SPE 145987

Ten Year Evolution and Field History of Design Changes for a Torque and Drag Reduction Performance Drilling Sub John E. McCormick and Chad D. Evans, SPE, Weatherford International, and Cameron Kirkpatrick, SPE, The University of Texas at Austin

Copyright 2011, Society of Petroleum Engineers This paper was prepared for presentation at the SPE Asia Pacific Oil and Gas Conference and Exhibition held in Jakarta, Indonesia, 2022 September 2011. This paper was selected for presentation by an SPE program committee following review of information contained in an abstract submitted by the author(s). Contents of the paper have not been reviewed by the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessar ily reflect any position of the Society of Petroleum Engineers, its officers, or members. Electronic reproduction, distribution, or storage of any part of this paper without the written consent of the Society of Petroleum Engineers is prohibited. Permission to reproduce in print is restricted to an abstract of not more than 300 words; illustrations may not be copied. The abstract must contain conspicuous acknowledgment of SPE copyright .

Introduction The need for advances in the robustness and versatility of downhole tools is increasing as well geometries become more complex. With more challenging wells being drilled everyday even the most advanced tools need to be improved upon. This is typically where research and development (R&D) engineers play a vital role. They work closely with operations personnel to create and constantly improve downhole tools. As service companies typically are the R&D arm of the oilfield, tools and improved designs are primarily market driven. This paper will explore the development of a mechanical friction reduction tool (MFRT), which is sometimes necessary in ERD and complex geometry wells to overcome torque and drag limitations. A tool starts as a concept that will overcome an obstacle when drilling or completing a well or ensure the safety and cost effectiveness of an operation. The R&D engineers take this idea and develop it into a first generation tool. They then meet with their managers, fellow engineers and operations managers to discuss the concept, the functionality, and the technical and financial viability of the tool. The engineers then go back and refine the design of the tool, after which is scheduled another design review meeting. This process undergoes several iterations, with more technical and managerial personnel involved each meeting, until a final design is agreed upon. This design review process is intended to manage the progress of the design, keeping check on the financial, functional, and implementation aspects of the tool. Once the first articles have been manufactured and field trials run, these tools are run commercially. Close watch is kept on the use of the tools and any issue and need for improvement are addressed by the design team. Constant assessments of the tools performance are carried out. This paper describes the development of a tool over a ten year period using the MFRT as a case study. We look at specific changes made to the tool, why these changes were implemented, and field trial results for the different modifications. Overview of the Mechanical Friction Reduction Tool (MFRT) The MFRT design was acquired from a start-up company. This concept for the drilling tool to reduce torque and drag was good, but the drilling tool was designed from a cementing perspective by a company that specialized in casing accessories. A series of immediate changes were needed. Other modifications came as a result of using the tools in more strenuous environments andinput from operations personnel. The MFRT is a sub based tool consisting of a 4 ft long mandrel with a clamp-on casting. The casting consists of 6 pods offset at 60 degrees each, which contain 2 rollers per pod. The rollers are used to reduce axial drag. Between the casting and the mandrel, or sub body, is a bearing sleeve which reduces rotational friction, or

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torque. The sub rotates with the string while the casting and sleeve remain stationary. Figure 1 illustrates the design.

Fig 1. Design of the Torque and Drag Reduction Sub The MFRT is designed to be a very versatile tool. It can be used in almost any downhole operation when jointed drill pipe is used and is particularly helpful in drilling and liner hanging operations where torque or lack of string weight is the limiting factor. The tools can be run in cased and open hole. Generally tools are run 1 per stand, though applications with exceptionally severe side loads may necessitate 3 per 2 stands. Because the MFRTs are sub based and 4 feet long, they can often be racked back in the derrick without removing the tools from the string. Where torque or drag is a limiting factor in a well, these tools are a very effective way for operators to achieve drilling goals. The next section describes several of the changes made from field experience that the MFRT has undergone over the past 10 years. Design History and Changes The tool was originally designed by a company specializing in casing accessory tools and based on the idea that it would be primarily used in cased hole. This presented a problem in many applications; in these applications the MFRT tools would become the weak link in the string. In order to increase the integrity of the tools within the drill string the material processing steps were changed in order to produce a tool that has a minimum yield strength of 120,000 psi. Other material changes included varying the alloying elements in the carbon steel used to manufacture the sub. This change was made to increase the strength of the material. Initially, the material being used for the mandrels had acceptable yield strength; however, this strength existed primarily at the original outer surface of the material. The strength decreased as the distance from the OD of the material decreased in the radial direction. Thus the internal strength deteriorated the further away from the outer-most portion the material was located. Any machining of the material was necessary during or after manufacture would remove the higher strength material. The jar grade material still has a drop off in strength when moving into the material from the outer diameter, but the drop off is linear instead of exponential, and has a very small slope. Thus the strength is virtually the same and does not depend on the type or amount of machining work performed. These changes were made after obtaining the original IP from the start-up company in order to bring the tools material up to par with their other downhole tools such as drilling jars.

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Another immediate and the most readily apparent change made to the MFRT was the switch from a casting design of 4 roller pods to a design with 6 roller pods. Each pod became smaller in size when the change from 4 to 6 occurred (Fig. 2), however, the number and size of the rollers remained the same. In the 4 pod design tool each pod contained 3 rollers and were spaced 90 out of phase. The hinge pin was also redesigned to be made more robust.

Fig 2. Six Pod Casting Design of the Torque and Drag Redcution Tool The functionality of the tool was improved because the pods are now staggered (every other pod is on the opposite end of the casting from the other (Fig. 2) and spaced at only 60 out of phase. This means that the contact area between the rollers and the wellbore is now more evenly dispersed, thus the load that the tool must take is moved away from the center and dissipated more evenly across the mandrel. This improvement not only increased the functionality of the tool in cased hole scenarios but also allowed the MFRT to be run in open hole conditions. Another important modification that involves the MFRTs drag reducing rollers came after the pod design change took place. Originally, both in the 4 and 6 pod tools, the rollers were manufactured with either a circular or a flat shape. With this design the rollers were mainly loaded on their edges, this was especially the case with the flat rollers, which caused excessive roller wear and could even lead to casting damage as the rollers tended to mushroom and bite into the pods (Fig. 3 and Fig. 4). This lessens the drag reduction effect of the tool and decreases the life of the casting. Figure 3 shows a roller that has deformed by developing a lip around the edge. Figure 4 shows an undamaged roller security slot on the left and a slot on the right that has been worn by a deformed roller.

Fig 3 & 4. Mushrooming and Casting Wear

The severity of this damage was very dependent on the size of hole that the tools were placed in. The use of each size of tool is limited to a certain range of hole sizes. Even within this range of sizes the amount of contact between the flat or circular rollers and the wellbore varies with size. This variation can either increase or decrease the severity of the wear experienced with this design. One might think that the rational design change would be to design a tool where the rollers have the same radius of curvature as the casing. The problem is that this would require a specific tool for each combination of drill pipe and casing/hole sizes, which is impractical. For this reason the rollers are now designed with a radius of curvature which allows one size of MFRT to be run in several casing or open hole sizes without the severe side loading and wear experienced previously. Figure 5

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shows the same size casting in multiple casing sizes. As the casing size increases, the rollers have less contact area with the wellbore. However, with the elliptically shaped rollers the contact area is still maintained away from the edge of the roller, reducing the risk for roller wear and deformation.

Fig 5. Multiple Casing Sizes for the Same Sized MFRT Casting

Normal, or acceptable, wear to the rollers can be seen below in Figure 6. Under the loading commonly experienced by these tools, a wear line may develop in the middle of the roller, but this does not affect the performance or maintenance of the tool. The rollers are simply replaced post-job.

Fig 6. Acceptable Wear on the Sub

A recent improvement made with regard to the rollers dealt with their position in the pods. The rollers were moved up, radialy away from the mandrel, and above the inner diameter of the casting. This change prevents the rollers from contacting the mandrel even if the bearing sleeve is allowed to wear beyond its acceptable limit. This reduces both the cost and likelyhood of repairs to the coating significantly when the MFRTs are run in environments with loading near their published limits. The Equivanlent Circulating Densities (ECDs) caused by the MFRT are low with the offset, out of phase six pod design. The smaller and more efficiently spaced pods improve the flow paths of the drilling fluids. This is illustrated in the CFD image in Fig. 7.

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Fig 7. Flow Test on the Torque and Drag Reduction Sub

Another material change involves the hardfacing material applied to the OD of the journal area of the mandrel. This change has been one of the most significant made to the MFRT in recent years. In order to protect the journal area from significant wear due to high side loads, debris, cuttings and coarse mud particles being caught between the journal and bearing sleeve, a hardfacing must be applied. This material needs to be strong, hard (i.e. abrasion resistant) as well as ductile. Also, the material must have a very strong bond to the steel material of the sub and have very low porosity. Over the years, numerous amounts of test have been conducted on various hardfacing solutions, these tests include bond strength tests, fatigue tests as well as others. Overall the tests showed that some of the initial hardfacings used on the tools were adequate but could be improved upon. This is where a new application process, using a higher velocity than previously used, was introduced to mechanically coat the journal area. The type of coating has also changed, a special alloy tungsten carbide has been chosen as the best coating for the MFRTs. This coating and application process has proven to provide the best combination of ductility, strength, abrasion resistance and porosity of any other processes tested. Reworking of tools due to wear can be a costly process where hardfacing is concerned and many steps have been taken to produce a coating that will last the lifetime of the tool. More abrasion resistant coatings are necessary for operation in extremely abrasive formations or for use during casing exits. As deep, horizontal casing exits become more common, achieving the weight to set single-trip mechanical systems becomes difficult, necessitating the use of additional T&D reduction methods to ensure a successful operation. Since the MFRT is mud lubricated, cuttings travel into and out of the space between the sleeve and the sub body. Metal shavings from the casing exit, however, become embedded in the sleeve, causing wear to the sub bodies (Fig.8 and Fig. 9).

Fig 8 & Fig 9. Wear on the Sub Bodies

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A significant factor in fatigue failure is stress concentrations. Fatigue cracks can result in a parted drill string, which is a catastrophic mode of failure for a drill string. These tools are asked to go into very severe drilling conditions and not only survive, but make the operation achievable. MFRTs are placed in the most severe sections of wells when T&D forces threaten the ability of the rig to reach TD or complete the well. As with every downhole string component used in deviated wells, stress concentrations in often severe doglegs are unavoidable. However, the stress concentrations can be designed to impart a minimal negative effect. At the very beginning of the MFRT the mandrel had a 90 transition; this type of transition causes the highest stress concentration possible. This was quickly changed to a different transition, decreasing the stress concentration and providing a more stable transition. With the initial transition design of the tool, an issue was encountered on a drilling run and a design review was conducted where the tranisiton was redesigned. A series of FEA models of the MFRT mandrel body were produced and various changes were tested. The FEA models revealed that a specific type of transition produced the smallest stress concentration. For this reason the tools now feature this modification. An MFRT sub with a journal area that supports the casting and rollers is shown in Figure 10 and 11 bent under stress in a dogleg. The maximum allowable dogleg for tools can be determined by thorough testing to ten million cycles, or revolutions, without generating fatigue cracks. If a tool can withstand ten million cycles with no fatigue cracks, it is considered to be able to rotate infinitely at that dogleg with the loading paramaters used.

Fig 10. Stress Concentration Locally

Fig 11. Stress Concentration for the Entire TooThe bearing sleeve is one of the most important pieces of the MFRT. It is designed to withstand the abrasion generated by rotating the sub within the non-rotating casting while the tool is under extreme torsional, axial and radial forces associated with ER wells. As such, their design is critical to the success of the tool. Many iterations of the bearing sleeves have been used during the existence of the tools, from the relatively simple early designs to the highly engineered sleeves the tools feature today.Operators require a sleeve that performs under the extremely harsh conditions the tools are exposed to as well as a sleeve that will last for several hundred hours of drilling. Material testing for the sleeves has been extensive, with many different materials, including PEEK, being tested. The end results of these test has landed

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on a carbon-composite material which has been found to provide the best wear resistance and reliability of manufacturing. However, the trade off to using this material is its increased abrasiveness; this is where the increased attention to the hardfacing already mentioned becomes increasingly important. The bearing sleeves are designed to be the sacrificial member for wear. This keeps both the cost of the tools and repair costs low enough to keep the tools commercially viable. In the past there have been several instances of tools being run beyond the capabilities of the bearing sleeves. When this happens the sleeves wear thin and the tools cannot perform at their highest level. This also causes an increase in repair cost and may necessitate changing the sleeves during a bit trip. The sleeves can be manufactured with a wear indicator on their ID. Once this indicator is no longer visible the sleeves need to be replaced. The wear indicator grooves serve a dual purpose; they are also radial and thrust lubrication grooves (Fig. 12). Another design highlight of the sleeves include protrusions into the roller pod areas (Fig. 13), which help to keep the castings and the bearing sleeves as one non-rotating part once installed on the mandrel. This helps to reduce unnecessary wear on the OD of the sleeves and maintains the torque reduction capability of the tools.

Fig 12. Radial and Thrust Lubrication Grooves Fig 13. Protrustion Areas Conclusion Both with initial design and the redesign of a tool, there is a significant amount of engineering required to produce a tool that is capable of operating in the types of downhole environements that exist in todays oilfield. Even with the intelligent concepts and thorough designs of drilling and completion tools, modifications are needed either to correct problems with the design or to enable those tools to perform better and operate in even more severe environments. A set of standard running limitations for the MFRTs has been developed based on both engineering calculations and field experience. Hole size, side loads, and dogleg severities (DLS) are restricted to insure safe use. There are a range of hole sizes that each tool can be run in. These sizes are determined by the elliptical shape of the rollers.

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If the hole size is not within the appropriate size range, the rollers will be point loaded and risk deformation. The side loading and DLS both contribute to cumulative fatigue in the sub. The fatigue tests run for the tools determine the maximum DLS the tools can handle. Each run the tools are modeled by an applications engineers and a decision is made based on the results of the model to determine the benefits of running the tools. Standards for the tools have been developed over time, but because each run has its own unique complications, an engineer is consulted before each use.

ACKNOWLEDGEMENTS

The authors wish to thank Weatherford for their support and permission to publish this paper.

REFERENCES 1. Long, T.P., McCormick, J.E. and Frilot, M.A., Inaccessible Drilling Targets and Completions Operation Made

Possible by the Alleviation of Excessive Torque and Drag, IADC/SPE 125991, IADC/SPE Middle East Drilling Technology Conference & Exhibition, Manama, October 2009.

2. Mason, C.J., Williams, L.G., and Murray, G.N., Reinventing the Wheel Reducing Friction in High-Angle Wells, SPE 63270, SPE Annual Technical Conference and Exhibition, Dallas, October 2000.