USE OF COILED TUBING FANS OUT AMONG WELL SITES OF THE WORLD

Better operator understanding of coiled tubing improvements is generating a burst of applications at well sites around the world.

Prompted by economics, producers are using coiled tubing in a wide range of well maintenance and remediation procedures to lower costs and increase recovery.

Some more common workovers using coiled tubing-production tubing cleanouts and matrix acidizing, for example-not only are lowering costs but also are achieving better results.

Other less known uses-logging, recompletions, and reentry drilling-until recently were thought unreliable or impossible. But better management of tubing fatigue, better materials, and larger tubing sizes are combining to boost producers' confidence in the relatively old technology.

While coiled tubing still is defining its possible roles in well servicing, coiled tubing technology continues to evolve. In addition to its well servicing benefits, many believe coiled tubing eventually could transform oil and gas well drilling and completion procedures as well.

Economic drivers are inspiring most coiled tubing uses. Even the staunchest proponents acknowledge today's stronger, more resilient coiled tubing is not appropriate for all applications.

But aside from economic limitations, saying what coiled tubing can't do is risky, warns long time coiled tubing enthusiast Alexander Sas-Jaworsky II, a staff engineer for Conoco Inc., "because we're still defining the envelop."

Sas-Jaworsky said, "Some people today say coiled tubing can't be used for certain applications that have failed in the past. But some of those old applications might have failed only because the coiled tubing was used improperly. If used properly, coiled tubing has applications across the board."

MORGAN STANLEY STUDY

Sas-Jaworsky's enthusiasm for the potential of coiled tubing is shared by many.

A study of coiled tubing markets by Morgan Stanley Equity Research, New York, found the technology is growing faster than well servicing as a whole because it is yielding significant cost savings on well workovers. In addition, Morgan Stanley reported, coiled tubing technology promises to radically change the way oil and gas wells are completed and, perhaps ultimately, how they are drilled.

Morgan Stanley cited these advantages of coiled tubing, saying the method:

Allows the operator to perform a variety of well services through tubing without killing the well. Injecting fluid into a well to kill it for servicing risks damage to the reservoir's pretreatment flow characteristics.
Is particularly well suited to operations in horizontal or highly deviated wells because tubing can be pushed into horizontal or near horizontal sections of the wellbore to deliver treatment fluids or convey downhole tools.
Allows tubing to be safely reeled in and out of a well at high speed, yielding significant savings with faster trips.
Needs less space and time for setup than a drilling or workover rig. Coiled tubing's smaller footprint translates into less well site preparation and reduces the risk of environmental harm.

Among coiled tubing's disadvantages, Morgan Stanley listed operator unfamiliarity and the tubing's tendency for helical buckling downhole, a condition that makes it hard to determine tool positions and, if left unchecked, can cause lockup.

Despite such drawbacks, Morgan Stanley's analysis showed that coiled tubing in some cases can save an operator 50-70% of the cost of working over a well.

DEFINING OPPORTUNITIES

In more economically robust times, incentives likely would be inadequate to prompt many producers to use coiled tubing.

But today's shaky oil and gas prices are forcing producers to hold down costs in every way imaginable. Meanwhile, the pace of drilling activity-extremely low by historical standards-is forcing growing reliance on production from older wells.

With the savings promised by coiled tubing technology, efforts to develop well site capabilities are racing to keep pace with defining opportunities in the field.

"In today's business environment, producers have to get more oil out of each well and get it out cheaper," said Susan Gary, Schlumberger Dowell's regional coiled tubing specialist in North America.

"An operator can do things with coiled tubing that can help lower costs in several ways.

"Every day around the world we use coiled tubing to accomplish our purposes and get it in and out of the hole without incident. And uses of coiled tubing are booming in more directions than we can keep up with."

To help coordinate its coiled tubing services, Dowell in 1991 formed a task force that included its own specialists and those of well servicing affiliates. Since its formation, attendance at task force meetings has dwindled as Dowell put members of the group to work in areas around the world where coiled tubing has distinct advantages and its use therefore has grown.

Hampton Fowler, manager of coiled tubing and nitrogen products and services for Halliburton Energy Services, credits product improvements by coiled tubing manufacturers with expanding the ways in which the tubing is used.

Fowler says improved heat treating techniques have increased the yield strength of steel alloy coiled tubing as much as 100,000 psi, while reducing by 50% the rate of tubing fatigue.

In addition to high strength alloy steel, coiled tubing for commercial use is available in titanium alloys that offer high strength and a low modulus of elasticity while retaining excellent corrosion resistance and fatigue properties.

Sas-Jaworsky says Conoco by the end of first quarter 1995 expects to complete testing on a 10,000 ft spool of 1 in. ID, 1 1/2 in. OD tubing.

Meantime, coiled tubing is available in sizes of 3/4 to 3 1/2 in. ODs and within the next year is expected to become available in a 4 1/2 in. diameter.

MODELING FATIGUE

With improvements of coiled tubing itself has come a better understanding of the product.

While operating problems persist, the ability to model and predict tubing fatigue and eventual failure-in the past coiled tubing's greatest disadvantage-has improved immeasurably.

Dowell has developed a family of coiled tubing computer software to help plan and perform a broad range of operations. Some modules can help an operator evaluate well bore geometry, tubing and casing data, and reservoir and production data. During evaluation and design, various modules:

Determine whether the coiled tubing string chosen for a specific job can reach TD safely and within operating limits, verify whether coiled tubing tools will pass into and out of the well bore, calculate tensile and compressive forces on the tools, predict the likelihood of lockup resulting from severe buckling, and alert the operator to anomalies between predicted and actual values or conditions.
Create and maintain a database of well site pressures and bending encountered by each of the company's coiled tubing strings. That allows an operator to calculate the extent of coiled tubing fatigue or damage on each job and predict remaining useful life to assure a specific coiled tubing string can be run and pulled safely.
Simulate reactions downhole of liquids, gases, and solids in three-phase flow, allowing the operator to model effects of reservoir and wellbore forces to optimize pumping treatments.

During a coiled tubing operation, Dowell software acquires treatment data in real time, allowing the operator to closely monitor progress.

"By modeling forces encountered by coiled tubing, we can predict weight on bit for drilling or what kind of force an operator can apply with a tool," Gary said. "We often show an operator the data on a given coiled tubing string before sending it to the field to work on his well.

"So we think we have a grip on how to manage fatigue."

APPLICATIONS SPREAD

Equipped with better understanding of today's stronger, more resilient, and larger tubing, operators around the world have begun incorporating coiled tubing into a host of well servicing procedures. Among the recent uses:

Petroleum Development of Oman (PDO) last summer used 2 3/8 in. tubing to perform a coiled tubing completion-claimed to be the first in the Middle East-on a 2,400 m well in Oman's Zauliyah field. PDO plans at least two more coiled tubing completions in the region.
In the Gulf of Mexico, Shell Offshore Inc. last July installed a coiled tubing unit on the Auger tension leg platform, in 2,860 ft of water on Garden Banks Block 426, to remove a hydrate plug from the A-1 well. The plug was removed much faster and at a fraction of the cost of traditional methods.
In the U.K. North Sea, a unit of Texaco Inc. last July used 1 1/2 in. electric line coiled tubing on a combined five logging runs and nine perforating runs in a highly deviated well and a horizontal well northeast of the Shetland Islands. Working in an underbalanced mode, coiled tubing accurately positioned two perforating gun assemblies longer than 200 ft and three others longer than 100 ft. Texaco perforated more than 1,000 ft of horizontal or near horizontal well bore.

DRILLING APPLICATIONS

Production tubing cleanouts still account for most coiled tubing well site operations. But interest in coiled tubing drilling has begun building within the past year, and operators are achieving solid results with the technology.

Dowell's Gary says at yearend 1993 fewer than 50 wells had been drilled with coiled tubing. By yearend 1995, the number is likely to exceed 300, mostly involving reentries.

She said, "Besides the time saved in most instances, drilling with coiled tubing allows an operator to drill underbalanced while the well is producing, minimizing the possibility of damaging the productive formation."

Later this fall, Shell Western E&P Inc. is scheduled to complete a 115 well drilling project aimed at reactivating an idle steamflood project in California's McKittrick oil field (OGJ, June 27, p. 31). Sixty-eight of the wells are being drilled with coiled tubing, making it one of the largest operations of its type yet undertaken.

Another large coiled tubing drilling program was completed last spring on Lake Maracaibo by Petroleos de Venezuela SA's Lagoven unit. The 38 well program stemmed from a five well pilot aimed at controlling gas encountered while drilling above 1,500 ft. If gas is not encountered by that depth, drilling continues with a conventional barge mounted rig.

To drill safely through the shallow gas zones, Lagoven set up on the wells with barge mounted coiled tubing units, then backed off a short distance from the locations to begin drilling. The work has been so successful a 40 well program using the same procedure is slated to begin next month.

ARCO Alaska Inc. has used coiled tubing drilling to perform remedial work on about a dozen horizontal Prudhoe Bay wells in an effort to develop a dependable, low cost way of sustaining production. ARCO mills a window in the casing using coiled tubing to extend the horizontal sections. Milling windows in horizontal well casing is considered critical to economics of coiled tubing drilling for horizontal reentries.

Also in Alaska, ARCO has used coiled tubing for sand fill cleanouts, completion strings, running liners and packers, and injection profile control.

Donald Scheve, ARCO Alaska's coiled tubing technology adviser, says coiled tubing is the company's favored way of putting anything downhole in Prudhoe Bay field.

"We've run scabs over washouts with coiled tubing and cemented them in place," Scheve said.

Meantime, in Indonesia, Dowell has helped drill three 8 1/2 in. holes to depths of 300 ft with coiled tubing and has completed other wells with 6 in. diameters to depths of 1,200-500 ft.

CONCENTRIC CLEANOUT

Success of new coiled tubing applications is opening the door for other novel uses.

Last June in Canada, Calgary Coiled Tubing and Nowsco Well Service Ltd., Calgary, performed the first concentric coiled tubing well cleanout using a Nowsco designed jet pump tool. The treatment was prescribed after the candidate well's production had fallen to zero because of sand influx.

Nowsco developed concentric coiled tubing and a jet tool to work together to clean sand from horizontal wells with ultralow bottom hole pressure without placing excessive pressure on the productive zone. The well chosen for the first application had a measured depth of 4,000 ft and true vertical depth of 2,070 ft.

The concentric coil tubing string on the job consisted of 7,380 ft of 2 3/8 coiled tubing with 1 1/4 in. tubing inserted inside. The operator delivers power fluid under pressure through the smaller coiled tubing to Nowsco's jet tool, where it picks up sand in the well bore and a small amount of formation fluid before returning to surface through the annulus between the coiled tubing strings.

Nowsco modeled the job so pump rates and concentric coiled tubing running rates would achieve sand pickup at rates of 30-50 lb/min. Calgary Coiled Tubing and Nowsco recovered 4,500 lb of sand in the 4 day job.

Nowsco says the technology has been used successfully on six horizontal heavy oil wells in Canada.

PARAFFIN REMOVAL

Meantime, Tatham Offshore Inc., Houston, in mid-September was to begin using a coiled tubing procedure to try to remove paraffin buildup from flow lines connected to its No. 2 well on Ewing Bank Block 914 in the Gulf of Mexico.

The well was completed in August 1993 with a subsea tree and wellhead as a gas/condensate producer and tied into a gathering system built by Tatham affiliate Leviathan Gas Pipeline Partners LP. Initial production averaged 12.5 MMcfd of gas and 2,300 b/d of condensate.

But by last May, 2 Ewing Bank 914 had stopped producing condensate and started producing lower gravity crude at a rate of more than 5,000 b/d. As a result of the unexpected change, paraffin began building up, impeding production.

Tatham first tried clearing the lines by injecting paraffin solvents into the production and gathering systems. But the company ultimately concluded the chemical treatments would not clear the lines to an acceptable level and chose to try coiled tubing remediation.

If the coiled tubing procedure is not successful, Tatham might be forced to replace the well's flow lines at a cost of about $5 million. The company also might elect to install downhole equipment in the well to inject paraffin inhibitor directly into the wellbore, a $1.5 million preventative measure.

Donald S. Taylor, Tatham chief executive officer, said the chemical treatments would have been the least costly way of solving the paraffin problem.

"However," Taylor said, "the coiled tubing procedure, or alternatively the fine replacement, will assure the company that it will be able to fully restore production from the well."

Tatham expects to be able to return 2 Ewing Bank 914 to production by December.

GAS STORAGE

Halliburton's Fowler said improved coiled tubing workover and production tools and procedures should improve the efficiency and economy of gas storage well workovers.

Coiled tubing jetting cleanout in an underbalanced mode while a well continues producing has been proven effective to improve gas delivery when conventional treatments were less successful or could not be used because of the possibility of damaging the formation. Coiled tubing also has been used in cementing operations to plug and abandon wells or to seal off unwanted reservoir zones.

Fowler said using coiled tubing to perform small matrix-type acid treatments offers the following benefits:

There is no need to kill the well.
Smaller amounts of acid are needed in a coiled tubing acidizing procedure because property prepared tubing leaves no pipe scales or pipe dope to react with acid.
When used as the injection string, coiled tubing protects permanent well tubulars from exposure to acid and can withstand pump pressures required for placement.

Fowler said, "While not all gas storage well formations are appropriate for acidizing, certain sandstones with moderate amounts of clay and good consolidation can benefit from near-wellbore acid treatment to remove skin effects of damaging materials such as drilling muds and scale deposits that might be introduced during well completion or injection cycles."

RECOMMENDED PRACTICE

Efforts to evaluate and describe coiled tubing technology to develop a set of accepted industry-wide standards is gaining momentum.

A coiled tubing proponent for the past three decades, Sas-Jaworsky for the past 4 years has chaired a task group formed by American Petroleum Institute Committee 5 to create a set of recommended practices to standardize coiled tubing handling at the well site.

Coiled tubing recommended practices being prepared by Sas-Jaworsky's task group will not cover downhole operations because that technical area is administered by another API committee. Rather, the group's recommended practices will involve best practices for coiled tubing preparation and use and recommend equipment operating guidelines, including ways to deal with the most likely contingencies.

"The manufacturing portion of the coiled tubing industry has been satisfactory, and we are adopting what is currently being used by manufacturers," Sas-Jaworsky said. "Quality control information is being imported from various other API committees."

Sas-Jaworsky aims to have a rough draft of the coiled tubing practices document ready for presentation to API Committee 5 by February 1995. If approved, the document will be given an API number and issued within a few months.

"Coiled tubing is being used in so many facets at the wellsite, API felt it had to at least provide a base line understanding of the performance of the tube and coiled tubing service life relative to its interaction on the reel, through the injector head, and with other surface components," Sas Jaworsky said.

GREATER EMPHASIS

Meantime, DEA-67, a 36 month study of slim hole drilling and coiled tubing technology sponsored by the Drilling Engineers Association and coordinated by Maurer Engineering Inc., Houston, is preparing to enter Phase 2 with greater emphasis on coiled tubing.

Greg Deskins, Maurer senior project engineer, said DEA research is guided by the willingness of the group's 20 or so major company members to fund specific proposals. In Phase 1 of DEA-67, participants focused more on slim hole drilling than coiled tubing.

However, since DEA-67 kicked off in January 1992, Deskins said, the types of well site jobs conducted with coiled tubing have proliferated.

He said, "There are so many more applications now because the tubing is indeed a stronger beast than it was. It's a lot more dependable, and the larger sizes have opened many new doors."

The fact that coiled tubing seems capable of contributing to the effectiveness of many new and traditional well servicing operations could help explain some of the new interest in the tubing among DEA-67's 58 industry participants.

For example, coiled tubing drilling almost of necessity is equivalent to slim hole drilling, but the reverse is not true. Similarly, horizontal well technology, is a sum of technical component capabilities, among which coiled tubing appears to be becoming more important.

"Without coiled tubing, horizontal capabilities wouldn't be where they are today," Deskins said. "There really isn't any other practical way to work over, complete, or log a horizontal well."

Whatever the causes, sentiment among participants is expected to turn DEA-67 more toward coiled tubing drilling than before. Phase 1 included an exhaustive search of coiled tubing literature. Coiled tubing field tests are planned during DEA-67 Phase 2 to begin in first quarter 1995.

BETTER UNDERSTANDING

Conoco's Sas-Jaworsky acknowledges that coiled tubing has become a better product with more applications. But he attributes its expanding use more to better understanding among operators of the product's behavior.

Coiled tubing manufacturers began making better tubing from higher grade steels in the late 1970s.

But Sas-Jaworsky said it wasn't until the early 1980s that companies such as Quality Tubing Inc. and Southwest Pipe Inc. began promoting and selling stronger coiled tubing the industry perceived as more reliable. In the mid-1980s, operators began trying to use coiled tubing in larger diameters for more different types of well site operations.

However, operators continued to base applications and procedure designs on arbitrary limits set after bad experiences.

For example, before coiled tubing steel quality began improving, the industry set an arbitrary limit for coiled tubing internal pressures at 5,000 psi. Yet when manufacturers began introducing coiled-tubing made of higher grade steels, operators didn't budge from the old internal pressure guideline.

Sas-Jaworsky said, "We had pushed the safety envelop much farther out, service companies began achieving much better coiled tubing performance than with lower grade steels, and all of a sudden coiled tubing began getting a reputation for being reliable. Even though we knew we were able to do a lot of new procedures, we also remembered there was a limited product life and in certain applications or situations the life was much shorter.

"But we've only recently-within the past 5 years-started to apply true technical reviews in scientific evaluations of what is happening as we bend this product in and out of the wellbore, beyond its yield point while under high pressure."

By empirically testing coiled tubing while bending it under pressure, operators have proven consequences that previously were only inferred.

"Today,' Sas-Jaworsky said, "the industry has developed a better understanding of how to yield this product toward failure and what kind of functional life we can expect.

"To me, that improved understanding is the most significant factor affecting operators' decisions about whether to try a coiled tubing procedure."