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Drilling mud key for OCTG in corrosion intensive extraction

Keywords: Tags  drilling mud, OCTG, shale gas, Kevin C. Garrity, Integrity Solutions, Mears Group Inc., NACE International,

A rapid expansion in demand for oil country tubular goods (OCTG) for drilling and producing oil and natural gas is bringing with it new levels of corrosion that call for innovative treatment and the addition of corrosion inhibitors. The use of drilling mud as a lubricant in the extraction of oil and natural gas from increasingly harsh environments presents corrosion issues not always faced by petroleum engineers in the past.

Fracturing rock and drilling horizontally in the nation’s shale gas regions, such as North Dakota, Pennsylvania and Texas, can create new stresses on pipe and equipment, and there might be a need for metallurgical engineers to team with corrosion engineers to look at the use of various exotic materials--molybdenum, nickel and chromium--as alloying agents with carbon steel to create the extraction piping of the future.

“Shale gas can be tougher on pipe,” said Kevin C. Garrity, senior vice president of the Plain City, Ohio-based Integrity Solutions division of Mears Group Inc. “You are fracturing surfaces vertically and then drilling horizontally into pockets of gas. The material in the pipe is exposed to a lot of different environments in fracking, including different compositions of water, soil environments and fractured rock.”

Rosebush, Mich.-based Mears Group is a midsized corrosion engineering company that serves as a consultant to the oil and gas industries. Besides its Michigan and Ohio offices, the company has U.S. offices in California and Texas as well as facilities in Abu Dhabi, Australia and India.

Combating the corrosive nature of drilling mud has been an issue for years, and corrosion engineers have learned a great deal about how to add inhibitors to the drilling liquid to fight corrosion both inside and outside the pipe, Garrity said.

Drilling mud is basically a high-viscosity fluid that allows drillers to carry cuttings back to the surface; provides hydrostatic pressure to prevent formation fluids from entering into the borehole, keeping the drill bit from overheating and the drill clean; and helps suspend drill cuttings when the drill assembly is lifted out and lowered back into the borehole.

Corrosion engineers have known for years that the addition of bentonites and other clays to the drilling liquid can affect the pH of the hole being drilled. The pH scale of pure water is seven, which is essentially neutral; a pH of less than seven tends to be acidic, while a pH of more than seven is typically more alkaline.

“The more acidic it is, the more corrosive,” Garrity said, noting that acidic compounds often also contain compositions of soluble salts. “Chloride tends to accelerate corrosion. The best analogy I can offer is seawater. At 25 ppm (parts per million) chloride, it is very corrosive.” A pH on the higher end of the scale tends to inhibit corrosion of ferrous materials, but Garrity cautioned that if the pH is too high the pipe can be subjected to stresses and cracking.

The addition of inhibitors to drilling mud can help stem the damage caused by corrosion. Bentonite, phosphates, oxygen scavengers and other compounds added to the mix design of the drilling fluid are all helpful in reducing corrosion. “But it depends on what you are trying to extract and the environment where you are trying to extract it from,” Garrity said.

For example, corrosion engineers have long dealt with the natural gas pumped from wells in the traditional gas fields of New York, Ohio, Pennsylvania and West Virginia, he said. The natural gas liquids pumped from those wells, some of the oldest in the United States, contain a great deal of hydrogen sulfate, as well as the bacteria that can accelerate corrosion. Piping used in oil and gas extraction can suffer corrosion on the outside of the tube, but also can be corroded on the inside, since both oil and gas often contain moisture content as well as hydrogen sulfate.

“As an industry, we have had years and years of experience with designing technologies to fight that kind of corrosion,” Garrity said. “But the material performance is just not as well documented in fracking. There is just not as much experience dealing with that.”

Garrity, who has 37 years’ experience in corrosion engineering and is the immediate past president of Houston-based corrosion society NACE International, said the recent boom in oil and gas drilling has been a boon to NACE and its 30,000 members worldwide. “In spite of the recent recession, NACE has done very well in growing its membership,” he said.

As North America increasingly becomes the swing producer for global oil and gas production, American companies spend an estimated $276 billion per year on corrosion protection as they replace corroded equipment and material in everything from pipelines to bridges, NACE International estimates, and the consequential cost of things such as oil spill cleanups and plant shutdowns bring that total to nearly $600 billion annually. The society points out that the application of existing technology could reduce those costs by as much as 40 percent.

However, corrosion engineers are always acutely aware that “corrosion is a never-ending battle. Anything you do to improve performance characteristics, you have to look for unforeseen consequences. It involves constant monitoring,” Garrity said, noting that corrosion engineers want to get shale gas extraction right. “When we visit legislators to get the message out about corrosion, one of the things they often ask about is shale gas. We want to make sure this resource doesn’t turn out to be an environmental or safety concern.”

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