IADC/SPE 151356

Understanding How the Placement of an Asymmetric Vibration Damping Tool Within Drilling While Underreaming Assemblies Can Influence Performance and Reliability Alan Kabbara, John McCarthy, Timm Burnett, Ian Forster and NOV Downhole Ltd

Copyright 2012, IADC/SPE Drilling Conference and Exhibition This paper was prepared for presentation at the 2012 IADC/SPE Drilling Conference and Exhibition held in San Diego, California, USA, 68 March 2012. This paper was selected for presentation by an IADC/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 International Association of Drilling Contractors or the Society of Petroleum Engineers and are subject to correction by the author(s). The material does not necessarily reflect any position of the International Association of Drilling Contractors or 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 International Association of Drilling Contractors or 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 IADC/SPE copyright.

Abstract This paper describes the work, on test rigs and full-scale drilling rigs, carried out with respect to placement of an Asymmetric Vibration Damping Tool (AVDT) within drilling while underreaming operations. An AVDT, by virtue of the forward synchronous motion imposed on the drill string, offers benefits in minimizing down hole vibration related tool failures and therefore maximizing ROP. Of interest in using the AVDT is the tendency to minimize stick slip by means of the parasitic torque it generates. This is of particular importance during underreaming operations. While underreaming, stick slip can result in low ROP and potentially an increased incidence of down hole tool failures. The use of an AVDT in these operations has been shown to significantly reduce stick slip. However, due to the forward synchronous motion caused by the AVDT, there is the potential to cause eccentric wear to BHA components in the vicinity of the AVDT. If allowed to progress, this eccentric wear can cause a reduction in down hole tool life and drilling performance. Eliminating eccentric wear would be beneficial in reducing repair costs, extending component life and further improving drilling performance. To minimize eccentric wear and maximize drilling performance, the placement of the AVDT within the bottom hole assembly is critical. This paper describes how the placement of intermediate stabilizers between the AVDT and the underreamer can minimize eccentric wear to the underreamer and the adjacent drill string due to the forward synchronous whirl induced by the AVDT. This approach allows the full benefits of the AVDT to be recognized while reducing the potentially damaging effects of eccentric wear to other BHA components. The work has drawn upon small scale rig testing, full scale testing at the Ullrigg test facility in Norway and from real-world drilling and underreaming operations in the USA.

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Introduction The AVDT has been shown to offer significant benefits in mitigating stick slip during drilling operations. This is of importance during underreaming operations where stick slip is commonplace. With reductions in stick slip, there will be accompanying reductions in lateral vibration, reductions in tool failure rates and increases in ROP. However, the AVDT derives its stick slip mitigation performance by inducing a benign forward synchronous whirl motion in the drill string. This motion opposes the more damaging backward whirl motion and generates a beneficial friction al torque that serves to damp the spin up phase of stick slip. Because of the generated FSW motion, it is therefore important that placement of the AVDT with respect to the reamer is considered carefully in order to reduce the possibility of eccentric wear on the reamer cutters. Reamer Tool Reamer tools typically comprise three moveable blades within a tool body housing. There are numerous methods of tool actuation and de-actuation, but typical methods are hydromechanical where the hydraulic pressure increase required for tool actuation and de-actuation is achieved by dropping a ball , which then lands upon a nozzle (McCarthy, SPE/IADC 128951, 2010). Some tool designs use electronic chips dropped from the surface for activation and de-activation, but this technology is in its infancy. During reaming operations, torsional vibration and stick slip is common. With stick slip, lateral vibration is usually increased, and resulting vibration modes can lead to costly tool failures and reduced ROP. AVDT The operation of this tool was covered in a previous paper (Forster, SPE/IADC 128458, 2010). During drilling operations, vibration effectively reduces ROP, increases MSE and causes damage to drill bits and other elements of the drill string. One particular mode of vibration is the lateral mode, where the drill string or BHA assumes a whirling motion. Another damaging vibration mode is torsional vibration in particular, stick slip. There can be coupling of the lateral mode and torsional mode during stick slip i.e. there can be high lateral vibration associated with stick slip as will be discussed later. There are three types of whirl in the drill string: forward whirl, where the whirl is in the same direction as rotation; backward whirl, where the whirl is in the opposite direction to rotation; and chaotic whirl, which is a backward whirl where there are impacts between the drill string and drill bore (figs 1-3). Backward whirl is the most likely whirl to occur due to the inherent friction between the drill string and the bore acting in the opposite direction to rotation and when this occurs with discrete features on the drill string such as stabilizer blades, impacts/shocks are likely.

Fig 1 Backward Whirl

Point A

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Fig 2- Chaotic Whirl

Fig 3 Forward Whirl

Because the whirl is in the opposite direction to rotation, backward whirl results in particularly damaging reverse bending stress in the drill string. This is due the string bend due to the backward whirl having to reverse approximately every revolution of the string best described by a point (A) on outside surface of the string bend moving from the outside surface of the bend to the inside surface of the bend approximately after half a revolution of the string moving from tension to compression. For the next approximate half revolution, the point will move back to the outside surface of the bend back to tension. The cycle will then repeat. The string will therefore be under reverse bending fatigue. When high modes of vibration are excited by, for example, a concentric stabilizer, the bent/deflected shape of the drill string is such that drill string stress will be increased and the high frequency will mean that any resulting fatigue damage will accumulate rapidly fig 4 (ANSYS, 2008). Where there are high frequency impacts/shocks also occurring, the drill string damage will be increased further.

Point B

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Fig 4 4th Lateral Vibration Mode

Conversely, because forward whirl is in the direction of rotation, the tendency for reverse bending to occur is reduced. Here, the point on the surface of the string that is on the outside surface of the string bend, will approximately remain on the outside surface of the bend point B. Point B will experience only a very low frequency reverse bend and will therefore be under minimal fatigue. Moreover, if the forward whirl is synchronous to the rotation, the reverse bending is eliminated forward synchronous whirl (FSW). Point B will remain static on the outside surface of the bend. It will remain in tension and is therefore not under fatigue. Forward whirl is unlikely to occur naturally and is required to be induced.

A tool that enables FSW to be induced in the drill string/BHA will be beneficial to the drilling operation, since it will oppose the natural tendency of the drill string to assume backward whirl. Such a tool will also create an additional beneficial braking torque on the drill string due to the FSW motion and, as a result, will also tend to damp torsional vibrations.

The AVDT discussed in this paper is such a tool. The tool comprises an eccentric stabilizer with two blades in an acute angled Vee configuration that has been shown in field operations to reduce BHA and drill string vibrations lateral and torsional vibrations (fig 5).

Fig 5 - AVDT Under FSW

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Centrifugal Force

Previously, the reasons for improvements in performance observed in field operations when using this tool had not been formally quantified since the tool was originally designed for the purpose of stabilizing bi-centre bits. It was therefore found necessary to study and quantify the behavior of the AVDT with the intention to study the performance limits of the AVDT and ultimately optimize the design.

The design of two test rigs to study the behavior of the AVDT was purposely small scale. This was in order to achieve a comprehensive understanding of the behavior, parameters and variables associated with the technology that may arise without the potentially prohibitive safety issues associated with a large scale whirling drill string. The test rigs also provided repeatable test results.

The optimum speed for the AVDT in the vertical was found to be at the first natural frequency fig 6. This vibration mode is benign when compared to the previous fourth mode it has low non reversed bending stress at low frequency. Fatigue and impact on the drill string is minimal. The optimum speed will increase with inclination, and also will reduce with compressive load. For the test rig, the an operating speed range with respect to the first natural frequency was studied - lower limit 70% of first natural frequency; and upper limit 150% of first natural frequency.

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Fig 6 First mode The centrifugal effects for the AVDT are due to the eccentric orbit of the BHA. The parasitic torque due to friction will result in reaction forces due to the contact point span on the tool blades. Although the reaction forces will tend to force the AVDT into a backward whirl, the centrifugal force will oppose these destabilizing forces and will act to sustain AVDT motion - fig 7. Tools with large contact point span blades will result in a smaller de-stabilsing force and hence will have greater stability. The trailing blade of the AVDT (effectively the tools asymmetry) has the effect of opposing destabilizing shocks.

For a concentric stabilizer, the centrifugal force does not oppose the force that is tending to send it into a backward whirl, and hence, it is inherently unstable. It also has no asymmetry with which to oppose destabilizing shocks.

The parasitic torque of the AVDT has a beneficial effect in damping torsional vibration as this torque increases with speed, it is of particular importance during the spin up/slip phase of the slick slip cycle.

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Reactive Couple

Centrifugal Force

Fig 7 AVDT tool forces

The effect of the AVDT Non lateral vibration was studied on the lateral vibration test rig and indicates that the lateral vibration associated with the FSW motion was up to six times lower that the typical vibration levels resulting from chaotic backward whirl associated with the concentric stabilizer fig 8a. Note that, although the slick assembly in blue also indicates low vibration levels, there is no guarantee that it will remain stable the plot suggests that there are brief periods of instability.

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Con stab

AVDT

Fig 8a Lateral Vibration Rig Test Result

The effect of the AVDT on torsional vibration was studied on the torsional vibration rig, by firstly, replicating torsional vibration, and then by engaging the AVDT via a clutch. This enabled a back to back test to be carried out using unchanged parameters and also removing unknowns such as the effects of stopping and restarting the rig.

The results show that when the AVDT is engaged at timestep 3500, the torsional vibration diminishes dramatically, as a result of the parasitic torque created by the AVDT during FSW causing a braking / damping effect during the spin up phase of stick slip (fig 8b ).

Fig 8b Torsional Vibration Rig Test Result The rig testing has also demonstrated a link between lateral vibration and torsional vibration/stick slip although the results are not presented here. The knowledge gained on the small scale test rigs was then been used in the design optimisation and field operation of full size AVDT. The AVDT placement software takes determines optimum parameters for the tool for a given placement in the drill string.

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AVDT Placement and Performance The AVDT is typically placed in the upper end of the BHA either within drill collars or within HWDP. Placement software allows the required rpm for a given placement to be determined. Also determined is the forward synchronous whirl section length i.e. the length of collars/HWDP over which whirl will occur. This will determine the separation of the AVDT from adjacent tools such as the reamer. Alternatively, if the calculated rpm is not achievable (usually too high), alternative placement may be considered i.e. using small outer diameter collars or HWDP; using a smaller size AVDT to achieve a greater orbit; using collars with an offset bore to increase the orbit; and using stabilisers to alter the FSW nodes. Typical improvement in torsional vibration, associated lateral vibration and ROP that have been achieved with the AVDT are shown in deep water field test results in the Gulf of Mexico in salt and salt exit (McCarthy, OTC 151356, 2011). In this application several wells were drilled through the same formation, with similar BHAs and bits and with similar inclinations. This allowed effectively back to back testing. In this application, it was possible to compare three non AVDT runs with two AVDT runs, and the results demonstrate the improvements that can be achieved in ROP figs. Here wells no AVDT 01-03 show high stick slip, low ROP, whereas, AVDT wells 01 and 02, show much higher ROP, lower stick slip. Even when 100% FSW is not achieved as is the case for AVDT well 02 during the build section there is still an significant improvement in stick slip and ROP over the non AVDT wells. These results are shown in Fig 9.

100% FSW

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Fig 9b Gulf of Mexico Results in Salt and Salt Exit

AVDT Reduced rpm

Fig 9c Gulf of Mexico Results in Salt and Salt Exit The AVDT wells also achieve performance improvement at lower rpm contrary to typical stick slip parameter mitigation methods i.e. reduced WOP and higher rpm. The typical improvements in the AVDT runs when compared to the non AVDT runs were as follows: AVDT 01 100% FSW 50% increase in ROP 70% reduction in stick slip 40% reduction in lateral vibration AVDT 02 - < 100% FSW through build section 40% increase in ROP 50% reduction in stick slip Lateral vibration comparable

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Placement Field Test 1 The first field test which was used to demonstrate the FSW motion imposed on other tools in the drill string by the AVDT was at the Ullrigg test facility in Norway. This testing was also described in a previous paper (Forster, SPE/IADC 128458 - 2010). The BHA comprised an AVDT , a reamer, a near bit stabiliser and a bit. The bit size was 12.25 inches. During the tests, high frequency data recording of lateral vibration was used at several locations in the string including at the bit (Barton, SPE 122208, 2009). The test demonstrated the effect of FSW motion on an off bottom bit with a near bit stabiliser. It should be noted that during the bit off bottom test the hole quality was poor i.e. it was 13-14 inches diameter and of variable quality and diameter. This would have the effect of exaggerating any lateral tendencies of the bit under FSW. With downhole vibration measurement and recording at the bit it was shown that when the AVDT achieved FSW motion there was a large reduction in torsional vibration/ stick slip, although the location of the near bit stabiliser resulted in the FSW motion being imposed and magnified at the bit with a resulting large increase in lateral vibration fig 10. The configuration was then replicated on the test rig, and identical lateral vibration increases were observed when the drill string rpm moved from below FSW speed to 100% FSW speed.

100% FSW

Fig 10a Ullrigg test

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100% FSW

Fig 10b Ullrigg test For off bottom applications therefore, it would be desirable to either reduce rpm to below FSW speed or to introduce a second bit stabiliser to ensure that FSW is not projected on to the bit i.e. minimise the pivoting action of the bit stabilisation. Placement Field Test 2 The second field test was carried out in a land application, and has involved several runs with the AVDT (NOV, 2011). All runs were through an abrasive hard formation. Prior to use of the AVDT , there was experienced significant torsional vibration/stick slip, shown in the run summary. Also shown in the run summary, was the improvement in torsional vibration/stick slip when the AVDT was used fig 11a.

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No stab

Fig 11a Land Application Placement In this application, the AVDT was placed 60-90 ft from the reamer and no stabilisation between the AVDT and the reamer. However, use of the AVDT without stabilisation did result in wear of drill string components due to FSW being projected down hole. The wear was on collars adjacent to the AVDT FSW section. The abrasive formation was also a contributory factor. The first revised configuration involved the use of a single stabiliser between the AVDT and downhole components, but this resulted in a pivot point for FSW to be projected downhole, and as a result did not reduce wear fig 11 b. The wear was on the reamer tool resulting in wear on one blade only.

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Fig 11b Land application Placement

Single stab

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FSW node Shift onto stab or below the stab

Original Node pattern

FSW projection Onto reamer

Fig 11c Land Application Placement The FSW node plot shows one possible FSW node shift scenario where the node is shifted below the stabiliser fig 11c. Another possible scenario would be for the node to be shfted onto the stabiliser as was the case for the Ullrigg tests, and also during the small scaled testing. In this latter rig test case, the rig had stabilisation points which were outside the natural FSW node points for the string. The rig operating in a whirl mode consistent with the stabiliser nodes being the dominant ones. The final configuration involved placing two stabilisers in tandem between the AVDT and downhole components, and this did successfully result in minimal wear for subsequent runs - fig 11d. In this case, the purpose of the double stabilisers was to minimise any pivoting, and hence the projection of FSW onto the drill string components below the AVDT.

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Fig 11d Land Application Placement

Double stabs

Discussion The placement and operating parameters of the AVDT have been shown to be critical in the successful operation of the tool in getting improvements in stick slip and ROP. The tool should be operated at an rpm that achieves 100% FSW and should be placed at a location that allows the tool to influence the stick slip and lateral vibration performance of the BHA. Equally, the placement of the tool should be carried out with regard to BHA components downhole, and, if the AVDT is close to adjacent critical components, additional stabilisation may be required to ensure that FSW is not projected on to these components.

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The field tests described in this paper demonstrate the effectiveness of the additional stabilisation. Conclusions The AVDT has demonstrated improvements in stick slip, lateral vibration tool failure rates and ROP. The AVDT results during initial small scale rig testing work has been confirmed during full scale field tests. The tool has the advantage of non moving parts or electronics for operation. For success full operation of the tool, pre job planning is critical both in terms of operating parameters (first mode, 100% FSW) and also in the placement of the tool with respect to the BHA stabilisation between the AVDT and downhole components. Future work should look at additional applications for the tool such a small bore applications, horizontal applications; dual AVDT applications (both closely spaced AVDT, and AVDT spaced further apart); and rotary steerable applications. Additionally, high frequency data recording should be adopted on AVDT applications where possible, so as to aid verification of tool performance. This has shown to be of importance during the Ullrigg tests. Current high frequency recording tools can record data at a number of locations in the drill string. In all future applications, feedback is essential in verifying the placement and parameters software. The feedback will also be of benefit in verifying stabilisation strategies.

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References

1. Forster, I., Macfarlane, A., Dinnie, R., NOV Downhole Ltd. 2010. Asymmetric Vibration Damping Tool Small Scale Rig Testing and Full Scale Field Testing. Paper SPE/IADC 128458, presented at the Drilling Conference, New Orleans, 2-4 February.

2. John P. McCarthy, Ian Forster, Timm Burnett, Alan Kabbara, National Oilwell Varco. Careful Planning and Application of an Asymmetric Vibration Damping Tool Dramatically Improves Underreaming While Drilling Performance in Deepwater Drilling. OTC 151356. OTC Brazil Oct 4-6 2011.

3. Barton, S., Clarke, A., Garcia, A., Perez, D., NOV/ReedHycalog, Mora, G., Carrion, C., Petroamazonas. 2009.

Improved Drilling Performance: Downhole Dynamic Logging Tools Break Paradigm in Ecuador. Paper SPE 122208, presented at the Latin American and Caribbean Petroleum Engineering Conference, Cartagena de Indias, Colombia, 31 May-3 June.

4. ANSYS Design Space/Work Bench FEA Version 11 2008. 5. McCarthy, J.P., NOV,Martin, W. H., Petrohawk, Truly Selective Underreaming: Adaptation of Field Proven,

Hydraulically Actuated, Concentric Underreamer Allows for Multiple Unlocking/Locking Cycles in a Single Run SPE/IADC 128951, presented at the Drilling Conference, New Orleans, 2-4 February.

6. NOV Downhole Ltd, Field Tests 2011.

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Glossary BHA Bottom Hole Assembly CHO Concentric Hole Opener (Underreamer) AVDT Asymmetric Vibration Damping Tool HFDR High Frequency Data Recorder ROP Drilling Rate of Penetration SS Stick Slip RPM Revolutions per Minute FSW Forward Synchronous Whirl GOM Gulf of Mexico TD Total Depth MSE Mechanical Specific Energy WOB Weight on Bit

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