Coiled Tubing Technology

Anti-Stall Technology boosts the value of coiled tubing well intervention

A new way to prevent stalling in downhole drilling motors has been shown to improve the efficiency of drilling and milling in oilwell applications.

Vibj�rn Dagestad, Statoil ASA, Nils Reimers, Tomax AS, Katrine Eide and Magne Mykkeltvedt, PI-Intervention AS

Long horizontal wells were established as the best strategy for good field economy on the Norwegian Continental Shelf in the early '90s. The wells, however, required new intervention methods. Statoil turned to Coiled Tubing (CT) as the primary option, but as the wells became more advanced, friction made interventions troublesome and operations with downhole motors costly and unpredictable. Simultaneously, Statoil began looking for improvements in through-tubing technologies. One such development was the Norwegian company Tomax AS's proprietary Anti-Stall Technology (AST), Fig. 1.

Fig. 1. AST is based on a telescopic tool placed anywhere in the BHA. The tool regulates the force on bit as a function of drilling torque transitions through an internal helical thread and absorber system.

CONCEPT

The concept is based on controlling drilling torque by regulating the weight on bit to assist the driller attempting to do the same from surface. Placed anywhere in the BHA, the new tool function prevents stalls and extends the life of drilling tools through significant reductions in vibration and shock.

In brief, the tool works as follows:

� When the bit approaches stall, the quickly increasing torsion will activate the tool, causing it to contract.

� This contraction will offload the bit, which will then spin up to its original speed.

� With the bit back at full speed, the unit will gradually release accumulated torsion.

� The process can be repeated as often as necessary.

TESTING/QUALIFICATION

Statoil produced two identical prototypes for qualification tests as a part of an internal through-tubing technology project. The program included three shop/bench tests and eight full-scale offshore operations. The tests confirmed tool function according to the theoretical background. The first bench test was done with a 2 7/8-in. Weatherford drilling motor.

First, a reference BHA was run without the anti-stall tool, and then with the tool. The bench setup was intended to simulate poor weight transfer where the BHA was jumped quickly forward and the bit rammed into an obstruction. In this case, a metal ring represented the top of a bridge plug. The feed was provided by air cylinders.

In the reference run, shock loads of about 0.5 kdaN (1,100 lb) were enough to stall the motor. With the anti-stall tool included, the motor did not stall even with the maximum available pressure and shock loads of 2.4 kdaN (5,400 lb). Another, more surprising, observation was the high rate of penetration that was produced when the motor was drilling at 2.4 kdaN�about 1 in. in a total of 15 min. Comparing the pressure gauges on the pumps with the output chart for the motor showed that, while attempting to prevent the motor from stalling, the AST controlled the motor differential pressure at high levels with an output close to the maximum horsepower.

Bench observations showed that the anti-stall tool established a stable state almost immediately after the bit engaged the obstruction. Also, the accumulated torsion inside the tool was observed to balance constantly against the bit torque; hence, the tool could be expected to keep the bit on bottom. Measuring oil deposits on the telescopic surface showed that axial movements were limited to an average of 2 mm and a maximum of 5 mm, much smaller than the 5 in. available, and the bit appeared to run at constant speed.

In terms of internal orientation, this equals about a 4° average and a 12° maximum. The theoretical time to stabilize tool orientation is the time the reactive pressure takes to reach the pump, which equals the length of tubing multiplied by the speed of sound in fluid (about 5 sec. for 6,000 m).To test the AST's ability to stabilize the BHA and its orientation, the bit was run into the obstruction without flow. Pumps were then started with the bit on bottom. The AST reduced the BHA length and the force on bit just enough to allow the motor to start and accelerate to optimal speed. Again, the system used just a few seconds to stabilize and produce good penetration.

FIELD OPERATIONS

The prototype tools were deployed to Ekofisk field and operator ConocoPhillips to mill a long section of hard scale from a minimum drift of 13/4 in. up to 4 in. Due to efficient penetration in the initial run, a planned intermediate run was left out, and the second mill completed the job. The Final Job Summary noted that the motor did not stall when milling commenced. The anti-stall device was ordered for a second well from the same platform. Again there was a sharp reduction in motor stalls. Eight consecutive field deployments saw positive overall results: significant reduction in stalls, low wear on CT and BHA, minimum fatigue cycles on CT and jobs completed ahead of plan.

In the Norwegian sector, the new technology is improving CT campaigns. Interventions with motor and mill are now often completed in one run. AST has consequently become a part of the �Best Practices� operational document for initial supporter Statoil. Other operators are also using the technology on a permanent basis, and according to the service supplier, the new technology was used for all CT drilling operations on the Norwegian Continental Shelf, regardless of operator, by September 2006. At the same time, 34 wells had been serviced worldwide using AST, and the fleet was expanded to cover the full range of 111/16-in. to 43/4-in. OD. The tool is now being tested for regular rotary drilling.

A set of tools was sent to Through Tubing Solutions Inc. in Oklahoma City along with an AST field services specialist to evaluate the technology for the US market. The first job was in the Fayetteville Shale, where the tool was run to improve efficiency in drilling out composite bridge plugs left after well stimulation. The anti-stall tool was run behind a 27/8-in. motor and produced a reduction in stalls, Fig. 2. The field operator has continued using the technology in challenging gas wells. Another set of tools was sent to Canada for similar operations, and the same efficiency improvement was produced.

Fig. 2. Stabbing an AST above a 2 7/8-in. motor for the first Fayetteville Shale job.

ADDING RIG VALUE

An overall observation from the Midwest and Canadian intervention specialists was that the AST's ability to overcome the effects of friction and doglegs could move the window for CT operations to wells previously requiring a workover rig. Such an extension of CT capacity has been the prime driver for engineering and improvement of AST since the first operation. By analyzing tools received back from the field, the tools' mechanical properties have been optimized. Further engineering and simulations aim to reduce the effect of jarring on the tool in both directions, using a pressurized fluid-transfer system like a modern shock absorber, but one that will transmit high, fast impacts similar to that of a hydraulic jar. At the same time, the system would dampen less precise impacts and counteract natural frequencies resulting from bit rotation.

THE AUTHOR
	Vibj�rn Dagestad is staff engineer for through-tubing technology at Statoil ASA. He earned an MS in petroleum engineering from the University of Stavanger and has been with various field services companies before his present position. He can be contacted at: vidag@statoil.com
	Nils Reimers is CEO of Tomax AS and co-inventor of the AST tool. He earned a BS in petroleum engineering from the University of Stavanger and comes from a position as vice president, Drilling Equipment at Aker Kvaerner. He can be contacted at: post@tomax.no
	Katrine Eide is director of Quality and HSE at PI-Intervention AS, where she also has been in charge of the commercial AST market introduction. She holds a BS in petroleum engineering from the University of Stavanger. She can be contacted at: ke@pi-intervention.com
	Magne Mykkeltvedt is a sales and operations engineer for Downhole and Intervention Services with Weatherford. He can be contacted at: magne.mykkeltvedt@weatherford.com