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 Ur anium Mill Tailings Impoundments in Southwest Texas September 2020 Elise A. Striz
SUMMARY
The U.S. Nuclear Regulatory Commission (NRC) staff has evaluate d the potential for existing and future oil and gas drilling and production operations in th e Eagle Ford shale in Southwest Texas to impact existing urani um mill tailings impoundments. T he 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 hydrofracturi ng 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 impoundmen t. Specifically, given the geological isolation of the Eagle Ford shale, its location at d epths well below the base of usable groundwater, and measured fracture heights from existing oil an d gas wells, the staff concluded that hydrofracking in the Eagle Ford shale does not provide a p athway for contamination to usable groundwater under urani um mill tailings impoundments. The staff also concluded that seismic disturbances from hydrofracking oil and gas wells in th e Eagle Ford shale present no significant risk to the stability of uranium mill tailings impo undments. Specifically, staff found that microseismic studies within the Eagle Ford shale show the seismic events related to hydrofracking are below the magnitude of significant earthquake s that could affect a uranium tailings impoundment. Finally, staff concluded that although t here could be a potential impact to the surface soils, surface water and groundwater from oil and g as wells near uranium mill tailings impoundments, the risk is mitigated as the design, dri lling and installation of these wells are regulated by the Railroad Commission of Texas (RRC). The R RC also requires a review of all offset wells to determine if there are any potential pathwa y for contamination to overlying groundwater. Therefore, staff concludes that it may rely on th e 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 hydrofracturi ng 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 publica tions 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 fract ure pathways generated by hydrofracturing of these wells could potentially impact groundw ater 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 settin g and the published data on measured fracture heights in oil and gas wells in the Eagle For d shale, these operations present no significant incremental risk of contamination of gro undwater 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 belo w ground surface) in the area of interest in Southwest Texas where the uranium mill tail ings 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 F ord 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 t hese 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 Ea gle Ford shale in oil and gas wells are on the order of tens to hundreds of feet in the a rea 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 det ermined that given the minimal seismic disturbance from hydrofracking in the Eagle For d shale, these operations present no significant incremental risk to the stability of ura nium mill tailings impoundments.
Staff also determined that seismicity from Class II injection w ells that dispose of produced water from oil and gas operations would not create a significan t risk to the stability of uranium mill tailings impoundments. The specific findings to s upport these conclusions are :
- Microseismic and tiltmeter studies show the seismic events re lated to hydrofracking are below the magnitude of significant earthquakes (Fisher, 2014).
- Except in cases of very unique tectonics, hydrofracturing doe s 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.
2
- Warpinski reported that that the typical microseism event in hydraulic fracture treatments in the Eagle Ford shale have values around -2.5 M w (Figure 3, Warpinksi et. al, 2012).
Warpinski stated that practical terms, a microseism of this siz e would have a slip radius of around 20 feet and a minute displacement on the order of a g rain size (Table 1, Warpinski et. al, 2012). Therefore, this size of microseism ev ent does not extend across the full height of the oil /gas play in a typical shale environ ment.
- In a report on induced seismicity from Class II injection wel ls, EPA stated that of approximately 30,000 Class II disposal wells in the United Stat es, very few have produced seismic events with magnitudes greater than M4.0 2. EPA stated that earthquakes less than M4.0 are not considered significant enoug h to damage underground sources of drinking water or infrastructure as supp orted by Table 1 (EPA, 2014).
- 3. Potential impact to soils, groundwater, and surface water at an d near existing mill tailings impoundments from oil and gas wells within the Eagle F ord shale
The staff performed a review of the regulation of oil and gas w ells 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 i nterest (e.g. Karnes and surrounding counties in Southwest Texas). Staff determined tha t these regulations should prevent, detect and correct any impacts to surface soils, surfa ce water or groundwater at or near an existing uranium tailings impoundment. The specific fi ndings to support these conclusions are:
- The drilling and completion of hydrofracking wells in the Eagl e Ford shale is regulated by the RRC to ensure the well casings and annulus are designed, cement ed, and monitored to prevent and detect leaks to protect groundwater (Texas Administ rative Code, Title 16, Part 1, Chapter 3, Rule 3.13 and 3.53).
- The RRC regulates drilling mud pits and leaks and spills assoc iated with all oil and gas operations to prevent, detect and correct impacts to soils, sur face 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 ga s wells) to determine if there are any potential pathway for contamination to overlying ground water. Corrective action such as plugging and abandonment can be required as needed (Tex as 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 Worksho p, 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 i njection-induced seismicity from class II disposal wells: P ractical approaches, Washington, D.C.
https://www.epa.gov/sites/production/files/2015-08/documents/in duced-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 (F inal Report), U.S.
Environmental Protection Agency, Washington, DC, EPA/600/R-16/2 36F.
Warpinski, N., J. Du, and U. Zimmer, 2012, Measurements of hyd raulic-fracture-induced seismicity in gas shales, Journal of Production and Facilities, Society of Petroleum Engineers (SPE) Paper 151597.
4