Understanding Shale Gas Development
Shale gas development (SGD) is the process by which energy companies extract methane gas and other natural gas liquids from shale deposits, then process and deliver them to consumers like utilities or petrochemical facilities. While the industry has been extracting shale gas for many years, today’s process is called "unconventional" because it requires newer methods like horizontal drilling combined with hydraulic fracturing (often called “fracking”), to reach previously inaccessible resources in deep shale formations. Shale is a porous, impermeable rock that must be fractured to release the resources it contains, including gas.
The last few decades have seen a shale gas boom across the U.S. with the addition of an estimated 1.3 million oil and gas facilities—active production wells, gas compressor stations, processing plants—not to mention a web of other infrastructure, such as pipelines, underground storage facilities, injection wells, waste sites, trucking services, and petrochemical plants. SGD is occurring in at least 21 states across the U.S. and in other countries, including Canada, China, and Argentina. Several states and a number of countries have placed bans or moratoriums on hydraulic fracturing to protect public health and to better study the environmental impacts of the process.
SGD in much of Pennsylvania and in parts of West Virginia and Ohio is done in the Marcellus shale formation. Marcellus shale is a deep layer of rock that rises to the surface in Marcellus, NY, but lies about one mile (5,000 to 9,000 feet) underground in areas where gas is being extracted. It is the largest on-shore methane gas reserve in the world. Another gas-rich layer, Utica shale, lies several thousand feet beneath the Marcellus shale layer. Drilling for Utica shale gas is becoming increasingly common, especially in Ohio where it is less deep than in Pennsylvania.
Stages of the SGD Process
The complex process used to extract oil and gas trapped in shale rock formations includes many different stages that take place over a period of years. To see a detailed pictorial representation of shale gas development, see EHP’s handout: Illustrated Stages of Shale Gas Development.
Energy companies search for underground reservoirs of oil or natural gas using mapping, geological clues, seismic testing, and other methods.
Leasing and permitting
After an energy company determines that a locality has enough resources to explore, leases are purchased from mineral rights owners (where applicable), and permits are requested and issued by the state.
Well pad development
Access roads are constructed and the well site is developed. Next comes the well pad, a large structural platform surrounding the drilling operations that is typically between three and 10 acres. The well pad holds the drilling rig, storage tanks, and other machinery and equipment. The energy company uses the drilling rig to drill the well(s). A typical Marcellus well is drilled 5,000 to 9,000 feet vertically. Horizontally, Marcellus wells vary in length, with some going upwards of 15,000 feet. Several wells may be drilled on one well pad, with some well pads hosting 20+ wells. Note that companies are looking to supersize with larger pads containing numerous wells and longer laterals. Drilling produces waste in the way of drill cuttings and sludge, which may be disposed of in local landfills.
Preparation for hydraulic fracturing
After the well has been drilled, the drilling rig is removed and the well is ready for hydraulic fracturing. This process requires convoys of trucks to transport vast quantities of water, chemicals, and sand to the well pad. These will be mixed together, creating fracking fluid. This fracking fluid is then injected into the wellbore at high pressure to expand fractures in the shale, forcing sand into the fractures to hold them open. With these fractures open, gas can migrate to the wellbore and then to the surface, where it is collected.
When a well first goes into production, gas and wastewater return to the surface. The gas—and possibly natural gas liquids—is captured. The wastewater is held in pits, tanks, or pools, at the well site until it is treated, recycled, or disposed of in an underground injection well. The wastewater is a mixture of flowback and produced water. Flowback water contains the fracking fluid mentioned above. Produced water is water previously trapped in underground formations that is released with gas and/or oil. Produced water can contain naturally occurring (but still dangerous) materials that are present in the shale layer, including radioactive compounds, toxic organic and inorganic chemicals, and heavy metals such as arsenic. It also contains a significant amount of salt that was trapped in the shale, which is why it is often referred to as "brine."
Work activity near well sites generally slows at this point once the well is in production, though some wells are re-fracked periodically in an attempt to boost gas production from a site. If a well pad has multiple wells, activity on the well pad may continue. A well, which may be productive from 10-50 years, will continue to generate produced water for the life of the well.
Transportation and Processing
After the gas is accessed, it is processed and transported to end-users. The movement of gas from producing regions to end-users requires an extensive and elaborate transportation system. In many instances, gas has to travel a great distance to reach the point of use, requiring miles of pipelines, pig launchers, compressor stations, and trains.
The gas may have to be processed to purify the gas by removing common contaminants or to have the natural gas liquids—such as propane, butane, and ethane—separated into their individual products.
Underground storage facilities, created by converting previously used underground sites such as depleted oil and gas fields, water aquifers, and old salt caverns, are used to store gas to maintain a supply year-round.
Throughout the hydraulic fracturing process, several forms of solid, semi-solid, and liquid waste are produced. Class II injection wells are used to dispose of liquid waste brought to the surface from gas production. This liquid waste—mostly brine—contains compounds that can be carried by water, such as naturally occurring radioactive materials (NORMs), hydrocarbons, heavy metals, and other toxic substances.
Solid waste, including rocks, dirt, and drill cuttings (ground-up rock brought to the surface after drilling), along with semi-solid waste known as "sludge," is disposed of in municipal landfills, which are usually owned by private companies. This waste contains NORMs and other toxic compounds that were present in the rock and used during well development. Due to exemptions in The Resource Conservation and Recovery Act (RCRA), oil and gas waste is not considered toxic or subject to federal cradle-to-grave tracking. Toxic leachate from landfills that accept shale drilling waste is a growing problem.
Remediation involves plugging the well(s) and restoring the site when a well is done producing gas, or if an exploratory well does not produce adequate resources. The process of site restoration involves filling the well with cement, removing waste and equipment, and capping the well. Energy companies may also attempt to restore the original landscape as much as possible with plantings and other landscaping.
Other SGD Infrastructure
Many people think of well sites alone when considering the impact of SGD on their homes and communities; however, the SGD process involves many other activities and facilities that can affect those who live nearby. For example, increased truck and train traffic can be disruptive in communities by creating road congestion, accidents, air pollution, and damage to local roads. Likewise, SGD-associated infrastructure including pipelines, compressor stations, and processing plants all carry potential risks to health and the environment. For additional information about each step of the process, please visit FracTracker’s Explore a Fracking Operation - Virtually