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treatment at 3,000 m. All three Oklahoma cases demonstrate a reoccurring problem in induced seismicity studies: the seismic events are small, the regional networks are sparse, and the data quality is often too poor to fully confirm a causal link to fluid injection for energy development (see also Chapter 1).

INJECTION WELLS USED FOR THE DISPOSAL OF WATER ASSOCIATED WITH ENERGY EXTRACTION

In addition to fluid injection for specific kinds of energy development (e.g., water injection to produce steam for geothermal energy recovery, or fluid injection for waterflooding [secondary recovery]), water injection to dispose of water generated as a result of geothermal and oil and gas production operations is very common in the United States. Water that must be disposed of originates from production (see, e.g., NRC, 2010) or from flowback. Hereafter we refer to this kind of water broadly as wastewater; Chapter 4 clarifies the different kinds of water from energy production that are disposed of and the different classes of wells that are designated in the United States for this purpose. A recent study by Argonne National Laboratory estimated the total oil and gas fluid recovered from flowback after hydraulic fracturing operations and waste fluid produced during daily oil and gas production in the United States to be 20.9 billion barrels (about 878 billion gallons) of water per year (Clark and Veil, 2009). The majority (95 percent) of this water was managed through underground injection and more than half (55 percent) was injected for the purpose of enhanced recovery (Clark and Veil, 2009) (see the section Tertiary Oil and Gas Recovery [EOR] in this chapter). Just over one-third of the total wastewater volume (39 percent) or 6 billion barrels (252 billion gallons) was injected in disposal wells. Table 3.2 shows the water volumes produced in conjunction with oil and gas operations for various states. Importantly, other types of fluid may also be disposed of through underground injection (industrial wastes, for example, from manufacturing unrelated to energy production); these different kinds of underground injection are also discussed in Chapter 4.

The annual volume of wastewater in the United States is disposed of in many tens of thousands of injection wells. For example, in Texas, over 50,000 Class II7 injection wells were permitted as of 2010 (of which approximately 40 percent would be associated with disposal of wastewater and the remainder associated with waterflooding for secondary recovery; Texas RRC, 2010) (Figure 3.12).

Felt induced seismicity potentially related to Class II water injection wells has been identified at individual sites in Arkansas (see Chapter 4), Ohio, and Texas (Box 3.7). USGS

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7 Wells in the Environmental Protection Agency’s (EPA’s) Underground Injection Control (UIC) program are described and regulated under one of six “classes.” Class II wells are specifically those that address injection of brines and other fluids associated with oil and gas production and hydrocarbons for storage. EPA’s well class system and the UIC program are described in more detail in Chapter 4.



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