Over the past decade, advances in technology have allowed the oil and gas industry to extract tightly-bound natural gas from shale formations deep underground. The resulting drilling boom has transformed communities around the country, boosting local economies but also raising concerns about how the intense industrial activity is affecting air pollution, water quality and public health.

In late June, InsideClimate News reporter Lisa Song had the rare opportunity to tour several drilling and processing facilities and get an insider's look at how exactly the gas is extracted and turned into fuel.

All photos by Lisa Song unless otherwise noted.

Image: U.S. natural gas production by source, 2012-2040, in trillions of cubic feet. Shale gas is shown in green. Source: U.S. Energy Information Administration (EIA), Annual Energy Outlook, May 7, 2014.

According to the U.S. EIA, the shale play with the largest proven natural gas reserves is the Marcellus Shale in Pennsylvania, New York, Ohio and West Virginia.

Since 2008, more than 6,000 Marcellus gas wells have been drilled in Pennsylvania.

Image: A well pad in McKean County, Pa. owned by Seneca Resources, the exploration and production arm of National Fuel Gas Company.

Drilling is a 24-hour operation. Workers on this well pad work 12-hour shifts every day for two weeks at a time, followed by two weeks off. Most of the workers are contractors, not Seneca employees.

Image: Robert Boulware, Seneca Resources manager of stakeholder relations (on right) explains the drilling process.

The well is first drilled vertically thousands of feet beneath the surface. The drill then turns sideways so it can bore horizontally into the Marcellus Shale layer.

Once the drilling ends—which usually takes 10 to 14 days—the drilling rig will be removed, and the operator will begin the hydraulic fracturing (fracking) process to stimulate the flow of gas into the well. The combination of fracking and horizontal drilling has made the shale boom possible.

Image: well casings on the Seneca well pad. These pipes are used to line the inside of wells and secured in place with cement. They're needed to prevent the well from collapsing, and to prevent fluids inside and outside the well from cross-contamination.

It takes about five million pounds of sand to frack a well in the Marcellus Shale. The frac sand is pumped into the wellbore under pressure along with five million gallons of water and thousands to tens of thousands of gallons of chemical additives. The chemicals are a mix of benign and toxic compounds, including proprietary chemicals claimed as trade secrets.

Image: Bill Kappel, a hydrogeologist emeritus with the U.S. Geological Survey, holds a sample of frac sand as he explains the fracking process.

Pictured here is a piece of the Marcellus Shale.

Fracking creates new fractures in the shale and opens up existing fractures. The frac sand props open the fractures, allowing gas to flow through them and into the gas well.

Natural gas produced at the wellhead is transported to processing plants and consumers via pipelines. Compressor stations are located along pipeline paths to push the gas along.

This Rex Energy compressor station is in Carroll County, Ohio, in the Utica Shale. It's across the street—about 350 feet—from the front door of Theresa and Frank Brothers' home. The couple, who live with their three children (ages 16, 23 and 32), say the compressors run 24/7, and the family has struggled with the noise pollution since March 2014.

Theresa Brothers stands in her front yard between her house and the compressor station, less than 200 feet from the Rex Energy facility. She's holding a sound level meter that registers 80 to 85 decibels—about as loud as a food blender or garbage disposal.

The Brothers said they were not warned or consulted about the compressor station before it was built, and that they've called federal, state and county officials for help, but were told the station complies with existing regulations. Carroll County has no zoning laws for natural gas facilities.

The Utica Shale in Ohio produces "wet gas" that contains methane (the main component of natural gas), water and natural gas liquids such as propane and butane. The liquids are separated from the natural gas stream and sold for use in home heating systems and chemical plants.

Methane leaks from wellheads, pipelines and processing plants throughout the natural gas development process. Because methane is a powerful greenhouse gas, these methane leaks decrease the advantage that natural gas has over coal from a climate change standpoint. It's unclear exactly how much methane is leaking into the atmosphere, and many studies are under way to better quantify the leak rates.

Image: the Kensington Processing Complex in Colombiana County, Ohio. The plant is owned by UEO Buckeye and removes natural gas liquids from wet gas.

The orange wind sock in the upper right-hand corner indicates the direction of the wind and acts as a warning during emergencies. If there is a sudden release of volatile gases, for example, workers can look at the wind sock to figure out which direction to go to reduce their exposure to air pollutants.

Image: the Kensington Processing Complex in Colombiana County, Ohio.

The natural gas liquids from the Kensington plant are shipped about 30 miles south to the Harrison Hub Fractionation plant, shown here, which separates out the ethane, propane, butane and other compounds. The chemicals are shipped via pipelines, trucks and rail to manufacturing plants and other facilities.

Image: the Harrison Hub Fractionation and Storage Complex operated by UEO Buckeye in Harrison County, Ohio.

Propane—a highly flammable gas—is stored in this Harrison facility storage device, nicknamed the "Death Star" by plant employees (the name is a reference to Darth Vader's spherical space station from Star Wars). Propane is used in home heating systems and gas fireplaces.

The shale gas boom has triggered the conversion of many coal-fired power plants to run on natural gas. The Dunkirk Power Plant in New York, pictured here, will be expanded to a 435-megawatt gas plant. It's expected to begin operations in 2015.

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