ML23102A013
| ML23102A013 | |
| Person / Time | |
|---|---|
| Issue date: | 09/30/2020 |
| From: | Elise Striz NRC/NMSS/DDUWP/URMDB |
| To: | |
| Saxton J | |
| References | |
| Download: ML23102A013 (8) | |
Text
Potential Impact of Oil and Gas Wells in Eagle Ford Shale on Uranium Mill Tailings Impoundments in Southwest Texas September 2020 Elise A. Striz
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) staff has evaluated the potential for existing and future oil and gas drilling and production operations in the Eagle Ford shale in Southwest Texas to impact existing uranium mill tailings impoundments. The staff found that there are three potential impacts which should be considered:
- 1. Potential impact to groundwater below existing mill tailings impoundments from contamination through fracture pathways caused by hydrofracturing of oil and gas wells within the Eagle Ford shale.
- 2. Potential impact to the stability of existing mill tailings impoundments from seismic events created by hydrofracking of oil and gas wells within the Eagle Ford shale.
- 3. Potential impact to soils, groundwater, and surface water at and near existing mill tailings impoundments from oil and gas wells within the Eagle Ford shale.
The staff reviewed each of these impacts separately. The staff concluded that the oil and gas wells within the Eagle Ford shale in the counties in southwest Texas where the uranium mill tailings impoundments are located (e.g. Karnes) do not create a significant risk for contamination of groundwater under the mill tailings impoundment. Specifically, given the geological isolation of the Eagle Ford shale, its location at depths well below the base of usable groundwater, and measured fracture heights from existing oil and gas wells, the staff concluded that hydrofracking in the Eagle Ford shale does not provide a pathway for contamination to usable groundwater under uranium mill tailings impoundments. The staff also concluded that seismic disturbances from hydrofracking oil and gas wells in the Eagle Ford shale present no significant risk to the stability of uranium mill tailings impoundments. Specifically, staff found that microseismic studies within the Eagle Ford shale show the seismic events related to hydrofracking are below the magnitude of significant earthquakes that could affect a uranium tailings impoundment. Finally, staff concluded that although there could be a potential impact to the surface soils, surface water and groundwater from oil and gas wells near uranium mill tailings impoundments, the risk is mitigated as the design, drilling and installation of these wells are regulated by the Railroad Commission of Texas (RRC). The RRC also requires a review of all offset wells to determine if there are any potential pathway for contamination to overlying groundwater. Therefore, staff concludes that it may rely on the RRC of Texas to prevent, detect and correct any of these impacts before they affect the surface soils, surface water or groundwater at or near an existing uranium tailings impoundment.
IMPACT ANALYSIS
- 1. Potential impact to groundwater below existing mill tailings impoundments from contamination through fracture pathways caused by hydrofracturing of oil and gas wells within the Eagle Ford shale 1
Numerous oil and gas wells are located in the Eagle Ford shale in Southwest Texas in counties where uranium mill tailing impoundments are located as shown in Figure 1 (RRC, 2020). The staff performed a review of recent relevant publications on hydrofracturing and potential impacts to groundwater in the Eagle Ford shale in the area of interest (e.g. Karnes and surrounding counties in Southwest Texas) to assess if fracture pathways generated by hydrofracturing of these wells could potentially impact groundwater at a uranium mill tailings site. Staff determined that because of the depth of the Eagle Ford shale, its location with respect to usable sources of groundwater, its geological setting and the published data on measured fracture heights in oil and gas wells in the Eagle Ford shale, these operations present no significant incremental risk of contamination of groundwater under mill tailings impoundments in the area of interest. The specific findings to support these conclusions are:
- The top of the Eagle Ford shale is deep (4,000-12,000 ft below ground surface) in the area of interest in Southwest Texas where the uranium mill tailings impoundments are located (Table 6.3; EPA, 2016).
- The base of treatable1 groundwater in the Eagle Ford shale in the area of interest in Southwest Texas is 2,800-10,800 ft above the top of the Eagle Ford shale (see Table 6.3, EPA, 2016).
- The Eagle Ford shale oil and gas play in the area of interest is overlain by the Austin Chalk formation and underlain by the Buda limestone. Both of these carbonate formations isolate the shale and physically limit the vertical height of fractures as shown in Figure 2 (Fisher and Warpinski, 2012; Fisher, 2014).
- The fracture heights from microseismic measurements in the Eagle Ford shale in oil and gas wells are on the order of tens to hundreds of feet in the area of interest and therefore remain thousands of feet below base of drinking water aquifers as shown in Figure 3 (Fisher and Warpinski, 2012).
- 2. Potential impact to the stability of existing mill tailings impoundments from seismic events created by hydrofracking of oil and gas wells within the Eagle Ford shale The staff performed a review of recent relevant publications on the seismic impacts of hydrofracturing in oil and gas wells in the Eagle Ford shale in the area of interest (e.g.
Karnes and surrounding counties in Southwest Texas). Staff determined that given the minimal seismic disturbance from hydrofracking in the Eagle Ford shale, these operations present no significant incremental risk to the stability of uranium mill tailings impoundments.
Staff also determined that seismicity from Class II injection wells that dispose of produced water from oil and gas operations would not create a significant risk to the stability of uranium mill tailings impoundments. The specific findings to support these conclusions are:
- Microseismic and tiltmeter studies show the seismic events related to hydrofracking are below the magnitude of significant earthquakes (Fisher, 2014).
- Except in cases of very unique tectonics, hydrofracturing does not generate even moderate magnitude earthquakes (Fisher, 2014).
1 Treatable groundwater is defined by the State. In Texas, usable groundwater is defined by the RRC as having less than 3000 mg/l TDS; therefore, an underground base of usable water is generally defined as having a less than 3000 mg/l TDS, as that level is feasibly treatable.
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- Warpinski reported that that the typical microseism event in hydraulic fracture treatments in the Eagle Ford shale have values around -2.5 Mw (Figure 3, Warpinksi et. al, 2012).
Warpinski stated that practical terms, a microseism of this size would have a slip radius of around 20 feet and a minute displacement on the order of a grain size (Table 1, Warpinski et. al, 2012). Therefore, this size of microseism event does not extend across the full height of the oil /gas play in a typical shale environment.
- In a report on induced seismicity from Class II injection wells, EPA stated that of approximately 30,000 Class II disposal wells in the United States, very few have produced seismic events with magnitudes greater than M4.02. EPA stated that earthquakes less than M4.0 are not considered significant enough to damage underground sources of drinking water or infrastructure as supported by Table 1 (EPA, 2014).
- 3. Potential impact to soils, groundwater, and surface water at and near existing mill tailings impoundments from oil and gas wells within the Eagle Ford shale The staff performed a review of the regulation of oil and gas wells by the Railroad Commission of Texas (RRC) to assess the protection of soils, surface water and groundwater from impacts from oil and gas wells in the Eagle Ford shale in the area of interest (e.g. Karnes and surrounding counties in Southwest Texas). Staff determined that these regulations should prevent, detect and correct any impacts to surface soils, surface water or groundwater at or near an existing uranium tailings impoundment. The specific findings to support these conclusions are:
- The drilling and completion of hydrofracking wells in the Eagle Ford shale is regulated by the RRC to ensure the well casings and annulus are designed, cemented, and monitored to prevent and detect leaks to protect groundwater (Texas Administrative Code, Title 16, Part 1, Chapter 3, Rule 3.13 and 3.53).
- The RRC regulates drilling mud pits and leaks and spills associated with all oil and gas operations to prevent, detect and correct impacts to soils, surface water and groundwater (Texas Administrative Code, Title 16, Part 1, Chapter 3, Rule 3.8)
- Before drilling these hydrofracking wells, the RRC requires the oil and gas operator to evaluate all offset wells (abandoned wells, existing oil and gas wells) to determine if there are any potential pathway for contamination to overlying groundwater. Corrective action such as plugging and abandonment can be required as needed (Texas Administrative Code, Title 16, Part 1, Chapter 3, Rule 3.37).
REFERENCES Fisher, K., 2014, Hydraulic Fracture Growth: Real Data paper presentation at GTW-AAPG/STGS Eagle Ford plus Adjacent Plays and Extensions Workshop, San Antonio, Texas, February 24-26, 2014.
2 https://earthquake.usgs.gov/learn/glossary/. The magnitude is a number that characterizes the relative size of an earthquake. Magnitude is based on measurement of the maximum motion recorded by a seismograph.
3
Fisher, K. and Warpinski, N., 2012, Hydraulic-Fracture -Height Growth -Real Data, Journal of Production and Facilities, Society of Petroleum Engineers (SPE) Paper 1378897.
U.S. EPA, 2014, Minimizing and managing potential impacts of injection-induced seismicity from class II disposal wells: Practical approaches, Washington, D.C.
https://www.epa.gov/sites/production/files/2015-08/documents/induced-seismicity-201502.pdf.
U.S. EPA, 2016, Hydraulic Fracturing for Oil and Gas: Impacts from the Hydraulic Fracturing Water Cycle on Drinking Water Resources in the United States (Final Report), U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-16/236F.
Warpinski, N., J. Du, and U. Zimmer, 2012, Measurements of hydraulic-fracture-induced seismicity in gas shales, Journal of Production and Facilities, Society of Petroleum Engineers (SPE) Paper 151597.
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Figure 1. Oil and Gas Wells in Eagle Ford shale in Southwest Texas (RRC of Texas, 2020)
Figure 2. Fracture Height in Horizontal Well from Microseismic Measurements in Eagle Ford shale limited by Austin Chalk and Buda Lime Carbonate Formations (adapted from Fisher, 2012)
Figure 3. Fracture Heights in Eagle Ford shale from Microseismic Measurement in Horizontal Wells in Southwest Texas Counties compared to Aquifer Depths (adapted from Fisher and Warpinski, 2012)
Karnes County
Table 1. Magnitude of Earthquakes and Effects (adapted from EPA, 2014)
Magnitude Earthquake Effects 2.5 or less Usually not felt but can be recorded by seismograph.
2.5 to 5.4 Often felt, but only causes minor damage.
5.5 to 6.0 Slight damage to buildings and other structures.
6.1 to 6.9 May cause a lot of damage in very populated areas.
7.0 to 7.9 Major earthquake. Serious damage.
8.0 or greater Great earthquake. Can totally destroy communities near the epicenter.