Geology’s role in unconventional natural gas production (via hydraulic fracturing and directional drilling) often is overlooked by the public, but it is a key factor in ensuring that natural gas production is efficient, economic and environmentally responsible.
Not all shale gas formations in the United States are created equal, so it is important that the public understands the differing geology and the advancing technology, while regulators provide appropriate protections that the public has confidence in.
Last month’s (April EXPLORER) Washington Watch column compared the rules and regulations for hydraulic fracturing in different states. This month, for comparison purposes, the geology and hydrology of the Marcellus Shale formation, Barnett Shale formation and Wind River Basin are paired with some of their respective state regulations.
Regulatory differences are noted in the “Fact-Based Regulation for Environmental Protection in Shale Gas Development 2012,” The Energy Institute, by AAPG member Charles G. “Chip” Groat and Thomas W. Grimshaw.
♦ The Pavillion Field of Wyoming’s Wind River Basin is extracting natural gas from shallower reservoir rocks (primarily Upper Cretaceous to Eocene sandstones with interfingering conglomerates and mudstones) and deeper source rocks (primarily Cretaceous shales) at depths from 1,000-5,000 feet. There are no significant impermeable layers above the source rocks, so gas can migrate.
♦ The Barnett Shale of the Fort Worth Basin in Texas is a classic and immense source of natural gas production. Initially gas was extracted from shallower reservoir rocks (primarily Pennsylvanian conglomerates) and now with hydraulic fracturing, gas is extracted from source rocks (primarily Mississippian shales) at depths of 6,500-8,000 feet. Overlying relatively impermeable layers of limestone help to constrain the gas to the reservoir and source rocks.
♦ The Marcellus Shale in the Appalachian Basin, extending through multiple states, is a relatively new play. The source rock, Middle Devonian Marcellus Shale, is overlain by more impermeable shales and limestones, which helps to constrain the gas until hydraulic fracturing is used to extract it at depths of 4,000-9,000 feet.
Given the general geology of these basins, it is clear there is little possibility of any groundwater contamination in the Barnett or Marcellus Shale from hydraulic fracturing because:
There is a greater possibility of groundwater contamination in the Wind River Basin, because there the reservoir and source rocks are shallower and closer to aquifers being used for water consumption. Contamination would only be possible, though, through improper hydraulic fracturing operations, because regulations are in place to protect groundwater.
♦ Regulations in Wyoming require casing depths for hydraulic fracturing drilling to be “Below all known or reasonably estimated utilizable groundwater,” as regulators understand that usable aquifers and reservoir rocks occupy the same geologic units in a sometimes complex way.
♦ Regulations in Texas require a casing depth “set and cement sufficient surface casing to protect all usable-quality water strata.”
♦ Regulations in Pennsylvania and New York call for casing depths at 50-100 feet or into “bedrock,” “whichever is deeper,” recognizing that usable aquifers are shallow and the bedrock can serve as an impermeable barrier without casing to reach the much deeper source rocks.
While groundwater contamination from hydraulic fracturing in these three basins is unlikely, there is greater public concern about the amount of water needed for hydraulic fracturing and how to store flowback waters.
♦ Regulations regarding the amount and source of water used in hydraulic fracturing for the Marcellus Shale varies, based on different watersheds, water commissions and states.
The toughest restrictions are enforced by the Susquehanna River Basin Commission, which requires approval for any surface or groundwater used in hydraulic fracturing.
The Delaware River Basin Commission requires approval for any surface or ground water in excess of 100,000 gallons per day. The restrictions are strict because there are large population centers and large agricultural communities that tap into surface and groundwater resources.
♦ In Texas, regulations require disclosure of the total volume of water used in the hydraulic fracturing, but not the source of the water.
The Barnett Shale play is next to Dallas-Fort Worth, and there is concern about depleting the main aquifer used for drinking water in the area, so tracking water use will help with water resource management.
♦ Regulations for Wyoming mandate that the source and depth of water supply wells within one quarter mile of the drill site be identified before drilling, but no reporting of where and how much water is used for the hydraulic fracturing is required.
Again the primary concern is to avoid any possible contamination of local water wells for a relativly small population of water users.
The storing of flowback waters depends to some extent on the hydrology and climate of the three regions.
♦ In the humid East, regulations tend to be stricter because surface waters are used for drinking, and water in storage pits does not evaporate as quickly as in arid regions.
The strictest regulations in New York require flowback water to be stored in a tank.
Pennsylvania has the second strictest regulations requiring that a high-grade seal line the bottom of any above ground pits.
♦ Wyoming requires tanks only where groundwater is less than 20 feet below the surface – otherwise an operator can store the flowback water in a pit with a specific grade liner.
♦ Texas requires operators to prevent pollution of surface or ground waters from storage pits, but has no specific regulations for the pits.
As unconventional gas production (via hydraulic fracturing and directional drilling), which is common in the arid West and semi-arid Midwest, becomes more prevalent in the humid East, it is essential that the geology, hydrology and meteorology be considered when devising or revising regulations.
Generating energy resources while protecting water resources benefits from sound geoscience, but geoscience is not the only factor used in devising oversight policies.
Population density, economics, other industries, political boundaries, social issues and land and water use patterns are contributing factors that cannot be ignored.
Aaron Rodriguez is the AAPG/AGI spring intern at the American Geosciences Institute. He is a student at Southern Utah University in Cedar City, Utah.