Well Control And Intervention

Unconventional coiled tubing operation restores a Brunei offshore well

Pioneering gate valve milling saves time.

Nasser M. Al-Araimi, Brunei Shell Petroleum; Ben Gedge and Karl Burns, Weatherford; and Mahesh Mahajan, BJ Services

During routine wellhead maintenance, the Lower Master Gate Valve (LMGV) on Brunei Shell Petroleum Offshore’s (BSP) Well-A seized. Given that the well was online and producing, it was assumed that the valve had seized in the fully open position, even though there was no immediate way to confirm it. A number of attempts were made to free the valve by moving and turning the wheel handle, but without success. Calcium carbonate scaling was identified as a likely cause of both the valve seizure and the progressive production decline observed since completion. As a result, an HCl soak treatment was performed by pumping 20 – 25 bbls of 15% pre-mixed acid to the wellbore. This wasn’t successful.

The same acid soak treatment was repeated. Given the closed nature of the LMGV, this treatment used a series of smaller 1 – 1.5 bbls acid pills, which were pumped and left to soak on the top of the closed valve. Each spent pill was displaced after several hours via the flow wing outlet. This treatment was also not successful in restoring valve movement.

An alternate approach was required. The timing of this requirement was driven in part by the need to access and inflow test the Sub Surface Safety Valve (SSSV) beneath the LMGV. The approach chosen was to mill through the seized gate and seat assembly. This would allow through bore access to inflow test the SSSV and set plugs, as required, for tree repair, recovery or replacement.

OPERATIONS

Well-A was shut-in and inoperable with the LMGV seized in the closed position on the monoblock tree. The offline well caused a production loss of 200 cu-mpd (1,260 bopd), and with a monetary loss of $1.9 million for every 30-day period the well was offline. Given the production loss, BSP planned a well intervention to bring Well-A back online, as soon as the equipment and personnel could be mobilized to the platform.

The first action was to ascertain the best method to perform the well intervention. The intervention should ensure the highest chance of success, be cost effective, and take the HSE issues into consideration. To recover the well, two well intervention options were considered, instead of a full workover, which would have been very costly.

1. Mobilize a gate valve milling machine
2. Mill out the valve using a coil tubing unit, which was soon to be mobilized to the platform for a stimulation campaign on adjoining wells, Fig. 1.

Fig. 1. The CT unit used was a 1.75 in. coil 0.125 QT-800 with 1,500 m of coiled tubing.

The first option has proven successful in a number of locations around the world, particularly on geothermal operations. Christmas tree valve seizure is common on geothermal wells due to scale precipitation caused by high production and surface cooling.

However, the only available unit was in Europe. To mobilize the unit to Brunei would be expensive and time consuming. Plus, additional equipment like BOPs would also have to be mobilized with the machine, even if it was technically sound. This made for a very expensive option.

The second option was attractive from a cost and logistics viewpoint, since a Coiled Tubing (CT) unit was being mobilized to the platform. The only problem was that there were a limited number of case histories on similar coiled tubing operations, and only one North Sea case could be identified. The HSE issues were a serious consideration, due to the limited data from previous operations.

A Service Well on Paper exercise was conducted and found that the milling operation was possible, given that the specialist, high-torque, low-speed, downhole, through-tubing motors, were available in Brunei. Other key components of the equipment package, such as the anti-stall tool and carbide insert stainless steel gate valve mills could be procured quickly, and mobilized at low cost.

In addition, the HSE issues were addressed and found that the risks were no greater than a conventional coiled tubing operation, as long as certain safeguards were in place. This included additional gate valves above and below the coiled tubing BOPs and the use of a flanged riser. Also, the cost was acceptable, since the majority of the equipment and personnel were in country.

JOB DESIGN

The next stage was to design the job and write the operational program. The first task was to mill out the LMGV in the 3-1/8 in., 5,000-psi monoblock dual string tree and gain access to the well after the milling operation. Then, a wireline plug could be set to secure the well after milling. It would be necessary to maintain proper and effective well control at all times and be able to run a fishing string to retrieve the bottomhole milling assembly, should that become necessary, see Table 1.

Other design considerations included using the shortest possible BHA with the shortest possible motor. The motor must have the least possible eccentric movement and be low speed, high torque. Low RPM, coupled with minimal eccentric movement, gives a smoother and faster milling ROP. Less bit chatter leads to less wear on the downhole tools.

In addition, the BHA must incorporate tools that allow the coiled tubing to be manipulated without stalling the motor. The mill design must allow full bore access in a single run and the BHA must be designed to be finished if it becomes disconnected from the coil tubing.

One additional tool was needed – an anti-stall device. The anti-stall tool controls the force applied to the bit by contracting under high torque loads. When the torque drops, the tool expands back to its original length. This action stops the bit from jamming and produces a smooth, effective penetration rate, regardless of the stick-slip effects. Otherwise, constant pump shutdowns and bottom pick-offs would have occurred.

MILLING

The total milling time for the operation was six hours and the total job time was 24 hours. However, due to the milling being performed at the surface and over such a short interval, depth correlation proved to be difficult.

The stroke of the bumper sub and anti-stall tool could make a difference of one foot either way. The pressure change across the motor during the milling operation was 150 psi to 400 psi at a constant pump rate of 63 gpm. The drive fluid was water with an added friction reducer.

During the milling operation the shear release sub castellations were broken. The mill become stuck, which was probably caused by a non-centered crossover sub on top of the tree. The mill became stuck in the back pressure valve profile above the tubing hangar. As the anti-stall tool contracted, the torque applied to the top sub of the “shear disconnect” broke the castellations. After the break, the top sub of the broken castellations disconnected and was milling on the bottom sub half of the tool, Fig. 2.

Fig. 2. The shear sub used for the operation shows damage from broken castellations and rotational wear.

Initially, the CT crew did not see that the shear disconnect had broken and they continued milling. Once the crew realized what had occurred, a fishing job was organized. This was made more difficult by the GS profile on the lower half of the disconnect sub, which was worn off by the top sub milling action. The mill and lower half of the disconnect tool were successfully fished with slickline, using a tool string of sinker bars, spang jars and a non-releasable bulldog spear.

After retrieving the fish, a drift run was made with slickline to drift the SSSV. This was successful, and no debris was found in the well. By depth correlation, the crew found that the gate valve was successfully and completely milled out. On retrieval, the mill was in excellent condition. It sustained only one score mark three inches above the carbide cutting structure, Fig. 3.

Fig. 3. The used mill on the left shows minimal wear compared to the new back up mill on the right.

Table 2    Click image for enlarged view

RECOMMENDATIONS

After the job was completed, BSP conducted a review to identify the key learning and recommendations from this operation for future jobs. Regarding the mill, carbide should be laid on the 45° shoulder above the mill’s water courses to allow for back-reaming.

For the BHA, the design should ensure stabilization whenever possible for full gauge to the tubular’s drift diameter. A dampening device should be included in the string whenever possible. Options to consider are a bi-directional accelerator, a lubricated bumper sub or a thruster type device. The bumper sub should be left out of the BHA, because the expanding and contracting mandrel causes depth correlation problems.

No release subs should be run below the motor, because there is too much torque for any current tool to function correctly, and to avoid possible damage to the motor. Designing and building a release tool, which could withstand the motor torque generated, would be an advantage. If it could have been run on this job, any disconnect would have left only a short fish in the well, allowing a full fishing string to be run without restriction on slickline.

Regarding the CT unit, using a coil larger than 1-3/4-in. would give greater wall thickness and ID for much greater mechanical and hydraulic advantage. Whenever milling is performed at a depth shallower than 1,000 ft, the coil should be cut every 2 hours or 10 stalls, whichever occurs first. The coil should continue to be cut after every milling run. The job designer should review the available injector types to determine whether feed sensitivity, hence weight-on-bit control, can be enhanced to minimize stalling.

Concerning the planning process, job designers should ensure that trials reflect planned operations as accurately as possible. They need to recognize that even small changes may have a significant impact. If a change from trial-developed parameters is unavoidable, then they should build in further safety factors. In this case, a conservative staged execution, i.e., progressive pilot hole, could have been an option. Designers should consider the available coil tubing connector types to ensure that the optimum design has been selected for this demanding application. Issues of stress concentrations and surface stress raisers need to be considered; compare and contrast among the vendors.

Select the bit on trial performance, but retain contingent options, if milling performance is erratic with a high number of stalls. Options include running a less aggressive mill or starting with a smaller mill size.

For future well maintenance, the operator should resume grease injection packing of christmas tree valve cavities for minimally actuated (3 times/yr) LMGVs on trees with known scaling tendencies. This injection packing should be carried out after each use to prevent scale build-up and seizure.

CONCLUSION

A limited history was available to aid planning this challenging, unique operation. However, this was a very successful operation, with only minimal difficulties given its potential complexity and lack of case histories for guidance.

No defects were identified in the preparation of the coil or connector equipment. Shallow milling produced cyclic reactive forces that led to failure of the coil by initiating a stress corrosion crack at a connector dimple. The crack propagated and weakened the remaining coil until failure occurred, when the stripper was tagged on BHA recovery to surface. A range of factors contributed to the incident. The torsional yield rating of the coil was not exceeded; rather, cyclic fatigue loading in a seawater environment formed and propagated an external surface stress corrosion crack.

Using the coiled tubing and pumping package, which was already on the platform, provided the lowest possible cost option, while maintaining full well control and HSE considerations. The well was recovered with minimal downtime after the milling operation was completed, then put back on production. 

ACKNOWLEDGMENTS

The authors thank Brunei Shell Petroleum, Weatherford and BJ Services for permission to publish this paper. The efforts of Abd-Nasir Abd-Rahman, Zulkapli Abdullah and Zulkepli Abd-Hadis of BSP during the gate valve milling operation are appreciated.

This article is derived from SPE 96685, which was presented at the 2005 SPE Annual Technical Conference and Exhibition held in Dallas, Texas, US, Oct. 9 – 12, 2005 and is reprinted with permission.

REFERENCES

  ¹  BJ Services Coil Tubing Operations Policy and Procedures Manual.

  ²  Brunei Shell Petroleum Coil Tubing Operations HSE case [BSP-73-CTU-HSE Case-001].

  ³  Brunei Shell Petroleum Well Engineering Manual for Coil Tubing Standards and Procedure [Volume 4, BSP-73-029].

  ⁴  C. Kruger, T. Sælensminde, and S. Myrmel, “Milling of Isolation Valve with Wireline Conveyed Technology,” SPE/IADC, SPE 92024 (2005).

  ⁵  Control of Substances Hazardous to Health Guidelines (2002).

  ⁶  Eirik Sorgard, Egil Ostvik, Njal Gronnerod, John Villar, Tom Salvesen, and Lars Tjaland, “Coiled Tubing Milling and Temporary Plug and Abandonment Operations,” SPE 54472 (1999).

  ⁷  MacDrill Operators Handbook Manual for Drillability, Torque and Hydraulics.

  ⁸  Marine IMDGC HSE Case [BSP-14.05-Report-SMRHSECASE01].

  ⁹  Shell Exploration and Production HSE Guidelines - EP 95000.

¹⁰  Weatherford Anti-Stall Tool Guideline and Performance Curve.

THE AUTHORS
	Nasser Al-Araimi has 12 years of industry experience, joining Petroleum Development Oman in 1994, as petroleum engineer. Al-Araimi was transferred to Brunei Shell Petroleum Company (BSP) in 2003 as head coil tubing and production support services, looking after all CT, stimulation and production services support. Recently, he joined the BSP corporate liquid forecast at the business planning and economics group. He holds degrees in Petroleum Engineering and Master in Business Administration.
	Ben Gedge has 35 years of industry experience. In 1999, Gedge joined as business development director for Underbalanced Services in the Eastern Hemisphere based in Dubai and then Kuala Lumpur. In 2004, he moved to his current position in Brunei as Weatherford Country Manager and Shell EP-Asia focal point.
	Mahesh Mahajan is senior district engineer for BJ Services Co. Previously he has worked for ONGC and Shell at various positions. He has 23 years’ past experience in the well services area including well stimulations, coiled tubing and completions. He has co-authored six SPE papers and has served several SPE forums as Working Committee member. He earned BE, Hons degree in Mechanical Engineering from University of Indore, India.
	Karl Burns has 21 years of industry drilling experience. In 2000, Burns joined Weatherford as district product line manager for Intervention Services in Indonesia. In 2004, he was transferred to Brunei to set up a new thru-tubing facility for Weatherford, where he is currently based.