Mechanical oscillator frees stuck pipe strings using resonance technology

This mechanical oscillator can handle dynamic hook loads up to 160,000 lb. It has successfully retrieved stuck coiled tubing and liners (Fig. 3).

Resonant vibration technologies provide an alternative method for freeing stuck coiled-tubing, liners, and drillstrings.

Situations where the axial reciprocation of pipe; the use of large tensile, compressive, or percussive forces; or where pipe/well bore friction prohibits movement of the string may respond favorably to the use of pipe vibration.

Although vibratory technology for recovering stuck members was first patented almost 40 years ago, very little has been done to construct equipment suitable for oil field use until the mid-1980s. At this time, Pool-Resotek developed applications involving liner, tubing, and drill pipe recovery. The success rate for each was 62%, 19%, and 25%, respectively.¹

According to a 1986 IADC/SPE paper, "The use of resonant standing-wave energy for the extraction of stuck tubulars is a new and promising technique that has been tested in more than 70 wells since 1984. The bulk of the experience with this technique has been in the area of shallow gravel packed liners to 3,500 ft. The deepest drillpipe extracted was at a depth of approximately 7,800 ft, while the deepest stuck tubing retrieved was at approximately 9,000 ft."1

The resonant vibration system consists of three basic components (Fig 1. [100,905 bytes]):

• A mechanical oscillator with suspension for isolating the rig structure
• A work string for transmitting vibrational energy
• A stuck member or "fish" to be recovered.

The eccentric weight mechanical oscillator generates an axial sinusoidal force that can be tuned to a given frequency within the operating range of the oscillator. In its simplest form, a pair of eccentric weights are mounted in a common plane on parallel shafts and timed to rotate together in opposite directions. This results in a net sinusoidal force output that is on a line perpendicular to the plane of the oscillator shafts.

The net force output generated by the oscillator acts on the work string to create axial vibrations. The vibrating work string transmits and delivers power developed at the oscillator to a region where the stuck member is located. At resonant frequencies, none of the oscillator output is consumed in accelerating the work string so that the energy developed at the oscillator is efficiently transmitted to the stuck area, with the only losses being those attributed to frictional resistance.

The vibrational energy received at the stuck area works to effect the release of the stuck member through the application of large percussive forces, fluidization of granular material, dilation and contraction of the pipe body, and a reduction of well bore friction or hole drag.

Advantages of the technology

The most substantial and apparent advantages of this technology are:

• The operation is conducted from surface utilizing equipment suspended above the wellbore.
• The procedure can be quickly applied with very little preparation and often while activities are under way for initiating conventional fishing measures.
• The procedure is low risk with respect to further deteriorating the situation.
• The results are often immediate, and considerable time can be saved in resolving a stuck pipe situation compared to conventional means.

Pipe vibration offers a unique means of freeing stuck downhole tubulars from a well bore. Operating a pipe-vibration system under resonant conditions causes certain phenomena to occur that are highly beneficial to the freeing and recovery of stuck tubulars:

A resonant vibrating system stores a significant quantity of energy much like a flywheel. The ratio of the energy stored in relation to the energy dissipated per cycle is referred to as the system's Q. A high energy level allows the system to transfer energy for a given load at an increased rate, much like voltage increases will allow a flashlight to burn brighter.

Only resonant vibrating systems will achieve this energy buildup and exhibit corresponding efficient energy transmission characteristics, assuring large energy delivery and corresponding force application to a stuck region of pipe.

Under resonant conditions, a string of pipe will transmit power over its length to a load at the opposite end. The only power loss consists of that necessary to overcome resistance in the form of damping or friction. In effect, power is transmitted in a similar manner to the rotary drilling process. The difference is that the vibrational motion produces axial translation instead of rotation.

The load accepts the transmitted power as a large force acting across a small distance. Resonant pipe vibration can deliver substantially higher sustained energy levels to a stuck tubular than any conventional method including jarring. This achievement occurs because it eliminates the need to accelerate or physically move the work string mass. Under resonant conditions, the power is applied to a vibrating pipe string in phase with the natural movement of the pipe string.

Technically, Poisson's ratio is defined as the ratio of lateral strain to axial strain in a body. This implies that when an elastic body is subjected to axial strain, such as when a length of pipe is stretched, its diameter will contract. Similarly, when the length of pipe is compressed, its diameter will expand.

Because a length of pipe undergoing vibration experiences alternate tensile and compressive forces occurring as waves along its longitudinal axis, its diameter will expand and contract in unison with the applied tensile and compressive waves. This means that for alternate moments during a vibration cycle, the pipe may actually be physically free of its bond.

Fluidization is another effect that promotes the freeing of stuck pipe. Fluidization is the action of granular particles when they are excited by a vibrational source of proper frequency. Under this condition, granular material is transformed into a fluid-like state that offers little resistance to movement of bodies through the media. In effect, it takes on the characteristics and properties of a liquid.

Practical considerations

Pipe stress is the limiting factor in applying vibratory energy just as it is in applying other forms of energy such as rotary power when drilling. Pipe stress is a function of both amplitude and frequency of vibration as well as the tensile load (Fig. 2 [105,214 bytes]). A broad operational range is available, but the choice of one is limited by the other if stress is maintained constant.

Currently, the hook load capacity is limited to 160,000 lb. Vibration Technology LLC's mechanical oscillator (Fig. 3 [10,023 bytes]) can handle dynamic hook loads up to 150,000 lb. It is 15 ft long, 4 ft 6 in. wide, and 2 ft 6 in. in depth. It is powered by a Detroit 8V-71T diesel engine. The oscillator weighs 5,000 lb and the power unit weighs 7,000 lb. It uses a 41/2-in. tool joint connector.

Tubular connections

Geometry of tubular connections has a direct bearing on stress concentration factors that, in turn, directly affect allowable stress. One of the most significant contributors to stress concentration is an abrupt change in section, including areas of thread and coupling engagements.

Casing and tubing threads without shoulders are the weakest with regard to fatigue strength, and stresses should be limited to 10,000 psi. Shouldered connections, including most premium casing and tubing threads, as well as conventional tool joints, can generally withstand fluctuating stresses of 20,000 psi or more.

Fatigue stress

Because of the oscillating nature of the transmitted wave alternating from tensional to compressional forces, metal fatigue is a primary consideration in determining safe operating limits. The fatigue, or endurance limit, of a metal or alloy is the stress level below which failure presumably will not occur in an infinite number of cycles.

Oil field tubulars generally have a fatigue limit of approximately 50% of their ultimate strength. In the consideration of fatigue stress in conjunction with normal stress, a Goodman or Soderberg diagram may be helpful in determining acceptable levels of overall stress.

Attenuation

Well bore geometry is the most significant factor affecting friction, and therefore damping of the vibratory energy. Intuitively, a straight and vertical well bore is most desirable, but is rarely the case.

Studies and data concerning vibrational energy damping in a well bore are scarce; however, the information that is available indicates that damping is not too significant up to depths of 6,000 ft in the presence of either water or mud, and may not even be a limiting factor up to depths of 10,000 ft.2 The effect of well bore geometry on damping is a subject that requires more study.

Recent examples

A recent example where a mechanical oscillator vibrated free a coiled-tubing string occurred in Southeast Texas. The work was initiated and completed in 1 day on May 19, 1997, for Chevron USA Production Co. The time included the rig-up and rig-down of all equipment.

The pipe vibration process was conducted from surface. Neither downhole intervention nor cutting of the coiled tubing string was required. Prior to the use of this technology, the operator had exhausted all conventional means of recovery.

Before the coiled tubing became stuck, a routine washing operation involving fill circulation using a 11/2-in. string inside a 23/8-in. tubing string, was positioned below a production packer set at 10,500 ft. Nitrogen was used during the circulation for hole cleaning purposes. As the work continued, the coiled-tubing string became temporarily stuck. The operator worked the pipe free by axially reciprocating the coiled-tubing string through back-and-forth movements.

Afterward, the string was pulled into the well tubing, circulated out, and the washing operation recommenced. (Normally a safe and cautious procedure.) Nevertheless, the coiled tubing became stuck again in the perforated zone and could not be worked free.

Conventional means were employed to free the stuck coiled tubing at this point, including flowing the well in an attempt to blow away any obstruction, spotting beads in order to reduce friction, and circulating with and without the use of nitrogen.

The next step would have been to cut the stuck coiled tubing at surface allowing for wire line access and subsequent downhole intervention measures. The alternative to this step was to employ pipe vibration from the surface in order to free the stuck coiled tubing.

One of the highly desirable aspects of the vibration process is that it does not require the coiled tubing to be cut or damaged. By using a coiled tubing bail, it was possible to attach the oscillator to the stuck coil using a clamping mechanism that does not distort or damage the coiled tubing.

The hook-up allowed the stuck coil to be worked under tension and compression as well as to be vibrated without the usual fatigue damage associated with an injector-head gooseneck.

The procedure consisted of scanning the frequency spectrum in order to identify the available resonant frequency range. Then it operated under one or several resonant frequencies in order to deliver a sustained level of energy to the stuck region. In this case, 5.9 hz was selected and the operation continued. Within 10 min, a change developed in the vibrating system, and the resonant frequency dropped to 5.8 hz indicating that a longer length of pipe was being acted upon in the system.

The vibration process was then stopped, and the crane operator was directed to pull upward on the stuck coiled tubing. The equipment was elevated with no increase in weight and the pipe came free. Because only a small amount of upward overpull was placed on the stuck coiled tubing, no noticeable change had been observed on the weight indicator.

After transferring the coiled-tubing string back to the injector, the equipment was rigged down, the well was circulated again, and the coiled-tubing string was withdrawn from the well.

Liner recovery

The mechanical oscillator was also used to free and recover a liner set through a 90° hole deviation. The work was completed on Apr. 24, 1997, on a well located in South-central Oklahoma for Oryx Energy Co. The job was finished in less than 8 hr, including the rig-up and rig-down of all equipment. The operation consisted of removing a 488-ft long, 51/2-in. liner set at a depth of 2,000 ft.

Downhole tubulars stuck across a long-hole interval normally do not respond well to jarring. In addition, washover operations have problems and risks that reduce the chance of a successful retrieval. Because of these and other considerations, the operator chose to use pipe vibration as the means for recovery.

A 31/2-in. tubing work string was used to engage the liner with a conventional casing spear and stop. The liner was engaged and the workover rig pulled more than 100,000 lb in an effort to move the stuck liner with no results. The rig was then used to pick up and an hydraulically driven mechanical oscillator over the well bore. The oscillator was attached to the work string and the slips pulled so that the oscillator suspended the work string and provided the necessary overpull to keep the spear suitably engaged with the liner.

The consideration of induced wall friction forces related to a large overpull led to a relatively small overpull of approximately 10,000 lb. The work string and coupled liner were then axially vibrated to identify resonant system frequencies. Having detected resonance at approximately 12 and 16 hz, pipe vibration was continued for about 30 min in order to loosen the liner.

The overpull was then increased to about 20,000 lb. Again, resonant conditions were observed and maintained for approximately 30 min to further loosen the liner. The overpull was alternately increased and decreased over the next 2 hr in a manner similar to that described above while the resonant vibration parameters were monitored for change.

A decrease in the measured resonant frequency indicated changing conditions, and the decision was made to increase the overpull to 70,000 lb. The oscillator was started and the work string vibrated at about 16 hz. Within 2 min the liner began to move.

After moving upwards 10 ft, the oscillator was set back and the rig continued to pull the liner with an initial 30,000 lb of drag, rapidly diminishing to the mud-supported weight of the work string plus liner.

Cased-hole applications

Tubing and liner sticking induced by mud, scale, or sand responds well to axial movement of the tubing. Because sand is an unconsolidated media, it will experience fluidization with corresponding decreases in skin friction that allows tubing to be pulled free.

In the case of mud or scale, the high-energy tubing movement tends to shear away the hardened media and thus creates a granular residue with significantly less holding force.

Mechanical sticking is usually characterized by a wedging action or mechanical interference of the free passage of a tubular, or possibly an upset section such as a tubing joint connection. In this case, a high force application can dislodge items such as junk along the side of a joint or upset, and the fluidization process can move and rearrange any restraining media, helping to defeat the wedging action. By changing the directional bias on the work string, attempted movements can be either up or down hole.

Seal assemblies become stuck in a receptacle probably due to conditions of heat and pressure over time. In effect, it is similar to a vulcanization process that causes the sealing elements to adhere to the wall of the receptacle. The application of a high tensile force will progressively break the bond over the length of the seal, and the assembly may be pulled free.

Open-hole applications

Keyseats are the open-hole equivalent of mechanical sticking inside casing. The application of a high energy level force, generally in a downward direction, is the appropriate procedure for solving this type of situation. The pipe vibration delivers a rapid series of tensile and compressive waves that result in a percussive action at the stuck point, similar to a single blow produced by a surface jar.

Differential sticking is one of the most common modes of pipe sticking in many areas. The mode of sticking is purely one of frictional resistance. The most appropriate means of solving the problem is to reduce mud weight and hydrostatic head holding the pipe against the wall of the hole.

In addition, chemical additives that destroy mud cake seals are very beneficial, especially when they are applied in coordination with vibratory energy. These actions, along with the application of high energy levels work together to liberate the pipe.

Hole collapse is a condition that may be related to swelling shale problems or similar problems that result from poor rheological properties or inadequate hole cleaning. In either case, the pipe can become stuck with reduced or blocked circulation.

Here, the application of vibratory energy applied to the stuck area will allow the cuttings to be agitated and fluidized. This greatly increases the probability of re-establishing or maintaining circulation. The process of fluidizing granular cuttings may allow the pipe to be pulled because there is a dramatic reduction in friction.

In the case of horizontal drilling, dependent on the radius of curvature, it is often impossible to use conventional fishing tools and procedures. Resonant vibration of the pipe string provides a means of recovery and may offer the only alternative to sidetracking.

References

1. Gonzalez, Orlando J., "Retrieving Stuck Liners, Tubing, Casing, And Drillpipe With Vibratory Resonant Techniques," SPE paper No. 14759, IADC/SPE Drilling Conference, Dallas, Feb. 10-12, 1986.
2. Bodine, A.G., "The Sonic Pump For Wells," ASME paper No. 59-Pet-18, Petroleum Mechanical Engineering Conference, Houston, Sept. 20-23, 1959.

The Authors

Buck Bernat is a principal in Vibration Technology LLC, serving as field operations supervisor and safety officer. In addition to his oil field experience, he has had environmental assessment and remediation, industrial health, and safety occupations. He has a BS in geology from Louisiana Tech University.

Henry Bernat is founder and general manager of Vibration Technology LLC. Bernat has extensive expertise in pipe recovery operations. He spent 28 of his 30 years in the oil industry with Tri-State Oil Tools where he worked in engineering, operations, and management for both foreign and domestic operations. He has a BS in mechanical engineering from Louisiana Tech University. He is a member of ASME and is a registered professional engineer in Louisiana.

Copyright 1997 Oil & Gas Journal. All Rights Reserved.