Technology Roundtable: Coiled Tubing

Coiled tubing has become increasingly popular in Russia in recent years. What is the current level of utilization at the moment?

Baker Hughes: Current market data estimates a CT unit count of roughly 250 in Russia & the CIS, which is up 24 units from the 2012 count of 226. This is 12.5% of the total world CT count.

Schlumberger: Indeed, coiled tubing technologies are becoming more prevalent across Russian oil and gas fields. The high level of Schlumberger resource utilization is a reflection of this, in addition to the fact that we are currently engaged with all major Russian oil and gas operators for this service provision.

Weatherford: Coiled tubing (CT) is still greatly underused in Russia in comparison to other countries because 1) workover (WO) rates are extremely low and 2) CT service companies have failed to convince oil companies about the advantages of CT interventions. However, with the increase of Horizontal drilling, market have been picking up for the past 2 years in job quantity and variety. Weatherford CT fleets are constantly used on wells that require interventions to ensure minimum near-wellbore reservoir damage. Clients are really becoming more aware of the CT added value.

Trican Well Service: Coiled tubing remains in high demand in Russia. This is related to increasing volumes of fracturing operations where CT is used for cleanout stage. The obvious advantages of CT, including underbalanced operations, lower time for mobilization and work performance, better safety and well control, etc, are becoming clear for the majority of operators so the level of utilizations grows rapidly. This fact is supported by not only the increase in capacity of the major service companies but also the number of new companies that re appearing on the market. It is also worth mentioning that the scope of work is changing with well designs moving more towards the complex multizone completions for the selective stimulation and production. For these applications the role of CT is widening, with well intervention, completion tools, downhole motors etc.

CT is often used to carry out similar operations to wireline. What are the main benefits of CT over wireline?

Baker Hughes: CT has overall better penetration in horizontal wells that require a tractor or other devices to get downhole. CT also allows pumping operations to enhance the job while running in or out of the hole. Telecoil specifically offers real time logging while using CT.

Schlumberger: While wireline and coiled tubing services can perform several common applications the fundamental difference between the two services lies in the fact that coiled tubing allows operation in a wider range of well conditions, in addition to the ability to pump fluids and access highly deviated and horizontal wells. The Schlumberger ACTive* family of live downhole coiled tubing services builds upon these benefits by providing a means to acquire and validate downhole data, monitor job progress, reduce risks and enhance production.

Weatherford: CT is much more than just another conveyance mode, and the variety of operations in CT goes way beyond those that wireline can achieve. CT allows continuous operations with underbalanced wells, and new applications for existing and new CT tools are invented almost every day. Weatherford’s Thru-Tubing product line offers hundreds of tools and a vast array of solutions. We offer packers to enhance wellbore isolation and stimulation, fishing and milling tools to help with removal or remediation of wellbore obstacles, and a comprehensive selection of tools to help with perforating, logging casing, and tubing exits. Almost all of these services can be provided via WO rig. All of these operations can be conducted to ensure little to no impact on the reservoir, while maintaining a small footprint on the surface.

For many years, CT was primarily used for post-fracturing cleanout, but for the past few years, Weatherford has started doing milling, abrasive perforation, fishing, tubing cutting, and other high profile interventions in Russia.

Trican Well Service: The increased number of horizontal wells and complicated well profiles demand a special approach for intervention techniques. Often, conventional wireline techniques are not appropriate to convey tools to producing levels. On the other hand, Coiled Tubing is well suited to conduct such operations in extended reach wells. Advantages over drill pipe or wireline conveyed logging include the rigidity and strength for deviated or high flow-rate applications allowing access not only to horizontal sections or the well, but also passing the sever doglegs in the well profile; logging cable is protected by CT pipe for tough or aggressive conditions; continuous circulation ability, e.g., for lubricating fluids or nitrogen gas for maintaining underbalanced condition of the well especially in formations with low pressure where otherwise the inflow is not possible.

CT has the potential to save operators money on workover and reentry operations. How is this achieved?

Baker Hughes: Workover rig operations incur higher costs per day and they also span over a significantly longer amount of time than CT operations. CT operations enable live well interventions that do not require typical workover or well kill operations and the associated near wellbore damage. Speed and economy have emerged as clear advantages relative to keeping a well online versus shutting in/well kill to enable historical workover interventions.

Schlumberger: Russian oil and gas operators are under constant pressure to reduce lifting costs and coiled tubing services have proven to be the most effective solution for workover and re-entry operations. This is achieved by reducing operational time of for well intervention applications, in addition to minimizing the impact of well intervention on the producing reservoir. A good example of this is the role that coiled tubing plays in the complex fracturing cycle of multilayer wells in which hydraulic jetting perforation, wellbore clean-out and nitrogen lifting services are combined. This is proven with the Schlumberger AbrasiFRAC* abrasive perforating and fracturing service. With the AbrasiFRAC technology the overall cycle of well preparation post-fracturing activity is reduced from 30 to 15 days, when compared to conventional workover methods. Due to the minimum impact on the producing reservoir, productivity index improvements of 13-14% have been experienced. One operator confirmed the lifting cost improvements enabled by the introduction of the AbrasiFRAC service and as a result, the company has made the technology a standard for most of its operating fields.

Weatherford: CT interventions are faster and safer than WO operations. The CT string is a continuous pipe that can be run in and out of the well with continuous circulation and without any downtime for connections. This continuous pipe also limits the interaction of workers, effectively eliminating the health and safety issues associated with pipe handling. Also, as mentioned above, CT operations can be carried out with underbalanced wells in order to minimize fluid invasion in the near-wellbore region. Well control that is achieved with continuous pipe significantly reduces the release of undesirables into the environment.

The diversity of CT allows us to perform different types of operations into a hole in one run, reducing trips to save money and reduce risk. For example, combined, single-trip operations include cleanout and gas lift, cleanout and milling, abrasive perforation and cleanout, acid treatment and cleanout, etc.

Trican Well Service: Coiled tubing technology has several significant advantages in comparison with the other methods. The capacity to create a continuously underbalanced state with low bottomhole pressure for the flowback of unconsolidated formation solids, a variety of downhole operations performed in one (or more) run (s) and a decrease in treatment time. Using coiled tubing for this purpose improves well productivity, boosting fracture conductivity and ultimately decreasing the number of workover operations associated with downhole equipment failure caused by solids production. Typically, CT efficiency is estimated by two major parameters – time spent on the well and the costs for the operator in comparison with using workover rigs. In recent years due to the changes in technology and operational efficiency of both techniques the differences in these parameters has decreased significantly. Time spent on each can be similar, as are mobilization and overall expenditures due to the growing supply of CT in the market. However, the principal advantage of CT for the operators is its impact on the production. The overall cycle of well intervention with a minimized, or even absent negative influence to the reservoir allows wells to produce quicker and at their maximum potential. This is possible due to the pumping capacities of CT including the nitrified, foamed or even pure nitrogen gas pumping and reliable well control.

How do you ensure well safety and control during CT operations?

Baker Hughes: Baker Hughes management systems, procedures and processes ensure that safety is paramount in the way we conduct our well intervention operations. With a combination of our constantly evolving processes and the use of pressure control equipment with up to date strict API standards, thorough maintenance and certification processes, we provide reliable pressure control during CT operations. The Baker Hughes track record in pressure control has established us as a market leader and we continue to evolve and improve with a “zero incidents” philosophy.

Schlumberger: Schlumberger has implemented its coiled tubing operations standard that complies with and exceeds the national regulations. The objective of the standard is to prevent injuries of personnel involved and minimize potential damage to equipment used in coiled tubing operations. This standard sets the criteria for personnel competencies involved in the operations, as well as the specification for assets and blow-out prevention equipment, based on the well risk category. Outlined in the standard are detailed procedures for contingency operations to reduce the severity of a potential incident. The standard must be followed at all levels of the organization as part of the Schlumberger service quality and HSE management system that ensures safe service delivery and superior service quality.

Most recently, Schlumberger released the CoilScan real-time CT pipe inspection system, which minimizes non-productive time by providing real-time pipe inspection during CT operations. The system identifies CT pipe defects prior to entry into the wellbore by combining real-time dimensional measurements to proactively address CT pipe failures. The CoilScan system is a portable device that is attached to the CT reel and rigged up at surface, and is the last component put on the CT unit. Real-time dimensions are used to update the CT fatigue life, in addition to the CT working envelope. The CoilScan system provides a complete view of what is happening with CT pipe at all times to enable our customers to consider all aspects of pipe management and act accordingly to mitigate failures.

Weatherford: CT fleets come with blowout preventers and hydraulic pressure pumps that allow you to control the well at any point during operations.

A typical CT spread includes a data-acquisition system which allows us to see and control pressures, weight, rates and other working parameters in the control cabin in real time.

Trican Well Service: Well safety and control during the CT operations are achieved by two factors – reliable special equipment and industry standardized work procedures. The equipment part includes the three major components: blowout preventers that act as the principal well control while rigging up and down on the well and the main contingency device when CT is in the well, stripper or hydraulic seals that keep the well closed or in control while the CT is in the well, and the double flapper check valves in CT bottomhole assembly that prevent the flowback from the well through the CT itself. The second part is related to industry accepted and recognized procedures that all the field personnel should follow during the operations. In Russia there is no state standard for CT operations yet so the service companies use the standards accepted worldwide and adapt to existing local regulations.

CT offers a wide range of downhole applications from circulation, logging, perforation, drilling, well bore cleanout, well circulation, completions and production. Which of these services do you offer in the region and which are the most heavily in demand?

Baker Hughes: In Azerbaijan, the most popular services requested by customers include wellbore cleanouts, real time logging by Telecoil, N2 well unloading, plug setting in horizontal sections, cementing jobs, including P&A (plug & abandon) and squeeze cementing, fishing and milling etc. We are currently in the process of deploying additional cutting edge CT technologies like concentric CT used for cleanouts on low bottom hole pressure wells and Snapshot for live well deployment perforating. We are also looking at completion services through CT this year (deploying screens or scab liners on CT). In Kazakhstan, we are also using CT units for acidizing, milling, cleanout, fishing, through tubing workover, nitrogen lift, and memory logging.

Schlumberger: Schlumberger Well Intervention Services has the capacity to support all of these applications. In Russia, we currently provide all of the mentioned applications. The only exception is with coiled tubing drilling, and this is due to the specific conditions of this application. We are working towards establishing a workflow to accommodate for the local conditions in Russia.

We also recognize the operations related to the fracturing cycle—such as hydraulic jetting perforation, post-fracturing clean-out and nitrogen lifts—have the highest demand in the Russian market.

Weatherford: Weatherford’s Coiled Tubing and Thru-Tubing product lines offer various kinds of services. We are technical leaders with local experience in most of them. As mentioned above, we have the widest variety of solutions in our portfolio, readily available equipment in the country, as well as all the required expertise.

We have been training local operators for the past seven years. We have had a fully operational base for three to five years while successfully implementing these technologies in the region.

Trican Well Service: As the Russian workover market grows in volume and in sub segments, more and more applications are be requested. Just a few years ago the majority of the market was related to cleanout and kickoff operations. Today, milling and drilling operations related to multizone fracturing completions are being requested by the operators. Sand jetting perforation technology, ISOJET, for fracturing is utilized on wells in West Siberia allowing simultaneous operations of fracturing on several layers and perforation and isolation of zones with CT. With the growing volumes of horizontal wells being drilled, production logging using CT with installed cable, perforation and associated services are becoming common for operators. The requests to run inflatable packers, perforation guns, speciality tools for extended reach wells and even downhole cameras are gaining in popularity as well. Overall, CT applications are growing with the increasing complexity of the well design and the growing experience of operators who apply new technologies appearing on the worldwide market.

CT can be used to drill a well from scratch. Is this technology employed in Russia and what are the benefits and limitations of CT drilling?

Baker Hughes: Most of the CTD technology (CT drilling) applications are actually related to underbalanced sidetracking or hole extension cases in already drilled wells, rather than drilling new wells from scratch. Its advantages include the trip in/out speed, the ability to make hole without having to stop and add a pipe joint and of course continuous pumping, making it an ideal candidate for underbalanced drilling, which results in reduced formation damage. Also the small footprint of the drilling unit and its environmentally friendly character add to this. Its limitations are mostly related to the set down weight available by a CT string which results in low ROP, the pump rate limitation due to pipe size, and the high cost of tubulars due to limited footage. This is due to greater fatigue induced by high pumping pressures. CT drilling is most popular in countries with highly established CT experience and know-how, like US (particularly Alaska), Canada and Norway.

Schlumberger: The application of drilling a well from scratch utilizing coiled tubing is known and used worldwide. This application is economically more viable within certain geological and surface conditions—such as relatively shallow high pressurized reservoirs or formations highly sensitive to drilling mud—in addition to applications that require a smaller footprint for surface equipment. In conditions such as these, the application of coiled tubing drilling of wells from the surface has undisputed value. The application provides the advantage of a smaller footprint, improved well control, and continuous monitoring and control of the downhole drilling regime.

Weatherford: CT drilling is a completely different business that requires different types of CT units with much bigger strings. Here in Russia, a few companies have tried in the past, but it is not really economically viable. The cost of CT drilling vs. rig drilling has not been competitive in Russia.

Trican Well Service: CT drilling is attracting the attention of operators all over the world. In some instances it really adds value, but in other cases not so much. It can be divided into several specialized niches – starting from vertical overbalance grass roots drilling to more complicated underbalanced drilling of horizontal wells.

In Russia, Coiled Tubing drilling has it potential benefits mostly in the following applications: close to water drilling, in-fill drilling, carbonate formations and horizontal wells. There are numerous fields in W. Siberia that have experienced high water cut after fracturing stimulation has been attempted. The WC increase is either due to poor cementing practices where communication between the frac and a water layer is established via the annulus, or by the fracture breaking into the water zone because of small or non-existing barrier towards the water zone. Formations that are close to water zones and for the lack of a barrier to fracture growth are the foremost candidates to CTD. Formations with low pressure are regular candidates for under-balance drilling. This may not be the preferred selection criteria in Russia as wells typically get fracture stimulated. In case that fracture stimulation is not feasible, CTD in underbalanced mode, combined with either open hole/slotted liner completion or cased completion with underbalanced or perforation technique, will ensure the least possible damage to the formations. Carbonate formations are another good example where CTD may be very beneficial. The natural fractures and fissures are typically plugged by the drilling mud lost in the fissures or the filter cake that blocks the smaller fissures. Complete mud losses are not unusual for fissured carbonates. In lower pressure reservoirs the mud and filter cake is hard to remove and the damage will not be only deep but also permanent. Drilling underballanced will alleviate the problems with mud and mud cake damage. Either sandstone, or in particular carbonate horizontal wells should benefit from underbalanced drilling, where good vertical permeability exists. Horizontal wells in sandstone reservoirs are typically drilled with RDF muds and do not represent a large issue if drilled with geosteering and are following up with good workover practice. Low reservoir pressure formations will nevertheless benefit from the underbalance drilling.

CT reentry drilling can be used to target previous untapped hydrocarbons reserves. Is this technology being adopted in Russia? Should the utilization be higher?

Baker Hughes: See above

Schlumberger: Coiled tubing re-entry drilling technology was implemented in Russia successfully through several projects. The application is more widely used compared to coiled tubing drilling from the surface. There are several recent coiled tubing re-entry opportunities in both Russia and Central Asia that Schlumberger is pursuing. As previously mentioned, we are in the process of establishing a workflow to accommodate for local conditions, in addition to defining project deliverables.

Weatherford: For the same economical reasons mentioned in the previous question, (CT drilling costs vs. sidetrack drilling rig costs), CT sidetrack drilling business is not really developed in Russia.

Trican Well Service: As of today, CT drilling – both new horizontal wells and reentry – has been adopted only by Surgutneftegaz. Undoubtedly, in future, CTD will take its niche on the oilfield service market in Russia but will remain a specific application that will develop with the acquired experience by service and operating companies. CTD projects require deep analysis and thorough study, and involve close interaction and cooperation between the customers (in terms of work scope) and the service providers (in terms of required resources). It is worth also capturing the principal requirements for such projects: infrastructure support for CTD operations – the ability to conduct coiled tubing drilling operations when supported by an established infrastructure, familiar and equipped for coiled tubing operations is difficult; compilation of data for a comprehensive study and candidate selection – quantity, quality, and format of data for the development of study/candidate selection; field support and structure for operations. The cost of coiled tubing drilling daily operations will be high, allowing very little tolerance for delays or inefficiencies of core support; locating and testing Russian made components and services to reduce the operational costs; implementation of and adherence to personnel training program for placement of national staff into supervisory positions.

Fracturing through CT can target specifics zones and increase production. What success have you had with fracturing through CT in Russia?

Baker Hughes: We have not done fracturing through CT in Russia. However, Baker Hughes has performed many multistage fracturing through CT in other part of the world, including U.S and Canada. Besides CT unit and stimulation pumping, we have SureSet hydraulic sand jet perforation and packer system, or OptiPort sleeve and packer system to create multiple access to formation and isolation for each stage fracturing.

Schlumberger: Due to local geological conditions—relatively deep layered reservoirs, low formation pressure—fracturing through coiled tubing applications have been deemed either uneconomical or technically inefficient in Russia. Schlumberger does offer a variety of joint completion, fracturing and coiled tubing technologies to the Russian market that have been successfully implemented and exhibited proven value. One example is the use of a variation of the AbrasiFRAC service, which was specifically designed for use in the Russian market to enable through-tubing conveyance to be conducted for production enhancement applications.

Weatherford: The CT string sizes used in Russia generally will not allow fracturing through the coil to be performed. The CT strings mainly used in Russia are 1-1/2 in. and 1-3/4 in., which would not allow a high enough flow rate to fracture effectively when pumping down the coil. However, recent developments in CT fracturing technology have led to hundreds of fracturing operations pumping the proppant down the annulus. These newly developed bottomhole assemblies are part of what Weatherford can offer to the market in Russia. Currently in Russia, CT can be used to open new perforations (using abrasive sand) and new zones. This technique is regularly applied by Weatherford in Siberia, but due to the size of the fracturing strings, the CT strings must be pulled out before the zone is fractured using conventional methods.

Trican Well Service: I believe that Trican Well Service is the first company to implement CT fracturing in Russia. The first project was undertaken on a coalbed methane formation and included 10-12 fracturing operations on the well with 73 mm CT pipe. The next area where CT fracturing will be applied is the wells completed with Trican’s proprietary technology – Burst Ports System (BPS). The main principle of the technology is that the hydraulically activated BPS casing collars containing pre-milled ports are placed into the horizontal well casing or liner string and subsequently straddled by Trican’s exclusive selective fracturing tool (C2C™)* run on coiled tubing. The BPS collar is pressured up causing the ports to burst at their designated pressure point, and allowing the fracture to be pumped into the targeted zone. The C2C tool is then released and moved to the next burst port collar location, where the process is repeated. Application of CT for this type of operation is limited by the size of the work reel contacting 73 mm pipe, however the solution has been found in combination of CT pipe and frac jointed tubing. In this case, frac jointed tubing is run in the well with C2C tool and hangs off on the wellhead. CT pipe is connected to frac tubing with a specially designed rotary connector and run in hole for fracturing operations. The length of CT allows the well to be kept in a controlled state and continuous movement between the zones eliminates the time spent for jointed tubing handling, creating safer operations. Thus this system gives the operators the advantages of the full drift of casing left intact for future workovers, frac intervals, the ability to re-enter and squeeze off or treat selected zones. This is a quicker and more cost-effective solution than anything currently on the market.

Wellbore clean has been one of the most common CT applications globally; is it a common procedure in Russia? What solutions do you have for this area?

Baker Hughes: Wellbore cleanouts (Sand cleanouts, mud cleanouts, scale cleanouts etc) are indeed the most common CT applications worldwide and in the region. Baker Hughes has the most comprehensive suite of proprietary tools in the market for wellbore cleanouts. The Tornado with its backward/forward jetting switch capability has been the market leading tool in sand cleanouts, the Rotojet has been proven as the most effective rotary jetting tool in cleaning hard scale deposits on tubulars. Both tools have extensive track records worldwide as well as in Russia Caspian region. And of course, Concentric CT (CCT), a cutting edge Baker Hughes technology which we are currently in the process of deploying in Azerbaijan. The technique incorporates a venturi jet pump mounted on CCT, and is specifically designed to remove solids and/or fluids from the wellbore without placing hydrostatic loads on the reservoir. In sand vacuum mode, strong jets fluidize and mobilize solids into the vacuum as well as prevent a stuck pipe scenario. The jets also aid in opening clogged slots by loosening fines that may be blocking flow. When the vacuum tool  applies suction, a localized pressure-influenced-zone is formed in the very near wellbore – thus providing an isolated “point-load” drawdown on the formation. It is the ideal solution for low pressure wells or long horizontal wells cleanout with hundreds of track record worldwide.

Schlumberger: Considering that proppant fracturing for production enhancement is the most commonly used technique in oil and gas producing regions throughout Russia, wellbore cleanout after the fracturing treatment has become a highly demanded coiled tubing application. Schlumberger offers a wide variety of specially developed solutions for this application, including downhole tools and chemical agents to improve clean-out fluid carrying capabilities or debris dissolution. The combination of these technologies, in conjunction with the ACTive family of live coiled tubing services that enable the adjustment of operational parameters in real time based on the actual wellbore condition, provides us the assurance of effective and optimized wellbore cleanout operations.

Weatherford: Wellbore cleanout and nitrogen kickoff are very common operations in Russia, especially after well stimulation. All major operators in Russia now understand the added technical value to such operations. Weatherford is performing quality wellbore-cleaning operations on a 
daily basis.

Questions for Coiled Tube Manufacturers

How do you ensure your manufacturing standards along the entire length of the tube?

Tenaris: There are many engineering and manufacturing processes that ensure that the entire length of each coiled tubing string meets engineering specifications, even though it is only possible to destructively test samples from each end of the tube. All of these processes are defined, controlled and audited within an overall quality management system, which begins with the specification and procurement of the steel. The mechanical properties and chemical composition of the steel are specified to a very narrow range. These properties are confirmed by testing at both the steel mill and at the tube mill. Steel strips are then selected, based on matching mechanical properties and chemical composition, to create the complete steel strip used to create the tube. The steel strips are inspected during the assembly phase of manufacturing to confirm that they meet surface quality, dimensional and mechanical property requirements. During milling, minor variations between strips are compensated for by adjusting certain milling parameters. These parameters are based on an extensive database of more than 65,000 previous test results, producing uniform mechanical properties along the length of the tube. Continuous non-destructive testing (NDT) is performed during the milling process to ensure weld integrity. Follow-up inspection is used to determine the cause of any NDT indications. Then the tube is hydrostatically tested to a pressure equal to 90% of the pressure that would cause the tubing to yield for a minimum of 30 minutes. Finally, all steps of the manufacturing and all test results are reviewed and certified, prior to authorizing the release and shipment of the tube.

What factors should be considered when selecting the grade of metal used within the tube itself?

Tenaris: Our customers choose their tube grade based on a range of operational considerations: the depth of the wells, the types of jobs that are being performed, the environment within the wells and compatibility with their equipment. As a manufacturer of coiled tubing, we create a detailed set of specifications for steel that will achieve optimum and consistent performance for each grade. Extensive testing is first performed to develop a steel specification for a grade. This testing includes thousands of laboratory tests, followed by extensive field trials to ensure performance. Then, an equally rigorous process of testing and qualification must be performed to identify and qualify a steel supplier who can consistently achieve the demanding requirements of the steel specification. The supplier qualification process is very specific, restricting qualification to a specific supplier’s mill and to a specific manufacturing process route within that mill.

How do you ensure the weld integrity when connecting the sections of tube?

Tenaris: It is necessary to join the strips end-to-end prior to milling because the final length of the coiled tubing string is greater than the length of the steel strip. Since the bias weld, which connects one strip to the next, remains in the tube, it is essential that this weld have no defects, and that it and the adjacent area meet all of the specified properties for that tube. The process begins by developing a weld procedure specification (WPS) for each grade of tubing. The WPS must be qualified by thorough testing and documented according the standards of ASME Section IX. Each bias weld must be made by a welder who meets both the training and proficiency requirements, in accordance with the WPS. Each bias weld is dimensionally inspected, tested for mechanical properties, and then radiographic inspection is performed to confirm the weld is free of defects. The surface finish is then polished to achieve a roughness that is less than the overall steel strip. Periodic testing is performed to ensure mechanical, metallurgical and fatigue resistance are within norms.

Questions for Tractor Service Providers

When should a tractor be deployed with CT, and why is it sometimes needed?

Welltec: The typical reason why a tractor is deployed on CT is to increase the reach of the coil. When the required depth cannot be reached during pre-job simulations, a CT tractor should be considered.

The mechanical limitations of CT are well documented and in highly deviated wells the failure mechanism is typically referred to as ‘lock-up.’ A lock-up consists of a sinusoidal buckle followed by a helical buckle brought on by the frictional forces overcoming those of gravity and the CT’s momentum.

In the following sketches the difference of forces acting on CT in a vertical well versus a deviated well becomes clear. Depth is plotted against surface weight indication and as the CT is running in hole, the weight increases almost linearly (box A). However, as the angle starts to build (box B), the surface weight increase rate drops as frictional drag from the casing / tubing acts on the ever increasing surface area contact. As the well increases angle to horizontal around 7,000 ft, the additional CT being run into the well ceases to contribute to the surface weight indication (box C).

And as drag continues to increase, the weight begins to drop off. Finally around 9,100 ft the force of friction overcomes the downward movement of the CT and lock-up occurs.

What benefits does a tractor bring to a project when it is deployed?

Welltec: The benefits vary depending on whether you are referring to an e-line tractor or a CT tractor.

The CT tractor as described above will increase the depth which CT can access thereby improving the operations as a whole. If the coil is being used for acid stimulation, the placement strategy can be implemented more effectively driving up the well production vs the alternative which we be to bullhead from the point of lock-up. Similarly for other applications where fluids are needed to be spotted such as cleaning, washing, etc.

For e-line tractors, the benefits are that you can deploy e-line tools in horizontal wells without having to bring out a rig or a CT unit. This allows the operator to manage his available ‘large’ assets better while still accomplishing workovers and interventions in highly deviated wells.

How can you maximize the payload a tractor can handle?

Welltec: The maximum payload that a tractor can push is a function of:
1) Well conditions
a. Environment – OH versus CH
b. Deviation
c. Friction factor
d. Minimum restrictions

2) Surface equipment
a. Available rig-up height
b. E-line cable type

Increasing force come from increasing the tractor size, the cable and the number of wheel sections. These all translate into more available power downhole.