Shales challenge gas processing design

The rapid growth in the unconventional shale plays has created boom times for firms that design and build natural gas processing plants. But that expansion has come with multiple challenges, two processing experts told the Marcellus-Utica Midstream conference.

A fundamental question involves whether to use off-the-shelf designs that can be installed quickly—but may be less efficient—or custom-designed plants that take longer to build and cost more—but can be tweaked to be highly efficient, varying by each particular field’s NGL cross section.

Another challenge gas processing design faces now is meeting increasingly strict methane emissions standards.

John Wilkinson, president and CEO of Ortloff Engineers Ltd., and Loren Pieper, vice president-processing technology for Valerus Field Solutions, discussed both challenges in their presentations and a questionand- answer session with the conference’s record crowd of 2,100 attendees.

Wilkinson noted that Ortloff ’s gas-subcooled process (GSP) has emerged as the dominant NGL extraction process in the midstream but pointed out “GSP is a 35-year-old technology, newer designs can give you greater flexibility.”

Still, the move to skid-mounted, off-the-shelf modules has made GSP a leader. “Standardization is the buzzword,” Wilkinson added. Retrofits once a new plant goes on stream can improve NGL extraction performance, he said. Flange connections from skid to skid make such retrofits easier to accomplish than with conventional, stick-built plants built on site.

Wilkinson compared various NGL extraction technologies, such as single-column overhead recycle and recycle split-vapor techniques. He compared them to GSP by such benchmarks as ethane and propane recovery efficiencies, relative compression power needed and CO₂ removal and tolerance.

CO₂ poses a particular problem to gas processors since techniques that are most efficient at ethane recovery require very cold temperatures that turn CO₂ into dry ice, which at the least hinders efficiency and at worst can damage equipment. He also noted compression always represents a major cost factor in designing and operating a gas plant. Higher NGL extraction rates require higher compression, so compressor efficiency is as important as the gas separation technique used.

He also covered Ortloff ’s “Glass Plant in a Bottle” technology that depends on fabrication at a central plant. This cuts costs and improves quality control, reduces plant footprints and reduces potential emissions by moving multiple functions into one unit. Simply reducing the number of flanges in a plant design can reduce potential methane emissions.

Wilkinson said there will be more plant retrofits in the future to increase ethane recovery as record-low ethane prices improve, “and the ethane market will improve,” he predicted.

In his presentation, Pieper said gas processing engineers have had to “quickly adapt to the shale gas revolution” and one response has been greater use of skid-mounted, “plug-and-play” modules built at central plants, shipped to plant sites and quickly assembled, echoing Wilkinson. He noted Valerus has manufacturing plants in Oklahoma, Texas, Louisiana, Ohio and Pennsylvania to serve the major, gas-prone shale plays located nearby.

He compared two similar compressor stations Valerus built for Marcellus operators. A conventional, stick-built station in Pennsylvania took seven months to build while a modular station composed of multiple skids went onstream in 67 days. “Skids can even be cryogenic,” Pieper said.

Just getting basic gas processing service started is a fundamental midstream challenge as new wells in new fields come on, he added. Pieper said it’s not unheard of for gas processors to move in skid-mounted, Joule-Thomson refrigeration units as a cryogenic precursor so gas processing can get underway and cash flows start for both the midstream operator and the gas producer.

Joule-Thomson plants have limited ethane recovery but do well at extracting heavier NGL, he said. Joule-Thomson technology can reach 85% propane recovery levels during what is, in effect, ethane rejection to the residue gas stream.

Later, cryogenic modules can be added to improve ethane extraction, Pieper said.