One of the biggest environmental challenges associated with hydraulic fracturing occurs not deep underground, but on the earth’s surface.
Flowback Fluid Handling
After the hydraulic fracturing process is complete for each zone, fracture fluids flow back up the well under formation pressure. These flowback fluids may represent as little as 3% or as much as 80% of the fracture fluids initially injected into the well. Traditionally, flowback fluids have been stored in lined pits.
The best casing and cement job won’t protect shallow groundwater from a leaking pit. That’s why it’s critical that lined pits are carefully constructed and comply with the environmental regulations governing them.
One way that some operators have been tackling the problem of leaking pits has been to use a closed circulation system. In such a system, fracture fluids are contained within metal tanks and pipes, and are never released into surface pits. It’s easier to detect and repair leaks in these systems but renting a large tank farm obviously adds significant cost.
A second problem associated with used fracture fluids is how to dispose of them when they’re no longer needed.
Disposal Techniques
The commonly-accepted way to dispose of used fracture fluids is to inject them into deep disposal wells or into Class II underground injection control (UIC) wells. This is the most common practice. However, the method is only available in regions where appropriate deep geologic formations are present and laws allow the practice. This is not the case in every state. See the chart below for descriptions of classes of injection wells and how many of each type exist.
Classes of Wells1United States Environmental Protection Agency, 2019, UIC Injection Well Inventory, https://www.epa.gov/uic/uic-injection-well-inventory (accessed December 1, 2022).
| Classes | Use | Inventory |
|---|---|---|
| Class I | Inject hazardous wastes, industrial non-hazardous liquids, or municipal wastewater beneath the lowermost USDW. | 903 wells |
| Class II | Inject brines and other fluids associated with oil and gas production, and hydrocarbons for storage. | 156,547 wells |
| Class III | Inject fluids associated with solution mining of minerals beneath the lowermost USDW. | 28,465 wells |
| Class IV | Inject hazardous or radioactive wastes into or above USDWs. These wells are banned unless authorized under a federal or state ground water remediation project. | 169 wells |
| Class V | All injection wells not included in Classes I-IV. In general, Class V wells inject non-hazardous fluids into or above USDWs and are typically shallow, on-site disposal systems. However, there are some deep Class V wells that inject below USDWs. | 549,322 wells |
| Class VI | Inject Carbon Dioxide (CO2) for long term storage, also known as Geologic Sequestration of CO2 | 2 wells |
In areas without this option, operators need to resort to other disposal techniques.
Sometimes, they will pay to dispose of the waste at a municipal waste treatment plant. Unfortunately, many of these plants are not currently well-equipped to deal with the high salt content of most produced water.
Another option is recycling – processing the fluids on-site or at a centralized location so that they can be reused for future hydraulic fracturing jobs. Recycling or reuse of produced fracturing fluids, brines, or connate waters has been gaining in popularity over the last several years.
Finally, in some places operators dispose of the fluids by applying them to the land surface. This method is more controversial and is only permitted in a few states.
Induced Seismicity
Injecting produced water and/or flowback fluid (used fracture fluids) into deep wells is currently the most common method for disposal. However, the method is not without downsides. If the UIC well is not carefully planned and permitted, the process has been demonstrated to lead to induced seismicity – earthquakes related to human activity. There is evidence linking injection well activity to seismic events in multiple locations across the United States, such as Texas and Oklahoma.
Images: “Water recycling and purification tanks” by antikainen via iStock