An explosion and fire at a gas well injured three workers Wednesday night in Avella, authorities said. The blast, at the Chesapeake Appalachia LLC Powers site was reported at about 6:20 p.m., Washington County emergency officials said. Workers were transferring water used in a gas-extraction process called hydraulic fracturing, and several of the natural gas liquid storage tanks caught fire, said Katy Gresh, a spokeswoman for the Department of Environmental Protection’s southwest region. During hydraulic fracturing, also called fracking, water mixed with sand and chemicals, some of them toxic, is forced into the wells at high pressure, shattering the underground shale and releasing trapped gas. Residents in several communities where fracking occurs have voiced concerns about possible water contamination and other safety issues, but the gas companies said the procedure has been used safely for decades. Two workers were flown to Pittsburgh hospitals after Wednesday night’s explosion, and another was taken by ambulance, Gresh said. The extent of their injuries was not immediately known. The fire was reported extinguished at 9:30 p.m. Neighbors along Meadowcraft Road said they heard explosions miles away from the drilling site. “It was an explosion. I thought a C-130 crashed up here,” said Bob Dalesio. “They fly up here on Wednesday night. That’s how bad it was.” “I was in my house and heard a big boom,” said Dana Cecchini. “I thought something happened upstairs. I looked outside and saw a big giant black cloud and flames.” Gresh said the three wells at the western Pennsylvania site appeared to be unaffected but that will be confirmed as an investigation begins Thursday to determine the cause of the blast. Company spokesman Stacey Brodak said emergency responders were keeping the natural gas liquids storage tanks cool. He said the wells had already been completed and there was no danger to the public or the environment. Brodak said he was unable to provide an update on the conditions of the workers, but “our thoughts and prayers are with them and their families for a full recovery.” Fire crews from the surrounding area battled the flames along with the county hazardous-materials team. A gas explosion earlier this month in eastern Pennsylvania killed five people. The blast, in Allentown, sparked an inferno that burned for hours while crews tried to shut off the gas supply. A 16-year-old girl and a 4-month-old boy were among those killed.Eight homes were destroyed. Neighbors said the blast knocked shelves off walls, brought down ceilings and ripped a front door off its hinges. That fiery blast was another in a series of natural-gas disasters to raise questions about the safety of the nation’s aging, 2.5-million-mile network of gas and liquid pipelines.