm) in diameter, that is supplied reeled on a large drum.

(PC) Production casing- most internal string of casing used to secure a well to separate productive horizons from the rest of the rock and produce oil or gas from the well, or, conversely, to inject liquid or gas into the formations.

Coiled tubing equipment- a set of equipment and specialized tools to perform coiled tubing operations. It usually consists of, but is not limited to:

- Coiled tubing unit (CTU)

- Pumping unit (PU)

- Nitrogen unit (NU)

- Set of cleanout and circulation equipment;

- Required CT bottom hole assembly (BHA).

The Boycott effect- the effect of accelerated deposition of impurities in a vessel with inclined walls. It is observed in various dispersed media. It can cause dune formation, sliding, avalanche-like depositing of cuttings accumulated on the bottom wall of a directional well.

BOP- blowout prevention equipment.  A set of devices designed to seal the wellhead.

Wellhead X-mas Tree. Equipment of the oil or gas wellhead, which includes a pipe head, a wellhead stack-up, closing and regulating devices and is designed to seal the wellhead, control and regulate its operational mode, as well as to perform various technological operations.

Tubing hanger plate. An element of wellhead equipment in the form of a plate on which wellhead equipment is installed and tubing is hanged.

Casing head. A device for connecting the upper parts of casing strings - conductor, intermediate and production casing - after completion of well drilling, running production string and its cementing.



I have named this method of stuck pipe elimination - the "Snake Method". The following material will allow you to build an associative series and agree or reject this name. I will start from the end. This method was invented as a result of eliminating a complication at one of the wells in Ukraine. The coiled tubing was successfully retrieved, and after the necessary restoration and maintenance work, its operation continued as usual. I thank #YuriyNagorniak, #Віталій Попович («Вітер»), #Валерий Залецкий for their outstanding teamwork.


The problem:

Well interventions using coiled tubing equipment are associated with risks of losing coiled tubing mobility. The reasons can be both individual and complex factors. Such as:

- Difference in hydrostatic and reservoir pressures;

- Mismatch of geometric dimensions;

- Complicated wellbore geometry;

- Mismatch of coiled tubing parameters with downhole conditions;

- Wedging by foreign objects;

- Rock sloughing;

- Sedimentation of solid particles as a result of inadequate cleanout modes;

- Abrupt release of significant amount of solid particles under the influence of trapped pressure after stimulation;

- Boycott effect, etc.


Traditional methods of CT stuck pipe elimination:

The methods for coiled tubing stuck pipe elimination are quite diverse and largely depend on a clear understanding of the current conditions in the well and the processes that preceded the complication. The most common methods of eliminating this complication include the following:   

1.     Intensive cleanout;

2.     Use of friction reducers;

3.     Acid baths;

4.     Use of lubricating additives;

5.     The «float method»;

6.     The «zebra method»;

7.     The method of variable hydraulic oscillations;

8.     The method of maximum permissible loads;

9.     Method of cutting with in-pipe cutters;

10. Pulling tubing/drill pipe with securing and removal of coiled tubing segments, etc.

It should be noted that the methods of eliminating this type of complication are constantly evolving, largely depending on the qualifications of the performers and the speed of their reaction. I would suggest to follow the old, reliable, parental principle - "from simple to complex". Only when the known, "traditional" methods have not achieved the desired result should we move on to more complex methods. For example, the proposed one.


Well background, the preconditions for complications:

We will not discuss the causes of the complication in this article. I would like to briefly discuss the preconditions requisites and ways that led to the invention  and implementation of the discussed method of eliminating the complication, such as loss of coiled tubing mobility.

 In a well with reservoir pressures significantly below the hydrostatic pressure, there was an ongoing operation on cleaning the bottom hole, partially filled with sand, using nitrified fluid. When signs of deviations from the normal course of the process appeared (discrepancy between the actual weight of the СT and the calculated one, increase in circulation pressure, decrease in the intensity of the returning fluid flow to the circulation tank, decrease in the volume of sand at surface), measures were taken to avoid complications. We started to immediately pull the СT from the well increasing the cleanout intensity with the regular adding of viscoelastic fluid pills into the circulation flow. The pulling was followed by regular overpulls puffs, and when the weight indicator reached critical values, the pulling was stopped and the "traditional" methods of stuck pipe elimination were introduced. The circulation through the СT was subsequently almost lost. As can be understood from the fact that this article appeared, traditional methods did not lead to the proper result. In this case, the stuck pipe was caused by formation of a sand plug, either between the tubing and the CT or between the production casing and the CT.

I would like to dwell on this "either/or" uncertainty. As it turned out later, our initial ideas about the depth of stuck pipe were not correct. The fact is that, to date, the only widely used method for determining the coiled tubing stuck point is a calculation method based on variable loads. This mathematical method of determining the free point is quite approximate. In our case, the results obtained from three different, independent sources showed that the stuck point was in the tubing, so the primary actions were aimed at impacting the calculated interval and, of course, proved to be ineffective. Nevertheless, further steps allowed us to avoid uncertainty and successfully eliminate the complications. The state of the well at the start of the Snake method implementation is shown in Fig.1.   

Стан свердловини на початок реалізації Snake-метода


Equipment and tools necessary for further operations:

No special, additional, expensive equipment is required for further operations. The entire list is included in the standard required equipment kits for coiled tubing and workover crews, and only some elements need to be improved or additionally manufactured in any nearby tool shop. In order to simplify the process of perceiving this idea, I will visualize some key elements.

Ліквідація прихоплення_

Ліквідація прихоплення

"Snake method". Step-by-step instructions:

In my previous article "Flowing a well: Simple solutions for difficult challenges", I expressed an opinion about the importance of having a center for creative thinking in the company's activities. Here is another example of practical implementation of this approach. The starting point for further development was one of the methods of eliminating stuck wireline cable that has been used since the time of our great-grandparents. Nevertheless, the coiled tubing is tightly stuck, and the "traditional" methods did not lead to proper results, so we will continue.

Health, safety, and environmental protection measures are of paramount importance and do not require a separate reminder. I will only give one piece of advice. Before the actual performance of any work, I advise you to hold general meetings with involved performers, conduct "work on paper", where you discuss HSE issues in addition to technical, technological and organizational matters. Not in the "read-and-sign" mode, but really in the "active involvement" mode. This is very important. I will omit some details to reduce the subject, but the general course shall be understandable. Let's move on.

1) Slack off the coiled tubing weight to "0". 2) Close the "pipe rams" and the "slips" of the BOP, secure them. 3) Breakup the connection between the stripper and BOP, lift the injector head above the BOP by L≈0.5-1 m. 4) Put the CT clamp above the BOP (Item 13). 5) Using a manual pipe cutter (Item 12), cut off the CT under the injector (monitor the injector head traction pressure and stripper pressure to avoid СT jumping off the injector). 6) Put the injector head on the CTU platform. 7) Breakup the BOP, X-mas Tree in a secure way, make up the lifting sub on the Tubing hanger plate of the Casing head. 8) Rig-down the CTU from the wellhead, rig-up the Workover rig, proceed with the necessary preparations and standard activities. 9) Set the boom crane to a position that will allow simultaneous operation of the workover unit (tubing control) and the boom crane (CT control).  Install the crane scales (Item 3) on the hook of the boom crane. 10) Breakup the tubing hanger plate in a secure way, rig up the workover BOP. 11) Using the coiled tubing reamer (Item 10), clean the CT, make up (Item 11) the LITT connector (Item 8) on the visible section of the stuck CT. The well schematic before further operation is shown in Fig.2.     

Ліквідація прихоплення_ (2)


I want to dwell on the procedure of stuck interval determination. To be precise on where the stuck point is, in the tubing or somewhere below? During breaking up the tubing hanger (on the first tubing joint) and by pulling the entire tubing string (Fig. 3), it is necessary to check its own weight and free movement. If the CT is trapped in the tubing, the weight indicator will show the tubing weight plus the CT weight, and the free movement of the entire string will occur. If the CT is stuck below the tubing shoe, the weight indicator will show the entire weight of tubing + CT with additional overpull (maximum CT load limits shall not be exceeded). I anticipate your questions about the possible sticking of the tubing itself. Please, be patient, everything will clear up soon*.

If the stuck interval is within the tubing space, the next steps are traditional and well-known. The process participants jointly decide whether to rescue the CT string (if there is anything worth to fight for) by using an internal cutter or to pull the tubing string with the CT cut in sections. The final decision depends on a number of factors, such as:

- total operating time of the CT string;

- length of the CT to the stuck point;

- possibility of using internal cutters;

- waiting time and cost of necessary components;

- cost of alternative options, taking into account the duration and possible lost profits, etc.

«Snake-метод» ліквідації прихоплення гнучкої труби


In the event of sticking below the tubing shoe, we will allow «Snake» to do its job. To reduce the chance of tubing falling into the well, further work should be carried out with two lifting bails. 12) Drift a long cable traction through the tubing joint (from the optional kit) (Item 4), then feed this tubing joint into the lifting bail, put the ear of the cable traction on the hook of the crane scales, and, by simultaneous operation of the workover unit and the boom crane, lift the tubing joint above the drill floor (Fig. 4).

13) Make up the connector of the cable traction (Item 4) onto the connector of the stuck CT string (Item 8). 14) Using the boom crane, pull the CT  to  1-2 tons above own weight and take note of the readings on the indicator of the crane scales. 15) Take off the CT clamp (Item 13). 16) Put the tubing joint into the collar make it up tightly and check the connection. 17) Controlling the process both by the crane scales indicator (CT weight) and by the workover unit weight indicator (tubing weight), run the tubing string in hole until  the upper end of  CT string appears above the tubing collar.

* Let me return to the issue of possible tubing sticking. Do you remember this uncertainty during determining the CT sticking interval? If the tubing is stuck, there will be no downhole movement of the string. The task will become more complicated, but it will not be a stalemate. In order not to overload this article, we consider the movement of the tubing string to be free (as it actually was), and leave the option with stuck tubing for further discussions.

18)  Repeating steps 12-17 run the tubing string in hole to reach the "head" of the CT sticking interval. 19) Use tubing from the additional set, an modified circulation swivel (Item 15), a rigid rod (Item 7), a short cable traction (Item 5) and a CT sub (Item 9), make a two-channel circulation system.


«Snake-метод» ліквідації прихоплення гнучкої труби.


Pay special attention to the condition of the sealing element. In this case, a suitable car crankshaft oil seal was selected (Fig. 5.1).

20) Put the swivel into the lifting bail, hang the cable traction ear on the hook of the crane scales, and by simultaneous operation of the workover rig and boom crane, lift the tubing above the drill floor (Fig. 5).

Ліквідація прихоплення_укр_page-0010
21) Make up the rigid rod connector (Item 7) with the CT sub (Item 9) onto the stuck CT connector (Item 8) (Fig. 5.2). 22) Using the boom crane, pull the CT to 1-2 tons above own weight and take note of the readings on the indicator of the crane scales. 23) Take off the CT clamp (Item 13). 24) Put the tubing joint into the collar make it up tightly and check the connection. 25) Assemble circulation lines through the manifold skid (Item 17) for the possibility to quickly change circulation modes, if necessary, from the direct circulation (circulation swivel – tubing string - annulus - choke manifold (Item 18) - circulation tank (Item 21)) to the reverse circulation (annulus – tubing string - circulation swivel – choke manifold - circulation tank).

Next, the sand plug will be circulated up by pumping fluid (if necessary, nitrified fluid) into the well annular, with sealing the annular at the wellhead by BOP pipe rams (until minor leaks appear). For tubing collars to pass through the BOP rams, it is necessary to briefly switch the circulation to direct mode with fluid splashing into the wellhead cellar (remember that we, as real engineers, took care of this in advance :)) An alternative way (I like it) is to use the 1КGОМ sealing equipment kit (insert №4). In our case, we couldn't quickly find such an equipment, so we used rams.

26) Controlling the process both by the crane scales indicator (CT weight) and by the workover unit weight indicator (tubing weight), run the tubing string in hole with circulation until connecting (Fig. 5.2) into the upper cap of circulation swivel (Fig. 5.1). Constantly monitor the intensity of the flow returning to the circulation tank, the amount of sand in the cleanout fluid. Depending on the actual downhole conditions, the person responsible for the operation should adjust the performance of the pumping unit/nitrogen unit, the parameters of the cleanout fluid, and tubing running speed. Circulate the well till clean returns before making next connection. 27) Disconnect the circulation swivel from the tubing string in the well. Using the crane, check if CT string is free by giving it an extra pull. 28) Secure the CT string at the wellhead with the help of a CT clamp, disconnect the rigid rod, and repeat steps 19-27 to remove the sand plug until the CT is released. 29) After releasing the CT string, check the free movement by lifting it to the maximum possible height using the crane. Rig down the workover equipment, rig up coiled tubing equipment, and pull the released CT string from the well.



This method allows to significantly increase the variety of possible steps in case of CT sticking in the well. Moreover, I see ample room for improvement. For example, in eliminating such causes of sticking as a "foreign object" or " CT exiting beyond the casing". By completely lifting the tubing and applying something more aggressive instead of the "tubing shoe", for specific tasks, to re-run the "Snake" and eliminate the cause of sticking, either by impacting or, with making certain changes to the design of the drill swivel, by rotary system. But this is another story and better "not in our village".

I deliberately did not focus on the financial benefit. It can be both very significant (taking into account additional costs for elimination of consequences of leaving the CT string in the well and the lost profit during the forced downtime of the well), or not worthy of attention (for example, if conditions allow, it is sometimes worth leaving the CT string in the well and continue well operation.  CT pulling can be done later if necessary). Each well, like a beloved woman, requires attentive and careful treatment, and then, perhaps, it will reciprocate your feelings.

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