ROARING BRANCH, Pa. — The frack job was frozen.

Deep beneath an icy Tioga County, Pa., farm earlier this month, an effort to extract natural gas from the Marcellus Shale shuddered to a halt. The culprit was not the 14-degree weather, but an innocuous material more often associated with beaches: sand.

The procedure known as hydraulic fracturing, or “fracking,” is designed to liberate gas locked in the shale by injecting pressurized fluid into a well to shatter the rock. But this frack job in north-central Pennsylvania was stalled: Sand contained in the frack fluid had clogged up the bore.

“These things happen,” said Greg Carder, a contractor employed by East Resources, of Warren, Pa., to frack the well.

Another contractor was summoned to dislodge the blockage. The one-day delay idled about 100 workers and the fleet of assembled machinery, adding tens of thousands of dollars to the well’s $4 million price.

In the grand scheme of things, the clog amounted to a minor glitch in the high-tech construction of a modern natural gas well.

Far bigger and costlier obstacles loom for hydraulic fracturing.

As the Marcellus gas bonanza accelerates, the practice of fracking wells to stimulate production has come under fire. Officials in New York City, worried that gas drilling threatens the city’s watershed, have banned Marcellus drilling while they review new environmental regulations proposed for the new technique. In northeastern Pennsylvania, opposition has stalled Marcellus drilling in the Delaware River basin.

Sen. Bob Casey, D-Pa., has cosponsored legislation requiring federal regulation of fracking. “This legislation will ensure that hydraulic fracturing does not unnecessarily jeopardize our groundwater,” he said.

The practice also is important for National Fuel Gas Co., the Amherstbased energy company that is making a big push into Marcellus drilling.

National Fuel controls nearly 740,000 acres of land in the Marcellus region of northwestern Pennsylvania and, following good results from its first three wells, plans to increase its spending this year on its own Marcellus drilling program by 50 percent from its initial plan. National Fuel’s current plans call for the company to spend $180 million to $200 million during the fiscal year that began in October.

The oil and gas industry says hydraulic fracturing does not threaten groundwater supplies and already is closely monitored by state regulators. They say federal oversight would retard domestic gas exploration.

“Fracking is a well-established technology, and National Fuel has been fracking wells for decades in its various drilling areas,” said Donna L. DeCarolis, a National Fuel spokeswoman. “We are fracking our Marcellus Shale wells at a depth of between 5,000 and 8,000 feet, well beneath the level of ground and well water sources that are typically only several hundred feet below the surface.”

Gas drillers say they have been hydraulically fracturing wells for more than six decades.

“Over the years, our company has drilled thousands of wells, and every one of them is fracked,” said Terrence M. Pegula, the chief executive officer of East Resources. “I can’t believe we have to sit now and try to explain it.”

Pegula acknowledged that the industry was unprepared for the attention hydraulic fracturing had attracted.

“I think we have done an absolutely horrible job of defending ourselves against people who are criticizing a technique that has been used for 60 years,” Pegula said.

Last month, East Resources agreed to donate $750,000 to the Susquehanna River Basin Commission to install a network of devices to remotely monitor water quality in streams near wells. The monitors will alert regulators to any serious spill.

The Tioga County location “is a very sweet spot in the Marcellus,” said Scott Blauvelt, East Resources’ general manager for environmental, safety, and health issues. The shale formation is about 200 feet thick and about 6,500 feet below the surface.

Unlike vertical wells that capture only a small part of the shale, horizontal wells are designed to turn laterally deep underground and follow the formation for great lengths. Gas drillers can drill several horizontal wells from one location and capture vast quantities of gas beneath an area as large as one square mile, rather than scattering dozens of vertical wells over the terrain.

But horizontal well sites are big. They must accommodate the machinery needed to pump millions of gallons of water and sand underground.

Workers from Universal Well Services recently began hydraulically fracturing the final 4,500 feet in the Marcellus.

On the well pad, the contractors assembled a fleet of equipment: 50 trailers to store some of the 3 million gallons of water needed for the job. Most of the water was purchased from a local municipal supply. About a third was recycled from previous jobs.

As Blauvelt describes it, hydrofracking is essentially an effort to push sand into tiny fissures in the shale, allowing the gas to escape.

“The water is just a vehicle to transport the sand down the hole,” he said. The company planned to use up to 5 million pounds of sand in the operation.

With 24 truck-mounted pumps, the slurry is injected under high pressure into the shale through holes in the steel casing. Sometimes, the wells “sand up.” The clog that formed earlier this month was dislodged by snaking a 2-mile-long pipe into the well and blowing nitrogen into the hole.

A “slick-water” frack typically includes surfactants to keep the sand suspended, plus polymer friction-reducers that speed the mixture. Biocides like bromine, a disinfectant used in hot tubs, may be added to prevent organisms from clogging the fissures.

The industry downplays the environmental effects of the additives, saying they amount to less than 1 percent of the mixture. But a provision of the proposed federal legislation would require drillers to disclose the frack additives.

Industry geologists say the fluid remaining in the shale stays there. In the Marcellus, a mile of impermeable rock separates the shale from the aquifer. The big challenge is managing fluid that flows back from the well once fracturing is done.

Flowback brine is saltier than seawater, and the Pennsylvania Department of Environmental Protection requires it be treated before disposal. But treatment is expensive, and the state is considering tighter rules to deal with the increasing volume of water.

News Business Reporter David Robinson contributed to this report.