appendix l injection well induced seismicity...

Stillwater Mining Company Benbow Exploration Portal & Support Facilities

Dean, Montana

Appendix L Injection Well

Induced Seismicity Potential

PLAN OF OPERATIONS FOR MINERAL EXPLORATION Benbow Exploration Portal and Support Facilities

H:\Files\091 STILW\13043\Injection Control Well Memo\M14 Weimer Benbow UIC Induced Seismicity Potential.docx\HLN\4/1/2014\034 2 4/1/2014 9:39 AM

Micro-earthquakes are typically not large enough to be felt or cause damage on the earth’s surface, and have a magnitude less than M 2.5 (Monroe and Wicander, 2005). The USGS National Earthquake Information Center website typically does not report earthquakes less than 2.5, although local seismic stations, such as those at universities and state geological surveys, may be able to detect smaller magnitude earthquakes (USGS National Earthquake Information Center, 2014).

CASE STUDIES Nicholson and Wesson (1990) list 27 acknowledged worldwide cases of seismicity that are associated with injection or withdrawal well operations. Twenty of these wells were for initial or secondary recovery of hydrocarbons, three were waste disposal wells, two were geothermal wells, one was a solution mining well, and one was a research well to evaluate triggering mechanisms of induced seismicity. Figure 1 is a plot of the available information regarding the depth of these wells and the injection pressures at which they operated. The depths of the wells averaged 6,980 feet and injection pressure averaged 2,017 psig. The maximum magnitude of earthquake induced by the operation of the wells ranged from M <1 to M 5.5. For comparison purposes, the maximum proposed injection operating pressure (265 psi) and maximum depth (1,700 vertical feet below ground surface) of the Benbow UIC well are also plotted on this figure. As of July 2013, in the United States, approximately 110,000 Class II underground injection wells are used by the petroleum industry to enhance production by hydraulic fracturing of impermeable reservoir rock, and 30,000 Class II underground injection wells are used for disposal of wastewater. Of these wells, most have no detected seismicity of magnitude equal to M 3, although the operation of a few wells does coincide with earthquake activity greater than M 3 (Ellsworth, 2013). While it is possible that deep well injection operation may induce small earthquakes in the immediate vicinity of the bottom of the well, many of these earthquakes are below the threshold for non-instrumental detection and below the threshold for damage. Nicholson and Wesson (1990) point out that it is reasonable to ask whether induced earthquakes of these magnitudes constitute a risk. The question that arises is whether small micro-earthquakes indicate the potential for larger, potentially damaging earthquakes, and specifically, whether such potential exists as a result of the operation of the proposed Benbow UIC well.

NATURALLY OCCURING SEISMIC ACTIVITY

Sminchak et al. (2001) found that deep well injection usually triggers activity in a seismically unstable area rather than causing an earthquake in a seismically stable area. To evaluate recent historical seismicity, a search of the available USGS National Earthquake Information Center archives centered on the Benbow adit site with a radius of 62 miles (100 km) for the 54-year period of record from January 1, 1960 to February 25, 2014 indicated 224 earthquakes were detected during this time frame. Most (117) of these earthquakes were associated with Yellowstone National Park and had magnitudes that ranged up to M 5.3. Seven earthquakes not associated with Yellowstone National Park were located 24 to 54 miles southwest to northwest of the proposed Benbow UIC well, and of these, four earthquakes had magnitudes greater than M 3 that ranged up to M 3.7, and hypocenters from 3.1 to 8.9 miles deep (Figure 2).


Hypocenters are those locations where an earthquake rupture initiates (MBMG online glossary). During the 1960 to 2014 time frame, no earthquakes were detected in the immediate vicinity (within 5 miles) of the proposed Benbow UIC well, and all earthquake hypocenters within a 62-mile radius were more than 10 times deeper than the proposed Benbow UIC well 0.3-mile maximum anticipated depth. While 54 years may be a significant period in human memory, it is less than an instant with respect to geologic time, and the available archives may not provide a complete understanding of the actual seismicity of the area.

TRIGGERS FOR INJECTION-WELL CAUSED SEISMICITY

Fluid Pore Pressure Of the several factors that distinguish seismically-active injection wells from the majority of aseismic wells, fluid pressure in the pores and fractures of rock plays a key role. Fluid pore pressure acts against the weight of the rock and against the forces that resist movement along faults, and can be an important component of the in situ (ambient) stress condition at depth. If the pore pressures are high with respect to the strength of the rock and the forces that resist movement along faults, the introduction of small changes in the in situ stress condition can “tip the scales” and enhance the probability for rupture (LBNL, 2013). The lack of high magnitude earthquakes in the area (outside of Yellowstone National Park) would suggest that in situ stresses are not close to the “tipping point” where injection pressures of 265 psi would induce seismicity.

Permeability and Porosity

The parameters that affect injection pressures are formation permeability and porosity. Rock formations with high permeability and porosity are more receptive to injected fluids (Sminchak et al., 2001). The pressure field generated by fluid injection is governed by the porosity, permeability and elastic constants of the aquifer (Nicholson and Wesson, 1990). Most of the cases of acknowledged induced seismicity are associated with water-flooding operations for the purpose of secondary recovery of hydrocarbons that entail large arrays of wells injecting fluids at high pressures into confined reservoirs of limited extent and low permeability (Nicholson and Wesson, 1990). To provide background, hydrocarbons are concentrated and confined within a geologic setting called a reservoir. During primary recovery, wells are drilled on spacing that will most efficiently permit optimum removal of hydrocarbons from the reservoir. The depth of the wells is reservoir-dependent and may be hundreds to thousands of feet deep. During secondary recovery, an array of existing wells is used to inject fluids (typically water or gas) in a manner that is designed to enhance the flow of hydrocarbons toward extraction wells (Zitha et al., 2011).

Depth of Well and Mounding In contrast, the proposed Benbow UIC well would be drilled to a comparably shallow depth (up to 1,700 feet from ground surface) into an unconfined (water table) aquifer and inject water into zones of significantly higher porosity and permeability. In unconfined aquifers, the pressure field generated by fluid injection is evidenced by the degree of mounding created by the addition of water. Calculations were made to evaluate the mounding caused by injecting 100, 300, and 500 gpm into the Madison


Formation, using an aquifer thickness of 1,050 feet and available published values of transmissivity (56,000 ft2/day) and storativity (3.5 x 10-4 to 2 x 10-3) (Greene, 1993). The calculations indicated that after a period of three years, a groundwater mound would form from 1 to 3.6 feet (which equates to 1.56 psi at the potentiometric surface) above the ambient water table elevation, which is one-tenth to one-sixth of the 23-foot natural groundwater elevation variation measured in the Madison Formation at the Benbow adit diamond drill hole in May and June 2013. These calculations are consistent with Nicholson and Wesson’s (1990) finding that aquifers of large spatial extent which can accept high volumes of fluid at low injection pressures rapidly dissipate pressure effects from the zone of injection and will not extend for any appreciable distance from the well.

Hydraulic Fracturing One researcher indicated that seismic events are unlikely to occur due to injection in porous rocks unless very high injection pressures cause hydraulic fracturing (Sminchak et al., 2001). One published source indicated the fracture pressure (the pressure at which the bulk rock fractures) of the Madison Formation in a Wyoming oil field is approximately 12,900 psi (U.S. Department of the Interior Bureau of Land Management, 2010). Another researcher indicated that while evaluating the Madison Formation for deep carbon sequestration in an oil field in Wyoming, the in situ reservoir pressure of 5,000 psi was increased by 1,500 psi (6,500 psi) before existing fractures were opened. No hydraulic fracturing of the rock occurred at these pressures (F. McCloughlin, Carbon Management Institute University of Wyoming, pers. comm., February 19, 2014). A third researcher described porosity, permeability and fracture zone characteristics of the Madison Formation of Wyoming that are similar to the porosity, permeability and fracture zone characteristics encountered while drilling the diamond drill hole along the Benbow adit (Thyne et al, 2010). The published pressure values are consistent with rock quality data obtained while drilling the diamond drill hole along the Benbow adit, where the average unconfined compressive strength of the Madison Formation was measured as 17,500 psi. Because the fracture pressure of the Madison Formation is two orders of magnitude higher than the anticipated 265 psi operating pressure of the proposed Benbow UIC well, hydraulic fracturing of the bedrock is highly unlikely to occur.

Favorably Oriented Faults According to researchers, failure rupture that would result in an earthquake can only occur if preexisting, favorably-oriented faults or fractures are available to meet the conditions for failure. To induce seismicity, the injection well must access hypocentral locations where potential events are most likely to occur (Nicholson and Wesson, 1990; Zhang et al., 2013). Such faults or fractures are hydrogeologically transmissive faults that are critically stressed at depth within the basement rock (Zhang et al., 2013). The term “basement rock” refers to igneous and metamorphic rocks that underlie the main sedimentary rock sequences of a region and extend downward to the base of the crust (MBMG online glossary). Faults have been mapped within three-quarters of a mile of the proposed Benbow UIC well. While faults can intersect Earth’s surface, the type of rupture that typically causes the release of energy we recognize as an earthquake commonly occurs at depths of at least 2.5 to 3 km (1.6 to 1.9 miles) below the earth’s surface where the lithostatic pressure becomes significant (M. Stickney Montana Bureau of Mines and Geology pers. comm., August 12, 2013). At shallow depths, the earth’s crust is relatively heterogeneous,

http://mbmgquake.mtech.edu/glossary.html#crust


and earthquake rupture initiations are stopped by those heterogeneities before they grow into larger earthquakes. For smaller magnitude range events (M 2-3), most of the events occur in the 1.9 to 5.6-mile depth (Sibson, 1982; Mori and Abercrombie, 1997). There are seismological, thermal, and mechanical constraints that are required before earthquakes can form at depth (Fagereng and Toy, 2011; Zhang et al., 2013). In contrast, the proposed Benbow UIC well would have a maximum depth of 1,700 feet (0.3 miles) which is one-fifth of the shallowest depth required for earthquake rupture.

CONCLUSIONS This initial assessment indicates that it is highly unlikely that the operation of the proposed Benbow UIC would induce seismicity. This assessment is based on these criteria:

• The proposed Benbow UIC well would inject into a highly permeable and porous unconfined aquifer, so that pressure effects from the zone of injection are not likely to extend for any appreciable distance from the well, especially not to the nearest mapped fault, three-quarters of a mile distant;

• The mounding caused by injecting 300 gpm water is one-tenth to one-sixth of the natural variation of the water table in the vicinity of the proposed Benbow UIC well;

• The pressures at which the well would operate are two orders of magnitude less than published values of the fracturing pressure of the Madison Formation;

• While Montana is seismically active, the earthquakes that have been detected within 62 miles (100 km) of the proposed Benbow UIC well within the last 50 years have hypocenters more than ten times deeper than the maximum anticipated depth of the zone of injection; and

• The maximum proposed well depth of 0.3 miles is shallower than the seismological, thermal and mechanical constraints that are required to form earthquakes, and one-fifth of the shallowest depth required for earthquake rupture.


References

Department of the Interior Bureau of Land Management, 2010. Rands Butte Gas Development Project Draft Environmental Assessment BLM- EA #WY-100-EA09-43.

Ellsworth, W.L., 2013. Injection-induced earthquakes. Science 12 July 2013: 341 (6142), 1225942

[DOI:10.1126/science.1225942]. Fagereng and Toy, 2011. Geology of the earthquake source: an introduction. From: Geology of the

Earthquake Source: A Volume in Honour of Rick Sibson. Geological Society, London, Special Publication 359, p. 1-16.

Greene, E.A., 1993. Hydraulic properties of the Madison aquifer system in the western Rapid City area,

South Dakota. USGS Water-Resources Investigations Report 93-4008. 56 p. Lawrence Berkeley National Laboratory U.S. Department of Energy, 2013. Induced seismicity primer.

http://esd.lbl.gov/research/projects/induced_seismicity/primer.html. McCloughlin, F. Carbon Management Institute University of Wyoming, pers. comm., February 19, 2014. Monroe, J.S. and R. Wicander, 2005. Physical Geology: Exploring the Earth, 5e. Brooks/Cole-

Thompson Learning. Belmont. 644 p. Montana Bureau of Mines and Geology Online Glossary of Earthquake Terms.

http://mbmgquake.mtech.edu/glossary.html Mori and Abercrombie, 1997. Depth dependence of earthquake frequency-magnitude distributions in

California: Implications for rupture initiation. Journal of Geophysical Research, Vol. 102, No. B7, p. 15,081-15,090.

Nicholson, C., and R. Wesson, 1990. Earthquake hazard associated with deep well injection—a report to

the U.S. Environmental Protection Agency. U.S. Geological Survey Bulletin 1951. Pierce, K, K. Cannon, G. Meyer, M. Trebesch, and R. Watts, 2007. Postglacial inflation-deflation cycles,

tilting, and faulting in the Yellowstone Caldera based on Yellowstone Lake shorelines. US Geological Survey Professional Paper 1717. 40 p.

Sibson, R.H., 1982. Fault zone models, heat flow, and the depth distribution of earthquakes in the

continental crust of the United States. Bulletin of the Seismological Society of America Vol. 72, No. 1, p. 151-163.

Sminchak, J., N. Gupta, C. Bryer, P. Bergman, 2001. Issues related to seismic activity induced by the

injection of CO2 in deep saline aquifers. First National Conference on Carbon Sequestration. U.S. Department of Energy National Energy Technology Laboratory Proceedings. 15 p.

http://esd.lbl.gov/research/projects/induced_seismicity/primer.html

http://mbmgquake.mtech.edu/glossary.html


Stickney, M. Montana Bureau of Mines and Geology, August 12, 2013 personal communication. Thyne, G., M. Tomasso, S. Bywater-Reyes, D. Budd, B. Reyes, 2010. Characterization of porosity and

permeability for CO2 sequestration models in the Mississippian Madison Group, Moxa Arch-LaBarge Platform, southwestern Wyoming. Rocky Mountain Geology, v. 45, no. 2, p. 133-150.

U.S. Geological Survey National Earthquake Information Center. Archived records search.

http://earthquake.usgs.gov/earthquakes/search/. Accessed February 25, 2014. Zhang, Y., M. Person, J. Rupp, K. Ellett, M.A. Celia, C.W. Gable, B. Bowen, J. Evans, K Bandilla, P.

Mozley, T, Dewers, T. Elliot, 2013. Hydrogeologic controls on induced seismicity in crystalline basement rocks due to fluid injection into basal reservoirs. Groundwater 51: 525-538.

Zitha, P., R. Felder, D. Zornes, K. Brown, and K. Mohanty, 2011. White Paper: Increasing Hydrocarbon

Recovery Factors. Society of Petroleum Engineers. 9 p.

http://earthquake.usgs.gov/earthquakes/search/


FIGURE 1. DEPTH AND INJECTION PRESSURE FOR THE PROPOSED BENBOW UIC WELL VERSUS DEPTHS AND INJECTION PRESSURES FOR OPERATIONS WITH

ACKNOWLEDGED CASES OF SEISMICITY

After Nicholson and Wesson (1990)

-

2,000

4,000

6,000

8,000

10,000

12,000

14,000

- 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500 6,000 6,500 7,000

Wel

l Dep

th, f

eet

Injection Pressure, psi

Proposed Benbow UIC Solution Mining Waste Disposal Research Geothermal Secondary Recovery-Gas Withdrawal

FIGURE 2. LOCATIONS OF EARTHQUAKES DETECTED BETWEEN 1960 AND 2014 WITHIN 100 KM OF THE PROPOSED BENBOW UIC WELL(1)

(1) Research indicates that the daily earthquakes which occur in the vicinity of Yellowstone National Park result from pulses of molten rock and/or hydrothermal solutions (hot waters) at depth that periodically inflate and deflate the Yellowstone caldera, much like “heavy breathing” (Pierce et al 2007). The depth (color) and magnitude (circle diameter) of each earthquake is indicated according to the USGS inset. Information for earthquakes outside Yellowstone Park is provided in the following format: date of earthquake (YYYY-MM-DD), magnitude M (moment magnitude scale), and depth of hypocenter in miles.

Stillwater Mining Company Benbow Exploration Portal & Support Facilities

Dean, Montana

Appendix M Project Timing and Sequencing

PLAN OF OPERATIONS FOR MINERAL EXPLORATION Benbow Exploration Portal and Support Facilities

ID Task Name Duration Start Finish

1

2 BENBOW EXPLORATION DECLINE PROJECT 1888 days Thu 8/1/13 Mon 10/1/183

4 ENGINEERING 640 days Thu 8/1/13 Sat 5/2/155 General Permitting 639 days Thu 8/1/13 Fri 5/1/15

10

11 Earthworks / Civil 221 days Thu 4/24/14 Sun 11/30/1419

20 Water Mgmt. & Treatment 176 days Wed 6/4/14 Wed 11/26/1423

24 Electrical 90 days Wed 6/4/14 Mon 9/1/1427

28 UG Decline 309 days Sat 6/28/14 Sat 5/2/1535

37

38 PROCUREMENT 255 days Tue 9/16/14 Thu 5/28/1541

45

49

52

53 CONSTRUCTION 1014 days Sat 5/2/15 Thu 2/8/1854 Earthworks / Civil 178 days Sat 5/2/15 Mon 10/26/1555 Mobilize Surface Contractor 10 days Sat 5/2/15 Mon 5/11/15

56 Pioneer Access Road - (2) Hoe's 4 days Tue 5/12/15 Fri 5/15/15

57 Clear Pad Area & Access Right of Way 12 days Sat 5/16/15 Wed 5/27/15

58 Grub Stumps & Pile Slash Pad Area 11 days Sat 5/23/15 Tue 6/2/15

59 Grub Stumps Pile Slash Access Road 6 days Wed 6/3/15 Mon 6/8/15

60 Strip Topsoil Pad Area 15 days Wed 6/3/15 Wed 6/17/15

61 Strip Topsoil Access Road 9 days Tue 6/9/15 Wed 6/17/15

62 Acess Road Cut & Fill 35 days Thu 6/18/15 Wed 7/22/15

63 Cut & Fill Portal Pad 40 days Thu 6/18/15 Mon 7/27/15

64 Replace Topsoil on Fill Slopes 12 days Tue 7/28/15 Sat 8/8/15

65 Install Drainage Pipes, Ditches & Berms 15 days Tue 7/28/15 Tue 8/11/15

66

67 Pioneer Waste Rock Access Road 3 days Sat 5/16/15 Mon 5/18/15

68 Clearing at Waste Rock Area 4 days Thu 5/28/15 Sun 5/31/15

69 Grub Stumps / Pile Slash Waste Rock Area 10 days Tue 6/9/15 Thu 6/18/15

70 Place Roadway Fill and Toe Berm 10 days Fri 6/19/15 Sun 6/28/15

71 Prep / Install Membrane, Sand, Drainage 50 days Mon 6/29/15 Mon 8/17/15

72 Buried Power Line Installation 22 days Mon 7/6/15 Mon 7/27/15

73 Install Pipelines (3 crews) 60 days Tue 7/28/15 Fri 9/25/15

74 Perc Pond Access Road & Excavation 30 days Tue 7/28/15 Wed 8/26/15

75 Perc Pond Water Control Piping 5 days Thu 8/27/15 Mon 8/31/15

76

77 Complete Remaining Civil Scope 30 days Sat 9/26/15 Sun 10/25/15

78 Civil Scope Complete 1 day Mon 10/26/15 Mon 10/26/15 10/26

Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 4 Qtr 1 Qtr 2 Qtr 3 Qtr 42014 2015 2016 2017 2018

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Project: Benbow Exploration Decline PDate: Wed 5/28/14

appendix l injection well induced seismicity...

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