The scourge of deepwater development
Rick Von Flatern
A cross-section of paraffin and scale blocked flowlines. (Photo courtesy Halliburton)
Paraffin has plagued the oil industry since its inception, a fact that makes the paucity of knowledge about the flow-stopping material all the more surprising. It is known to be a crystal that precipitates in flowing crude when cooled to below a certain temperature called the cloud point. And it is known that when the oil at the cloud point is significantly warmer than the pipe through which it is flowing, the paraffin will deposit on the pipe walls.
"That is about the sum total of what we know - that is the evidence," said Chevron engineer Jeff Creek, who is a member of the Deepstar flow assurance committee investigating deepwater paraffin problems. Deepstar is a multi-producer oil industry consortium seeking solutions to the deepwater drilling and production challenges. "We have some equations we use to forecast it deposition, but nobody is quite sure of why it should do anything."
Lack of sustained and probing research into the mechanics of paraffin deposition probably results from its long-time characterization as a nuisance, easily and inexpensively treated with chemicals and occasional wireline-conveyed scrapers.
But that was before the oil industry stepped off the world's continental shelves, where paraffin deposition occurs in difficult to reach, miles-long subsea flowlines bathed in near-freezing water. "The infrastructure is not there for deepwater," said Creek. "Most people would rightly like to go with miles of flowline tiebacks to existing shallower water structures on the shelf in less that 1,000 ft of water."
Solutions expensiveIndividual well production rates of tens of thousands of barrels per day, and miles of pipeline thousands of feet below the ocean floor upgrade paraffin blockage from an easily remedied inconvenience to an expensive, potentially catastrophic, event.
In that environment, traditional preventative measures and remedies such as pigging and through-flow-line tools pumped to the wellhead require production interruptions that are costly and time consuming. Chemical injection requires miles of umbilical that Creek said could run to as much as $1 million per mile. Insulation for miles of pipeline is equally cost prohibitive.
Still, while more exotic methods are being considered, variations on the traditional mechanical solutions are being offered. But operators concerned about untried tools are less than anxious to put them in their pipeline at the risk of thousands of barrels of oil production per day. To provide a testing ground for wax removal methods and tools, Deepstar has commissioned Radoil Tool in Houston to create a test loop that duplicates an average five-mile stretch of Gulf of Mexico pipeline.
Test loop ready
Using a model based on averaged sidescan pipeline surveys, Radoil engineers have nearly finished construction of a 900-ft, 6 5/8-in., coiled tubing-accessible flow loop that uses four 25-ft radius bends and numerous friction dogs to simulate five miles of GOM pipeline. It will be ready for the Deepstar committee's inspection on February 26, 1996 and ready to challenge the wares of some 30 interested vendors by March 15.
Vendors will be showcasing pigs, cutters, or any other mechanical device whose efficacy they wished blessed by the group. Radoil is prepared to convey tools to the plugged section or vendors may use their own method of transportation.
The tool operator is blinded from that section of the pipeline holding the standardized-paraffin blockage so that, just as in the field, he can determine his tool's progress only through instrument readings.
Novelty and innovation
The nature of deepwater subsea production demands more than updated versions of traditional paraffin scrapers. To promote that goal, Deepstar's flow assurance committee has designated a special subcommittee on novel wax remediation techniques. Among its areas of investigation are internal pipeline coatings, ultrasonic, microwaves, and magnetics. While none of the methods have proven overly successful yet, the subcommittee is keeping an open mind and, equally importantly, an open purse.
Coatings have shown little progress to date, but the investigation is young. The solution may lie in overcoming paraffin's uncanny, and as yet almost completely mysterious, proclivity for deposition. It has even adhered to Teflon, something which few other things are able to do, particularly when in flow.
"[Internal coatings] has the most promise and a chance to work," said James Coleman, Arco engineer and chairman of the flow assurance subcommittee. But even if a coating does succeed in fending off paraffin deposition, Coleman points out, there are practical questions about its suitability for deepwater flowline applications. "What happens the first time you pig it? Will you lose protection?"
Microwaves, too are in only the earliest stage of investigation. What is known is that they will not penetrate pipe. So, any microwave generator offered for the purpose must be transported to the blockage. It is a concept still very much in the realm of possibility and will likely be tested in the Radoil loop as a CT-conveyed tool.
Ultrasonic waves first showed promise when the subcommittee's attention was drawn to an acoustic generator that, as Creek put it, "pureed" a wax sample. But, when applied to the standardized wax to be used in the Radoil tests, the equipment had no effect. "We realized they must have had pretty soft stuff to start with, and there was no way this acoustic tool was going to help you at all," Creek said. But the Radoil group was credited with its first contribution to the cause. "It saved a lot of goose chasing."
Other ultrasound generators have met the same fate. One from the University of Texas, designed to remove filter cake from borehole walls, and another tunable generator, left the wax unfazed. But Coleman and his group have not written ultrasound off the list of possibilities and have instead allocated funds for two more approaches, including the possibility of using ultrasonic to vibrate the pipe itself to keep the paraffin from depositing.
And, Coleman believes, ultrasonic could be used to cause an homogenizing effect in the flow stream, effectively creating crystals upon which the paraffin would deposit rather than on the pipe wall, and remain in solution.
A joint industry project outside Deepstar is looking to ultrasonic as an adjunct to what is currently the only paraffin maintenance program in deep waters - regular pigging. Pigging has long been used to clear pipelines.
But interrupting the levels of daily production expected from deepwater wells is costly. When pigs are run more often than necessary, production is wasted. Conversely, when too much time is allowed between piggings, it can be operationally dangerous. Even very thin wax buildup can block a pig over a long section of pipe. Ultrasonic may find use optimizing pigging programs.
"Right now it's a guess," said Coleman. "By imparting a small amount of energy into the pipe that would change frequency with paraffin buildup, we could better determine when it was time to pig."
Temperature and chemicals
While the mechanism of paraffin deposit is not known. There is no disputing the role of differential temperatures. Paraffin will begin to precipitate in the flow stream at a certain temperature, but will only deposit on the walls of significantly cooler tubulars. But since heating miles of subsea flowlines is impractical, perhaps cooling the oil at some point before it enters the pipeline would work.
At least two methods for cooling the flow stream, according to Coleman, hold promise:
- Flash cooling: Super cooling it at the wellhead would force the paraffin to precipitate but not give it time to deposit and it would be carried through the pipeline as a slurry.
- Oil injection: A slip stream of cooled oil would be injected into the flowing wellbore fluids to act as a host for paraffin deposits, again removing the paraffin in a slurry before it could deposit on the pipe walls. There are reports the theory is being successfully used in Canada.
In a similar vein, rather than inject cooled oil, chemicals could be injected into the stream at the wellhead. As the mixture arrives at the host platform, a relatively simple separation process could extract the solvent and return it to the wellhead, where the cycle would be repeated. Texaco has reportedly begun looking into such a "solvent recycling" system.
There are few subjects so controversial in the world of paraffin remediation than magnetic treatment of the flow stream. Manufacturers of the tools seldom agree on why their tools work or to what extent they work. To the chagrin of the more sophisticated vendors of the technology, some purveyors of what the industry terms magnetic fluid conditioners (MFC) offer little more than what critics called "magnets wrapped around a pup joint."
Others offer software to design well-specific configurations. While testimonies to the efficacy of MFCs abound, critics say equally as much anecdotal evidence exists to dispute their claims. No one on either side of the argument offers more than theories as to why they work (or don't).
Manufacturers claim scientific explanations of why their tool works are elusive because the mechanism of paraffin deposition is itself not understood. Critics do not dispute that claim.
But scientists, at least at Deepstar, say they cannot recommend a tool be incorporated into a multimillion dollar deepwater pipeline until its manufacturers can explain in the language of physics and chemistry exactly why the tool works.
While they do not dispute their evidence is anecdotal, MFC makers bridle at any implication use of their tools reflect less-than-best practices and point to an impressive array of highly regarded major and independent oil companies using, and re-ordering, MFCs to mitigate paraffin problems. And one manufacturer, Dallas, Texas-based Magwell, has sufficiently convinced oilfield service industry giant, Halliburton, to serve as their worldwide distributor.
But without evidence of why, testimonials do not sway Chevron's Creek, though he says, neither is he anxious to dismiss the technology. "Some of my field guys tell me they have one (MFC) and cutting frequencies are being reduced," he said. "Cutting frequencies are not scientific but neither can you dismiss it out of hand."
Industry skepticism, said Magwell vice president Mark Varel, was caused by early, uninformed marketers who, mostly unwittingly, led customers to expect too much from the tool. "Every technology has a limit," Varel said. "And people have gone in and tried to sell this as a miracle product and that is where it really got screwed up. You get total suspension of wax depositions in fluids that are highly reactive and (in others) you get marginal reactions, which is to be expected. If you don't prepare your customer for this, you are courting disaster."
Taking magnets seriously
Coleman says Deepstar is taking MFC claims seriously. He and everyone on his committee would be very happy to solve their companies' deepwater paraffin problems with magnets. Magnets are a small initial, and low-maintenance, investment. "I can't go to my boss and say: okay, we have a five-mile long pipeline, and trust me, you don't have to pig it, you don't need chemicals, and you don't need anything except this magnet on it," said Coleman. "We're not there yet and we have told the magnet vendors that until we can design a magnet treatment that we are sure is going to work before the well flows, it is not going to get used in a subsea environment."
Getting magnets to the point they can be confidently included in subsea plans (even if not to the exclusion of other paraffin remediation measures but as a significant part of the solution ) means quantifying magnetic effects. And that means understanding the science. Coleman's group is trying to do just that. "It (MFC) is in my subcommittee and we are studying it and spending a significant amount of money on it," said Coleman. "There is, at least in the Deepstar community, a healthy skepticism. We are trying to understand if there is any science to magnetic treatment of fluids."
Toward that end, in September 1996, the subcommittee held a two-day conference dedicated to the subject of magnets. Attended by MFC vendors, Deepstar engineers, operators, and other interested scientists who heard at the seminar, according to Coleman, only more anecdotal evidence of magnetic effects on flowstreams. The workshop ended by commissioning research to be conducted at the University of Florida to determine the effects of magnets on paraffin-laden flow streams.
Even as the work was being awarded to the University's Dr. Sam Colgate, MFC vendors were dismissing it as meaningless. The professor, they claimed, could not duplicate in a laboratory the nuances found in the field and that made their tool effective.
The laboratory apparatus draws fluid from a single reservoir and propels it along a pipe to a tee where it is thermally treated. Half the fluid is shunted through a section of pipe with magnets and the other half through a blank. Downstream of the blank and the magnet, highly sensitive instruments measure flow stream characteristics. Both samples are then captured and run through all the molecular level analysis available to the university as well as other physical tests for such things as wax hardness.
To date, only one sample has gone through the test circuit. It has not been subjected to analysis other than what could be determined from the metering systems about differences in flow. "To this point, all the results have been negative," Colgate said. "Both samples behaved identically. I am treating these streams thermally to cloud point or plating. I am not so much interested in what is happening, as is it happening differently in one tube than the other."
Magwell's Varel believes the experiment is due to failure unless it incorporates proven technology such as his company can offer. "Why would they not want to know specifically how to effect a flow stream? It is more than passing fluid between two magnets," he said. "There are three critical factors to receiving optimum treatment form applied field treatments." His three critical factors are:
- Exposure time of the fluid to the magnets
- Velocity of the fluid
- Gauss strength of the magnets.
"If you get one or more of these three factors out of balance, the treatment received becomes less than optimum," he said. "Optimization of the technology can be controlled, but this is what they will miss at Deepstar."
But Colgate believes the vendor is missing the point. "I would say sure, to optimize the phenomenon would require attention to details like that," he said. "But, if the effect that causes their devices to work is based on such subtle things that a lab experiment totally misses all of them, then it can't be robust. It can't work over wide variations in the field and is probably of little interest to the industry."
Colgate says he is happy to discuss the experiment with vendors and has agreed to accept the offer of manufacturers to include their tools in the procedure. One vendor has taken him up on the offer. "I am going to do it," he said of introducing the tool into the apparatus. "It is not going to be a trivial matter, but I am going to do it."
Despite all that, there appears to be a common ground between skeptics and MFC promoters. Both are looking, though perhaps from different perspectives, at the influence of polar, inorganic material on paraffin formation and deposition.
"We are not effecting the wax but the surrounding fluids," said Varel who claims his tool creates what he calls nucleating sites where paraffin will deposit in the flow stream rather than on the tubing wall. "We make them available to the system which attracts the organics, and makes a higher suspension rate of organics in solution."
The theory, says Varel, explains why some fluids, those with high levels of inorganic polar compounds, will react more significantly to magnet treatments than others. As an adjunct to their tool, the company intends to introduce a chemical treatment that would raise the number of available nucleation sites in a given stream.
Such inorganic compounds precipitate from the flow stream familiarly in the form of scale. And experimental results, at the moment proprietary but to be released soon, seem to confirm the ability of magnets to alter scaling. And, in a scenario that bears a vague likeness to Magwell's theory, Deepstar has funded an investigation into the effects of polar components on paraffin deposition. Coleman compares the hypothesis behind the research to the mechanism of corrosion inhibitors that work by leaving a coat of nitrogen between the tubing wall and flowstream.
"Is that what is going on?" he said. "Does paraffin not really stick, and some other mechanism within the crude initiates sticking? In paraffin, there is paraffin, asphaltines, scale, oil, everything. Is that just trash, or is the asphaltine the deposition initiator?"
Whether magnets work is far from decided. One major oil company is ordering more of the tools for a field in which it has used them for several years. Another called research into them a waste of money.
In deep waters, once seemingly trivial matters have risen in the pecking order of challenges. So, it is with the once nearly innocuous problem of paraffin. The stakes are higher and the problem physically more difficult to reach than when deepwater meant 600 ft. It will be solved because it is a real obstacle to efficient deep subsea development.