A hydraulic fracturing has taken place on a well a portion of the injected fluids (typically about 5-25%) along with naturally occurring brine in the formation will flow up through the well to the surface, commonly called flowback fluids. During this period, which may last for several days to a week or more, the well is allowed to flow to allow hydrocarbons to begin to be produced efficiently. Once the well has been brought on-line and producing oil or gas, brine will be co-produced along with the hydrocarbons during a well’s productive life, and are known as produced fluids. A well will typically produce 5-10 barrels of fluids (or more) for every barrel of oil or for every million cubic feet of gas. The produced fluids typically have high concentrations of total dissolved solids (TDS). TDS may include higher concentrations of salts and metals such as barium and strontium, while containing lower levels of organics and radionuclides such as radium-226. The table below provides a summary of the flowback water quality from Marcellus wells in Pennsylvania.
|Parameter||Drinking Water Standards1||Approximate Median Concentration in Typical Pennsylvania Groundwater2||Approximate Median Concentration in Typical Marcellus Wastewater3|
|pH||6.5 to 8.5||7.50||6.60|
|Total Dissolved Solids||< 500||163.0||67,300|
|Total Suspended Solids||-||1.0||99.0|
|Sulfate||< 250||18.0||2.4 to 106|
|Nitrate-Nitrogen||< 10||0.50||0.1 to 1.2|
|Dissolved Organic Carbon||-||< 1.0||62.8|
|Dissolved Methane||-||No data available||No data available|
|Oil & Grease||-||< 5.0||6.3|
Source: Center for Rural Pennsylvania, Boyer et al (2012). 1 Pennsylvania Department of Environmental Protection, 2006. 2 Pennsylvania State University, 2011; Davis et al., 2004; and Thurman, 1985. 3 Hayes, 2009.
TDS levels may be 2 to 3 times the levels found in seawater. Therefore they must be properly managed so they can be reused or treated to meet state regulatory requirements before discharge where allowed. Innovations in recycling of wastewater for reuse in hydraulic fracturing have significantly reduced the volume of flowback and produced fluids that would otherwise be disposed of via surface discharge or disposal wells. Currently it is estimated that the industry is recycling about 80-90% of the produced fluids for hydraulic fracturing in some shale basins such as the Marcellus, while other basins may only see 10-20% of these fluids recycled for hydraulic fracturing. The benefit of recycling the produced fluids for hydraulic fracturing is that less fresh water is necessary to be withdrawn, and less water needs to be treated or disposed. The produced fluids are separated from the oil or gas in the field and stored in tanks and then often trucked to another site for recycling or treatment.
When the fluids are treated for recycling for hydraulic fracturing, they may either be treated in the field or transported to a centralized treatment facility via tanker truck. Typically the produced fluids are treated to the extent that they can be injected back down into the shale without plugging up the formation or otherwise negatively impacting oil or gas production. Often the treatment may be as simple as running the water through a filter to remove sediment, or using chemicals to remove metals via precipitation but leaving the salts in the solution. If higher levels of treatment are required to discharge to a stream or river then a desalination technique such as thermal distillation may be required to remove salts that would otherwise be toxic to fresh water species.
A shale gas well is producing 5 million cubic feet of gas per day and for every million cubic feet of gas generates an average of 8 barrels of produced fluids. How many barrels of produced fluids will this well produce in a day? How many gallons will it generate (hint: there are 42 gallons in a barrel)? Ultimately this well will produce an estimated 5 billion cubic feet of gas. What would be the total amount of brine generated from this one well? Provide your answer in both barrels and gallons.
As mentioned above, not all the produced fluids can be efficiently recycled and therefore at some point will require disposal. If treated to sufficient levels, these fluids could be discharged to a stream or river if the proper permits are in place. However, this is not common in many states. A more typical way to dispose of these fluids is through deep disposal well injection, a process regulated and permitted under the EPA’s Safe Drinking Water Act. For more info go EPA: Class II Oil and Gas Related Injection Wells. Wells that are used to inject any fluid into the earth are known as Underground Injection Control (UIC) wells, and are divided into six different classes. Class II wells involve oil and gas operations and include:
- Disposal wells
- Enhanced recovery wells
- Hydrocarbon storage wells
The graphic below shows Class II disposal and recovery wells. It is estimated there are about 180,000 Class II wells in the US, approximately 80% are recovery wells and 20% are disposal wells, collectively accepting 2 billion gallons per day of produced fluids.
Class II disposal wells (or Class IID wells) are used to dispose of oil and gas wastes, primarily produced fluids, i.e. saltwater, and are often called saltwater disposal wells. The injection rate and pressure for all injection wells must be carefully monitored to ensure fluids do not migrate into undesirable zones. When geologic faults are close enough to an injection well and the injection pressure is too high it can cause the fault to slip and thus cause an earthquake large enough to be felt at the earth’s surface. In the last several years there has been a significant increase in seismicity in some parts of the country, as shown by this USGS graphic.
The EPA provides a good overview on the facts and myths surrounding induced seismicity with injection wells and hydraulic fracturing.
Clearly we need to make sure that oil and gas development is occurring using the best available technologies to reduce the industry’s overall environmental footprint, yet meet our society’s energy needs. Another lesson will address some of the environmental impacts that oil and gas development can have and the best management practices to prevent them.