HomeMy WebLinkAboutCity of Akutan Geothermal Technical Feasibility Report - June 2011 - REF Grant 2195475Akutan Geothermal Resource Assessment
Commissioned by
City of Akutan, Alaska
As part of its
Geothermal Development Project
June 2011
Principal Investigator:
Amanda Kolker, AK Geothermal
Other Investigators:
Bill Cumming, Cumming Geoscience
Pete Stelling, Western Washington University
David Rohrs, Rohrs Consulting
Akutan Geothermal Resource Assessment
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Contents
Summary ........................................................................................................................................................................ 3
Objectives of Study ........................................................................................................................................................ 3
Introduction ................................................................................................................................................................... 4
Background and Previous Studies ................................................................................................................................. 4
Geologic Setting ........................................................................................................................................................ 5
Geothermics .............................................................................................................................................................. 7
MT Resistivity ............................................................................................................................................................ 7
New Data 2011 .............................................................................................................................................................. 8
1. Temperature Gradient Data ............................................................................................................................. 8
1a. Core Hole Drilling ............................................................................................................................................ 8
1b. End-of-Well Logs ............................................................................................................................................. 9
1c. Equilibrated TG Logs ...................................................................................................................................... 10
1d. P/T Data Analysis .......................................................................................................................................... 13
2. New fluid chemistry and geothermometry .................................................................................................... 15
2a. Sample Collection and Data Sources ............................................................................................................. 15
2b. Chemistry ...................................................................................................................................................... 15
2c. Geothermometry .......................................................................................................................................... 15
2d. Geochemical Model ...................................................................................................................................... 16
3. Core data ........................................................................................................................................................ 16
3a. Overview ....................................................................................................................................................... 16
3b. Rock Types and Primary Mineralogy ............................................................................................................. 17
3c. Secondary Mineralogy, Mineral Paragenesis, and Hydrothermal History .................................................... 17
3d. Permeability and Porosity of Well Rocks ...................................................................................................... 20
Resource Conceptual Models ...................................................................................................................................... 21
Future Drilling Targets ................................................................................................................................................. 25
Capacity Assessment ................................................................................................................................................... 27
Resource Existence and Size ................................................................................................................................... 27
Confidence in Resource Existence ........................................................................................................................... 27
Probable Resource Capacity .................................................................................................................................... 28
An alternative approach .......................................................................................................................................... 28
Monte Carlo Heat-in-Place Option .......................................................................................................................... 28
Resource Risks ............................................................................................................................................................. 29
Upflow Development Risks ..................................................................................................................................... 29
Outflow Development Risks .................................................................................................................................... 30
Conclusions .................................................................................................................................................................. 30
Recommendations ....................................................................................................................................................... 31
References and Bibliography ....................................................................................................................................... 32
Akutan Geothermal Resource Assessment
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Summary
The Akutan geothermal resource can be conceptualized as containing two major zones: an upflow zone
and an outflow zone. While the outflow and upflow zones likely represent one interconnected field,
they are distinguished here for the purposes of development. The upflow zone temperatures could
approach 572 °F (300 °C), and the reservoir probably consists of a brine liquid overlain by a small steam
cap. The outflow zone temperatures are lower, decreasing as the brine flows eastward. Fluids produced
by corehole TG-2 show evidence of chemical re-equilibration to lower temperatures, with cation
geothermometry providing a range from 392-464 °F (200-240 °C). The outflow fluids become
extensively mixed with cooler meteoric waters near the surface hot springs.
Alteration mineralogy in exploratory coreholes suggests two disappointing conclusions about the
outflow system: (1) the rocks in both TG-2 and TG-4 were at temperatures greater than 469 °F (250 °C)
in the geological past and have cooled to present temperatures; and (2) the part of the outflow
encountered by the wells appears to lack sufficient thickness and permeability to support commercial
development. Additionally, development of the shallow outflow would entail significant risk of rapid
cooling during exploitation as a result of either cold water influx from near-surface aquifers or injection
breakthrough. Exploratory corehole drilling encountered the outflow zone with fluid temperatures of
359 °F (182 °C) at shallow depths of 585’ (187 m). Recent data suggests that the 359 °F (182 °C) zone
produced in TG-2 is drawn from a nearby fault zone not located directly below the well. Although it is
possible that a hotter resource may exist slightly deeper than either of the current wells, this is unlikely
to be the lowest risk target for development.
Although TG-2 encountered the outflow predicted near its location, the two exploration coreholes did
not demonstrate an outflow resource that would be suitable for development. Given these drilling
outcomes and results of new gas geothermometry from the fumaroles, a well targeted to cross the 1500
ft2 (0.5 km2) fumarole field would have the highest probability of encountering commercial production
at Akutan. This target is likely to be >428 °F(>220 °C) and could be as hot as 572 °F (300 C). The depth to
the target will depend on the elevation of the drill pad but it is likely to be greater than 4000’ (1300m).
An important issue is the trade-off between the cost and practicality of constructing a pad closer to the
fumarole and drilling further directionally. A 380-428 °F (180-200 °C) outflow resource target about
2200’ (800 m) to the northwest of TG-2 might be preferred if its higher targeting risk and lower
generation per well were sufficiently offset by lower drilling and access cost.
Objectives of Study
This study has three primary objectives: (1) to report on data collection efforts for the Akutan
geothermal resource to date; (2) to provide the technical parameters needed when assessing the
feasibility of developing the geothermal resource for a combined heat-and-power project envisioned by
the City of Akutan and other stakeholders; and (3) to provide well targets for future drilling efforts. This
report synthesizes all the datasets collected on the Akutan geothermal field to date (listed on p. 4), and
provides an updated assessment of the Akutan geothermal resource based on all available data. Well
targets and recommendations for mitigating resource risks are given.
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Introduction
Akutan Island is located 790 mi (1271 km) southwest of Anchorage and 30 mi (48 km) east of Dutch
Harbor. The Island is home to North America’s largest seafood processing plant. The City of Akutan and
the fishing industry have a combined peak demand of ~7-8 MWe which is currently supplied by diesel
fuel. In 2008, the base cost of power in the City of Akutan was $0.323/kWh (Kolker and Mann, 2009).
Since 2008, the City of Akutan has led exploration and other assessment activities in an effort to
determine the feasibility of geothermal development on the island. The 2009 exploration program
included practical access assessments, a geologic reconnaissance field study, soil and soil gas
geochemical surveys, a remote sensing study using satellite data, a review of existing hot springs
geochemistry data, a magnetotelluric (MT) survey, and a conceptual model analysis. The 2010
exploratory drilling program included the drilling of slim-hole temperature gradient (TG) wells, fumarole
sampling, and chemical analysis of well and fumarole fluids. Follow-up production-size wells are being
planned for the near future.
Background and Previous Studies
As a volcanic island with accessible hot springs, Akutan has been the subject of geothermal resource
studies since 1979. The original exploration effort was limited to the immediate hot springs area and
included geologic mapping, shallow (<500’ / 150m) geophysical surveys, and fluid geochemical studies.
In summer 2009, the COA executed a follow-on exploration program including a geologic
reconnaissance field study, soil and soil gas geochemical surveys, a remote sensing study using satellite
data, a review of existing hot springs geochemistry data, a magnetotelluric (MT) survey, and a
conceptual model analysis. In summer 2010, an exploration drilling program was carried out with two
temperature gradient (TG) wells drilled in Hot Springs Bay Valley (HSBV). The following reports have
been written on the Akutan geothermal resource, in chronologic order:
1. Motyka, R., and C. Nye, eds., 1988. A geological, geochemical, and geophysical survey of the
geothermal resources at Hot Springs Bay Valley, Akutan Island, Alaska. Alaska Division of
Geological and Geophysical Surveys (ADGGS), Report of Investigations 88-3.
2. Motyka, R.J., S. Liss, C. Nye, and M. Moorman, 1993. “Geothermal Resources of the Aleutian
Arc.” Alaska Division of Geological and Geophysical Surveys (ADGGS) Professional Paper 114.
3. Kolker and Mann, 2009. “Heating up the Economy with Geothermal Energy: A Multi-Component
Sustainable Development Project at Akutan, AK.” Transactions, Geothermal Resources Council
Annual Meeting 2009. *Both paper and poster format available.
4. Kolker, Cumming, Stelling, Prakash, and Kleinholtz, 2009. “Akutan Geothermal Project: Report
on 2009 Exploration Activities.” Unpublished report to City of Akutan and the Alaska Energy
Authority, 37p.
5. WesternGeco, 2009. Magnetotelluric Survey at HSBV, Akutan, Alaska: Final Report – 3D
Resistivity Inversion Modeling. Unpublished report prepared for the City of Akutan, Alaska,
GEOSYSTEM/WesternGeco EM, Milan, Italy, 27p.
Akutan Geothermal Resource Assessment
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6. Kolker, Stelling, and Cumming, 2010. “Akutan Geothermal Project: Preliminary Technical
Feasibility Report.” Unpublished report to City of Akutan and the Alaska Energy Authority, 31p.
7. Kolker, Bailey, and Howard, 2010. “ Preliminary Summary of Findings: Akutan Exploratory
Drilling Program.” Unpublished report to City of Akutan and the Alaska Energy Authority, 32p.
8. Kolker, Cumming, and Stelling, 2010. Geothermal Exploration at Akutan, AK: Favorable
Indications for a High-Enthalpy Hydrothermal Resource Near a Remote Market.” Transactions,
Geothermal Resources Council Annual Meeting 2010. *Both paper and poster format available.
9. Rohrs, 2011. “Geochemistry of the Akutan Geothermal Prospect, Alaska.” Unpublished report to
City of Akutan, 36p.
10. Stelling and Kent, 2011. “Akutan Geothermal Exploration Project: Geological Analysis of Drill
Core from Geothermal Gradient Wells TG-2 and TG-4.” Unpublished report to City of Akutan,
24p.
11. Kolker, Bailey, and Howard, 2011. “The 2010 Akutan Exploratory Drilling Program- Preliminary
Findings.” Draft paper submitted to the Geothermal Resources Council for
publication/presentation at the GRC Annual Meeting, October 2011.
12. Kolker et al, 2011. “Akutan Geothermal Project: Summary of Findings from the 2010 Drilling
Program.” Unpublished report to the City of Akutan, 33p.
Geologic Setting
Akutan volcano is part of the Aleutian Volcanic Arc, which is Alaska’s most promising setting for
geothermal energy. Akutan volcano is one of the most active volcanoes in the Aleutians, with 32 historic
eruptions (Simkin and Siebert, 1994). Akutan volcano is a composite stratovolcano with a summit
caldera ~1 ¼ mi (2 km) across and 200-1200’ deep (60-365 m; Newhall and Dzurisin, 1988; Miller et al.,
1998). Most of the reported eruptions included small-to-moderate explosions from the active
intracaldera cone. An initial volcanic hazard review indicated that the proposed geothermal
development area was unlikely to be directly impacted by eruption activity consistent with the previous
1500 years, excepting ash fall that might cause temporary closure.
The HSBV walls are composed of ~1.4 Ma lava flows, with the SE wall being slightly older and containing
numerous dikes. The valley was glacially carved, perhaps during the last major glaciation ending ~8,000
years ago, and potentially reworked during a minor glacial event ending ~3,000 BP (Black, 1975). The
HSBV is composed of two linear valleys (the SE-trending Fumarole Valley and the NE-trending valley that
contains the hot springs; Fig. 1) joined at right angles, suggesting structural control of glacial flow. Soil
geochemical anomalies (Arsenic (As), mercury (Hg), and carbon dioxide (CO2) at the junction of the
Fumarole Valley and the HSBV also suggest that the valley junction is structurally controlled and could
be an important fluid conduit (Kolker et al, 2010). Hg, As and CO2 occur in anomalously high
concentrations near the hot springs as well.
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In March 1996, a swarm of volcano-tectonic earthquakes (>3000 felt by local residents, Mmax = 5.1)
beneath Akutan Island produced extensive ground cracks but no eruption of Akutan volcano. InSAR
images that span the time of the swarm reveal complex island-wide deformation, suggesting inflation of
the western part of the island and relative subsidence of the eastern part. The axis of the deformation
approximately aligns with new ground cracks on the western part of the island and with Holocene
normal faults that were reactivated during the swarm on the eastern part of the island. The deformation
is thought to result from the emplacement of a shallow, east-west-trending, north-dipping dike plus
inflation of a deep magma body beneath the volcano (Lu et al., 2000). Studies of 3He/4He ratios in
Akutan fumarole gasses indicate degassing of relatively fresh near-surface magma (>6 RC/RA; Symonds
et al., 2003). This implies that unlike many other composite stratovolcanoes, Akutan’s magmatic
plumbing system includes two lateral rift zones, similar to the classic rift zones at Hawaiian volcanoes
and elsewhere. These rift zones are aligned along the regional least-compressive-stress axis (John
Power, pers. comm.), and serve as active magmatic conduits at shallow crustal depths (Fig. 1). NW-
trending rifting appears to be providing the large-scale permeability as well as the magmatic heat source
- crucial for the development of an extensive hydrothermal reservoir beneath the volcano.
Figure 1. Topographic map of Akutan Island, showing the geothermal project area and pertinent geologic features.
Hot Springs Bay Valley (HSBV) is an L-shaped topographic low that lies at the center of the geothermal project area.
Akutan village
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Geothermics
Several thermal springs are located in HSBV, about 6 km from Akutan village (Fig. 1). Five groups of hot
springs with about ten vents have been identified, including tidewater springs on Hot Springs Bay beach
that are only exposed at low tide. Temperatures range from 129 to 205 °F (54 to 96 °C); and some have
been reported as boiling. A fumarole complex (often called the “fumarole field”) exists at the head of
HSBV to the west of the hot springs and covers an area of approximately 1600 ft2 (500m2).
Motyka and Nye (1988) concluded that the fumaroles are likely fed directly by gases and steam boiling
off the deep hot reservoir and that these fluids then mix with cool groundwaters to produce the hot
spring waters further down the valley, forming a geothermal system that is at least 2.4 mi (4 km) long.
Recent studies of the chemical composition of the fluids confirm that they become extensively mixed
with cooler meteoric waters near the surface. Fumarole gas geothermometry indicates that the
reservoir fluids attained a temperature of at least 518 °F (270 °C). Cation chemistry from the hot springs
and produced fluids indicates that the fluids are re-equilibrating to lower temperatures along the
outflow path, with cation geothermometry from the fluids produced by corehole TG-2 providing
temperatures of 211-232 °C. The silica geothermometry of 320 °F (~160 °C) indicates that the resource
close to the hot springs (probably <1500’ / <500m distance and depth) is likely to be 320-358 °F (160 to
180 °C) (Rohrs, 2011). This is consistent with active silica sinter deposition at the hottest springs. Based
on the springs, well TG-2 was expected to encounter a permeable zone with 320-358 °F (160-180 °C)
fluid, which it did.
The structure(s) controlling upflow of hydrothermal fluids is probably one or more NW-trending normal
fault(s). One such mapped fault cuts near-perpendicularly across HSBV (Fig. 2). All of the hot springs are
topographically lower than the fault’s surface trace, consistent with geochemical indications that they
outflow from an upflow near the fumarole. The fumarole field lies along a parallel linear feature, but no
fault has been mapped there. A perpendicular NE-trending fault may control the linear shape of the hot
spring locations, but that fault has not been conclusively identified with available data.
MT Resistivity
The resistivity pattern of the Akutan geothermal prospect has an overall geometry similar to that of
most geothermal reservoirs where a low resistivity, low permeability smectite clay caps a higher
resistivity, higher temperature, permeable geothermal reservoir. However, the resistivity values of >20
ohm-m within the low resistivity zone at Akutan are much higher than in the smectite zone of most
developed geothermal fields. Several models can explain such a pattern, including an unusually high
fraction of dense lavas causing weak alteration, or relict alteration that formed at higher temperatures.
A localized pattern of alteration near the hot springs is more conventional, with a <600’ (<200 m) thick,
5-15 ohm-m zone that represents a smectite clay cap overlying a higher resistivity geothermal outflow
(Figs. 10-13). Therefore, the overall resistivity geometry is consistent with the geochemistry. A tongue of
high resistivity at -300 m elevation in Figs. 10-13 trends from the fumarole to the hot springs. This is
consistent with a relatively resistive flow path that originates from a >428 °F (>220 °C) upflow near the
fumarole to a 358–428 °F (180–220 °C) outflow extending to the hot springs.
Unfortunately, steep topography and high winds prevented the MT from accessing much of the
prospective area. There are no MT stations over the fumarole area and so a well that targets an
interpreted upflow in its vicinity might target only the surface extent of the altered ground and
Akutan Geothermal Resource Assessment
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fumaroles. There are also no MT stations over a large part of the likely outflow path, making it difficult
to assess the risk of targeting a well on the accessible part of the likely north flank of the outflow.
New Data 2011
1. Temperature Gradient Data
1a. Core Hole Drilling
In 2010, two small-diameter temperature gradient (“TG”) core holes were drilled at locations given in
Fig. 2. Since the Akutan Geothermal area is roadless, the drilling operations were supported by
helicopter. Due to budget constraints, only two of the four planned holes were actually drilled; these are
marked with black arrows in Fig. 2. The 2010 exploratory drilling plan was designed to test whether the
shallow resource was potentially commercial. Within narrow budget constraints, the wells were
designed for long-term monitoring as well as a test of the shallow, accessible targets at the Akutan
geothermal field. The hole(s) were completed as temperature gradient wells and available for future
monitoring. A detailed report on the drilling operations, P/T survey results, and other data is provided in
Kolker et al. (2011).
Well “TG-2” was drilled to a TVD (total vertical depth) of 833’ (254 m). It was sited to test the outflow
aquifer(s). Between 585 and 587’(178 and 179 m), a highly permeable zone was encountered that
flowed geothermal fluid at 182 °C (359 °F). This productive zone was cased and cemented, sealing it off,
at which point it cooled to about 329 °F (165 °C). The structure hosting the flowing fluid appeared to be
a fractured, highly vesicular, flow margin. Due to the temperature and permeability of the formation at
relatively shallow depths, drilling this well was challenging. Although targeted to 1500’ (457 m), the well
was terminated due to drilling problems.
Well “TG-4” was drilled to the planned TVD of 1500’. It was sited at the southern part of the junction
between the two perpendicular valleys, to test the size and extent of the outflow zone. Since well TG-4
did not encounter substantial fluid flow, its location appears to be outside the margins of the outflow
zone, vertically or horizontally (or both). However, well TG-4 did encounter an anomalously high shallow
temperature gradient, implying close proximity to a geothermal source.
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Figure 2. Map of the Akutan Geothermal area, showing the four candidate exploration well locations that were
considered for the 2010 program. The two holes drilled in 2010 are marked with black arrows.
1b. End-of-Well Logs
After TD was reached, three P/T logs were recorded at 12, 24, and 36 hours after circulation ended for
each well. For every run, stops were made at 20 foot stations. Because these surveys were taken so
soon after the well was drilled, the temperature readings were still influenced by the cooling effects of
fluid circulation. Therefore these represent “unequilibrated” downhole temperatures.In order to
predict the equilibrated downhole reservoir temperature, we used the Horner method to extrapolate
the measured values to a longer period. The end-of-well elevation vs. temperature plot generated from
extrapolated Horner values is shown in Fig. 3. The MRT reading from the flowing zone is also shown as a
purple dot.
Akutan Geothermal Resource Assessment
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Figure 3. Estimated equilibrated Temp v. depth plot for TG-2, based on Horner extrapolations of downhole survey
data (see Fig. 11 and text for details).
Both wells show very high shallow temperature gradients, which is consistent with their proximity to the
shallow outflow zone. Following production, TG-2 shows a drop in temperature occurring just above the
casing shoe at 603’(181 m), corresponding to the hot fracture zone between 585 and 587' (178 and 179
m) that was cemented in. The apparent cooling is likely the result of drilling fluid and cement injected
across that entire area. TG-4 shows a relatively rapidly increasing temperature gradient until ~900’(274
m),transitioning to a slowly increasing temperature gradient from 900’-1500’. The fact that there was
no temperature reversal and that the gradient continues to increase suggest there could be a deeper,
hotter aquifer below 1500’(457 m) that was not penetrated by drilling. An injection test performed on
well TG-4 suggested that the well has generally poor permeability.
1c. Equilibrated TG Logs
While the end-of-well surveys were conducted with a memory tool, it was not possible to use a memory
type tool for the post-completion logging due to the small inner diameter of the Akutan TG wells (inner
diameter > 1.5 inches / 3.81 cm). Due to this and other unique conditions of Akutan TG wells (high
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temperatures at shallow depths, remoteness of the wellsites, among others), thermistor-type
temperature logging equipment was used, and downhole pressures were not recorded.
The survey for TG-2 was completed on May 22, 2011. Results from the equilibrated survey are shown
with the three build-up surveys in Fig. 4.
Figure 4. Equilibrated temperature profile for TG-2, plotted with the three end-of-well heat-up surveys. The end-of-
well surveys were taken 12 hours. 24 hours, and 36 hours after circulation; the equilibrated profile was obtained 9
months later in May 2011.
The new temperature profile shows a distinctly different shape from the end-of-well temperature build-
up profiles. Among the new features to note are: (1) The well was bleeding while the log was run,
resulting in a minor steam or two-phase section in the upper 60-70’ (20-25 m). (2) Apparent cooling of
the well since shut-in is noticeable in the upper 400’ (122m). This probably reflects a trickle of water
downflowing from around 200’ (61m) MD and exiting into the formation at about 415’ (126m) MD. It
can only be a trickle of water because the water is heating up as it flows down behind the casing. (3)
The highest temperatures occur in the permeable zone near 585’ MD (415’ / 126m elevation), with a
temperature reversal of about 9 °F (5 °C) below the permeable zone to the bottom of the well.
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0 50 100 150 200 250 300 350 400Elevation (feet ASL)Akutan TG2 Temperatures, oF
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The new data also shows that the permeable zone at 585’ MD (415’/ 126 m elevation) has fully
recovered in temperature. Notably, the static temperatures measured in this permeable zone are about
338 °F (170 °C), which is lower than the 359 °F (182 °C) temperature measured in this zone when the
well was flowing. Since the MRT reading does appear to be correct based on silica geothermometry, this
implies that the well was drawing in higher temperature fluids when it was producing.
A temperature survey was run in TG-4 by the City of Akutan crew on May 10, 2011 (Fig. 5).
Figure 5. Equilibrated temperature profile for TG-4, plotted with the three end-of-well heat-up surveys.
The temperature profile from the equilibrated survey differs very slightly from the end-of-well
temperature profile. The new profile shows that the top 800’ (244 m) of well TG-4 heated up slightly,
but the bottom temperatures remained extremely close to those measured during the end-of-well
surveys. This is not surprising in light of the fact that that well was relatively impermeable and exhibits a
temperature profile that shows heating primarily from conduction for the upper 800’ (244 m). By
contrast, the bottom of the hole is approaching an isothermal gradient. This suggests that the
conductive heating is from the side (i.e., from a shallow outflow zone at some lateral distance), not from
a hot aquifer below.
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1d. P/T Data Analysis
Although TG-2 flowed geothermal fluid at 359 °F (182 °C) during drilling, the equilibrated temperature
logs show a maximum temperature of 338 °F (165 °C) with a reversal at the bottom of the hole. This
implies that the 359 F fluid was not circulating in the immediate vicinity of TG-2 but rather was “pulled
in” from elsewhere due to the pressure drop caused by flowing the well. A likely scenario is that the
productive subhorizontal fracture at 585’ (178m) in TG-2 is connected to a subvertical fracture dipping
west (see Fig 11). When the subhorizontal fracture was produced, the subvertical one became a
temporary conduit for fluids in the outflow zone. It is unlikely that the source of the 359 °F (182 °C) fluid
is directly below Well TG-2 because of the temperature reversal recorded in the most recent log.
A comparison of the static temperature profiles in TG-2 and TG-4 shows the difference between the
shape of a convectively heated outflow profile in TG-2, and a conductively heated temperature profile in
TG-4 (Figure 6). Also, the temperatures in the upper 800’ (254 m) of TG-4 are generally lower than in TG-
2, indicating that TG-4 is further from the shallow outflow path. No strong conclusions can be drawn
from the temperature profiles as to whether additional high temperature permeable zones underlie
either well, but it appears unlikely based on the shape of the bottom of both well profiles.
Figure 6. Equilibrated well profiles for both Akutan TG wells, shown with a boiling point with depth curve for water
with 0.1% non-condensable gas content.
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Akutan Geothermal Resource Assessment
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The 359 °F (182 °C) temperature measurement during drilling of TG-2 does appear to be correct based
on silica geothermometry. If the well was drawing in higher temperature fluids when it was producing,
this suggests that TG-2 was drilled on the margins of a more permeable and hotter outflow path. The
higher temperature fluids drawn into TG-2 during the flow test suggest that the production zone is in
proximity to the higher temperature zone but that it has a relatively low permeability connection to this
zone. The slight temperature reversal of about 9 °F (5 °C) below the permeable zone is consistent with
the geologic model that the thermal features in HSBV represent a confined lateral outflow from a
geothermal reservoir located further west, or possibly north.
The temperature gradients for Akutan wells TG2 and TG4 vary widely, but compared to the continental
average of 1.65 °F/ft (30 °C / km; Fig. 7) they are very high above 600’ (244 m). This suggests that both
are within proximity of a very shallow outflowing resource.
Figure 7. Temperature gradients, in degrees Fahrenheit per foot, for both Akutan TG wells. The average continental
geothermal gradient of 1.65 °F/ ft is shown for comparison. The outlier data point at ~410’ depth can be ignored as
it reflects a transition between the part of the well affected by a small amount of downflowing water and the part
unaffected, and thus does not represent an accurate temperature gradient.
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2. New fluid chemistry and geothermometry
2a. Sample Collection and Data Sources
Two fluid samples were obtained from well TG-2, with the first obtained from the entry zone at 585’-
587’ (178-189m) measured depth (MD) during a well discharge. This production zone was subsequently
cased off. A second flow test of the well obtained samples from production zones between 603’ (184m)
and 833’ (245m) MD, which was the completion depth of the core hole. Because TG-4 encountered
poor permeability conditions, a sample of the fluids in the wellbore was obtained by flowing with an air
assist. The MD of TG-4 was 1500’ (457m) and had a cemented casing at 596’ (182m) MD. Therefore the
data obtained during the discharge of the wells vary in quality.
New gas chemical data are from samples obtained from the fumaroles in 2010. All other analyses used
in geothermometry calculations and chemical modeling were obtained from past reports (fluid analyses
from the hot springs, non-condensable gas analyses from the summit fumarole and from the hot springs
– see p. 4 for sources).
2b. Chemistry
Geochemical data were interpreted using a combination of binary and ternary diagrams and
geothermometry gas plots. The data set used to interpret the reservoir conditions consists of all of the
data obtained at Akutan. Data and interpretations are provided in full in Rohrs (2011).
The chemical analyses of the hot springs water shows that they are derived from a dilute, near-neutral
Na-Cl reservoir brine. The Akutan hot springs show slightly elevated HCO3- and SO4 concentrations,
suggesting mixing along the outflow path with dilute, steam-heated near-surface waters. Hydrogen and
oxygen isotopic data shows that the hot spring waters are derived from local meteoric water.
The chemistry of the fumarole gasses demonstrates a strong magmatic affiliation. There is no evidence
from the gasses that the reservoir water has mixed with air-saturated fluids along an outflow path.
Compared to many geothermal systems, Akutan displays enriched N2 concentrations, which in some
cases would raise concerns with regards to acid or vapor-cored conditions in the reservoir. However, the
other gas plots show that the gases are well-equilibrated and likely to be derived from a high
temperature neutral chloride reservoir. In addition, the gas concentrations in the flank fumaroles imply
that some fraction of gas is derived from equilibrated steam, indicating the presence of a localized
steam cap in the reservoir. The chemistry of the fumaroles are consistent with an equilibrated
geothermal system associated with an andesitic stratovolcano (Giggenbach, 1991). In comparison, the
gas from the summit fumarole originates from a more oxidizing environment and exhibits high H2S
concentrations. These all suggest a magmatic affiliation for the summit fumarole steam.
2c. Geothermometry
The hot spring and well discharge samples are well suited to chemical geothermometry using the silica
and Na, K, Ca, and Mg concentrations of the fluids. The estimated temperature of last equilibration
along the outflow path suggests that the fluids have equilibrated at ~338 °F (~170 °C) and ~392 °F (200
Akutan Geothermal Resource Assessment
16
°C) for the two samples from TG-2. This temperature is similar to the estimated entry temperature of
359 °F (182 °C) at 585-587’ (178-179m) MD in well TG-2 (Kolker et al, 2010). Cation concentrations in hot
spring and well discharge analyses show that the springs and well fluids are mixed or partially
equilibrated fluids. This is commonly observed along outflow paths where the fluids are re-equilibrating
to lower temperatures and mixing with near surface waters with elevated Mg concentrations (Rohrs,
2011).
The data from hot spring HS-A3 and the entry at 585’ (178 m) MD in core hole TG-2 suggest that the
fluids originate in a deeper reservoir with temperatures in the range of 428-464 °F (220-240 °C). This
compares to a temperature of 412 °F (211 °C) estimated from the Na-K-Ca geothermometer for the well
discharge. Geothermometers that apply Na, K, Ca, and Mg concentrations tend to partially re-equilibrate
to lower temperatures in the outflow zone, and so the deep reservoir temperature is likely to exceed
464 °F (240 °C; Rohrs, 2011).
Geothermometry estimates from flank fumarole gasses exhibits very good consistency, indicating an
origin from a mature, equilibrated neutral chloride reservoir. The gas geothermometry consistently
suggests reservoir temperatures of 518-572 °F (270-300 °C; Rohrs, 2011).
2d. Geochemical Model
The new geochemical data set confirms the previous interpretations of the resource distribution in
HSBV. The hot springs represent a shallow outflow from a high temperature neutral chloride reservoir
that exists further west. The chemistry of the hot springs indicates that they have experienced
significant mixing with cooler, dilute near surface meteoric waters. Because the fumarole gases show
little evidence of mixing with air-saturated waters, the upflow zone is likely to lie near the fumaroles.
Also, gas grid plots indicate that the fumaroles contain a component of equilibrated steam, suggesting
the possibility that a localized steam cap overlies the deeper geothermal reservoir. Geothermometry of
the well discharges and the fumarole gases indicate a likely deep reservoir temperature of at least 464
°F (240 °C) based on Na/K geothermometry, with temperatures possibly as high as 572 °F (300 °C) in the
upflow based on gas geothermometry. The geochemical data do not provide any constraints on the
reservoir boundaries to the west nor on the reservoir volume within the outflow area (Rohrs, 2011).
The non-condensable gas data from the fumaroles suggest that a steam cap may overlie the deep brine
reservoir. The chloride hot springs in HSBV represent shallow outflow from the reservoir. The outflow
becomes diluted by mixing with cool meteoric waters, especially in the near surface environment
(Rohrs, 2011). Thus, the geochemical data are very consistent with the geochemical outflow models
suggested by Kolker et al. (2009).
3. Core data
3a. Overview
Composite logs from Akutan TG wells TG-2 and TG-4 of the bulk lithologies, alteration mineralogy, and
temperature data are provided in Appendix A. Full lithologic logs were recorded at the wellsite during
drilling and are provided as an Appendix in the Summary of Drilling Findings (Kolker et al., 2011). The
core was then sent to Western Washington University (Bellingham, WA) for detailed laboratory analysis.
Akutan Geothermal Resource Assessment
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The goal of the laboratory analysis was to determine the hydrothermal history of the HSBV. Core
samples were selected based on zones of interest from drilling records, core photographs, and complete
coverage of the depth of core. Determination of specific mineral species was conducted through X-ray
Diffraction (XRD) analysis, Scanning Electron Microscopy (SEM), and petrographic observations.
Quantitative permeability studies of the core were not conducted, however qualitative observations
about the permeability of the field by visual observations of the core were recorded. Finally,
compositional analysis of 19 bulk rock samples were conducted by X-ray Fluorescence (XRF) at the
Geoanalytical lab at Washington State University in Pullman, WA. Methodologies for above studies,
detailed results, and discussions are provided in Stelling and Kent (2011).
3b. Rock Types and Primary Mineralogy
There are four main lithologies present in the Akutan core: basalt, andesite, ash tuff, and “lithic basalt.”
The most common lithology in the core is basalt lava. These flows appear to be subareal in nature and
contain plagioclase, clinopyroxene, rare olivine and primary apatite. The ash tuffs are fine grained rocks
lacking phenocrysts. Groundmass phases are plagioclase microlites, glass, and alteration minerals (see
below). In TG-2, these units are <3’ (1 m) thick. In TG-4, which is ~2 miles (3.2 km) closer to Akutan
Volcano, similar units are as thick as 60’ (18 m).
The rocks provisionally named “lithic basalt” were a puzzle during the on-site evaluation, and remain
enigmatic. The lithic basalt is composed of multiple different rock types, suggestive of some sort of
debris flow deposit, yet the matrix between the clasts is crystalline, indicating a magmatic origin. At this
time, the origin of this lithology is unknown.
3c. Secondary Mineralogy, Mineral Paragenesis, and Hydrothermal History
A graphical representation of secondary mineralization and clay replacement is presented in Stelling and
Kent (2011). The rocks in general appear to be only weakly altered. As a result of the increased porosity
near lava flow tops, these regions tend to be more altered and more readily brecciated than the main
body of the lava. Heavy Fe-oxidation was observed between flow layers. Alteration minerals occurred
interstitially, in fractures, in vesicles, and in contact zones.
Alteration assemblages in both wells are dominated by chlorite, zeolites, epidote, prehnite and calcite,
and this alteration appears to have happened multiple times in both wells. The presence of adularia in
specific locations in both wells indicates higher temperature and permeability conditions existed at
some point in the past. The presence of kaolinite in TG-2 indicates argillic alteration with lesser extent
and intensity. Illite was identified in both wells, although much more sparsely in TG-2.
Within the most recent propylitic alteration event in TG-2, the sequence of zeolite formation shows a
classic trend toward higher temperatures with depth. It is likely that this trend will continue below the
base of the well (833’ / 254 m). Figure 8 shows that some higher-temperature minerals (illite, epidote,
prehnite, wairakite and adularia) occur in regions that are currently much colder than expected for
these minerals. This suggests that the TG-2 region underwent higher temperature alteration in the past.
The presence of these higher-temperature minerals at unexpectedly shallow depths further suggests
that a significant portion of this older alteration sequence has been removed through erosion, possibly
Akutan Geothermal Resource Assessment
18
glacial. Overprinting of these minerals by lower-temperature alteration assemblages indicates the
sampled region has since returned to a lower-temperature alteration regime with reduced permeability.
Figure 8. First occurrence of indicator minerals with depth in core from Akutan well TG-2. Horizontal arrows
indicate formation temperature ranges for each mineral. Dashed lines indicate published values; solid lines indicate
the most commonly reported minimum temperatures.
The pattern of alteration in TG-4 is more complex than TG-2 (Fig. 9). That the depositional history of TG-
4 includes multiple alteration events does not necessarily mean that the two wells have had significantly
different thermal histories; rather, the most recent alteration event may have been stronger in the TG-2
region, overprinting more completely the alteration sequence observed in TG-4.
Akutan Geothermal Resource Assessment
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Figure 9. First occurrence of indicator minerals with depth in core from Akutan well TG-4. Horizontal arrows
indicate formation temperature ranges for each mineral. Dashed lines indicate published values; solid lines indicate
the most commonly reported minimum temperatures.
Akutan Geothermal Resource Assessment
20
Comparing the observations made in the two drill cores provides a basic sequence of alteration for the
HSBV geothermal outflow zone. Both cores show an alteration sequence progressing from an early
propylitic event, a narrow band of adularia-bearing propylitic alteration, followed by a later propylitic
event. The trend from moderate propylitic to high-temperature adularia-forming alteration and back to
moderate propylitic indicates that the shallow portion of the HSB field has reached its thermal peak and
has cooled moderately. Additionally, many of the higher temperature minerals occur at depths much
shallower than reported in other geothermal fields. Thus is it likely that 1) this region was hotter than it
is currently, and 2) the uppermost portion of the rock column has been removed and these rocks have
risen to their modern shallow depths.
3d. Permeability and Porosity of Well Rocks
The primary lithologies do not lend themselves to high primary permeability. The abundance of isolated
vugs filled with secondary minerals indicates that fluid flow through microscopic intergranular networks
has been important, but flow rates are likely very low. Vug filling is especially common in fine-grained,
detrital deposits (e.g., ash tuff), but clay alteration and fracture mineralization by carbonates and
zeolites reduces permeability in these rocks.
The primary fluid pathways appear to be associated with brittle fracturing and lithologic contacts, based
on the abundance and degree of alteration and secondary mineralization. Very fine grained deposits
(ash tuff) lack large crystals that would add structural control over the fracture patterns. As a result,
these rocks are prone to planar fractures at prescribed orientations (30o, 45o, 60o and 90o). The majority
of these fractures have some sort of secondary mineralization associated with them, and some of the
larger fractures are deeply altered to a variety of clays and other minerals. Because the tuff is more
susceptible to clay alteration, these fractures can seal before major secondary mineralization becomes
intense. However, these units are not very thick in the wells, so may not have extensive control over the
overall fluid flow in the reservoir. Permeability may locally increase at the top of lava flows where vugs
in vesicle-rich flow tops may collapse, but this was not observed in the core. Some heterogeneous
lithologies (e.g., “lithic basalt,” see below) contain entrained clasts of older material . Fluid flow within
these lithologies are concentrated and directed around the entrained clasts, which would likely result in
moderately increased permeability compared to intergranular flow.
The occurrence of the mineral adularia helps to elucidate the permeability. Although adularia occurs in
all lithologies in the HSB cores, the restriction of adularia to fractures highlights the importance of
secondary permeability, as it does in many fields worldwide. Adularia is strongly associated with zones
that once had high permeability but each occurrence of adularia in the core is in veins that are now
thoroughly sealed by mineralization. Therefore, the waxing of a higher temperature system and
subsequent waning has apparently reduced the permeability in the HSBV outflow system.
No evidence for large scale structures were encountered in Akutan geothermal wells. A number of
brecciated zones were observed in TG-4, but most were “sealed” with secondary mineral deposits and
therefore probably do not represent active faults. Minor slickensides observed in cores could be related
to a possible normal fault on the SW side of the valley near TG-4.
Akutan Geothermal Resource Assessment
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Resource Conceptual Models
Several conceptual models of the Akutan Geothermal Resource were presented in an earlier report
(Kolker et al, 2009). The acquisition of new data in 2010-2011 are consistent with the same basic upflow
– outflow model. The most important change in the conceptual model assessment is the increase in
confidence in the fumarole as the locus of a benign reservoir upflow that would be a suitable target,
based on the promising new gas geochemistry analyses. The new data significantly reduces the
probability that an economic reservoir would be found in HSBV.
The location of the high permeability outflow path is still only loosely characterized by two models,
although the upflow seems more closely connected to TG-2 than to TG-4. The alternative outflow
pathways continue to be either along the HSBV or along a northern trajectory from the fumaroles to the
hot springs. These two alternatives are explored in Figs. 10-13. Both conceptual models are based on the
notion that the Akutan geothermal system is a single resource comprised of two distinct features: a
high-temperature (>500 °F / >240 °C) upflow zone, and a lower-temperature outflow aquifer (~360-390
°F / 180-200 °C), as suggested by chemical data.
Figure 10. Map view of “Conceptual model 1,” showing outflow along HSBV. Isotherm contour placement is based
on downhole temperature data, chemical geothermometry, hot springs and fumarole locations, and MT resistivity
data (shown here at -300 m (~984 ft) depth). Profile line “CM1” corresponds to Fig. 11.
Akutan Geothermal Resource Assessment
22
Figure 11. Profile “CM1,” as shown in Fig. 10. This model shows outflow along HSBV. Isotherm contours based on
downhole temperature data, chemical geothermometry, hot springs and fumarole locations, and MT resistivity
data (shown here as 3D inversion model).
Conceptual model ‘CM1’(Figs. 10 and 11) shows a high temperature resource upflowing beneath the
fumarole field, and cooling along an outflow path that follows the L-shaped path of HSBV. The upflow
must be some lateral and vertical distance from well TG-4, since no trace of conductive heating from a
deep source was observed in the temperature profile of TG-4. Also, for this model to fit the observed
downhole temperature profiles in both wells, the outflow along HSBV can only be very thin (vertically
constrained low-permeability) and restricted to the shallow subsurface.
Because the 359 °F (182 °C) flow during the flow test completed while drilling is higher than the
measured static temperature, because there does not appear to be a downflow that would reduce the
temperature of this zone from 359 °F (182 °C) to 338 °F (165 °C) when the well is static, and because the
temperature reversal in TG-2 below this zone makes upflow from below the well unlikely, the produced
higher temperature fluid appears to have been “pulled in” laterally from a nearby source. This could be
related to the westward-dipping fault near TG-2 shown in Figs. 11 and 13. The rapidity with which this
hotter fluid was drawn in during such a short test implies that the 338 °F (165 °C) permeable zone in TG-
2 must be restricted in volume and at a higher natural state pressure than the 359 °F (182 °C) adjacent
reservoir.
Akutan Geothermal Resource Assessment
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Figure 12. Map view of “Conceptual model 2,” showing outflow beneath the mountain to the north of HSBV.
Isotherm contour placement is based on downhole temperature data, chemical geothermometry, hot springs and
fumarole locations, and MT resistivity data (shown here at -300 m (~984 ft) depth). Profile line “CM2” corresponds
to Fig. 13.
Conceptual model 1 ‘CM1’ does not resolve the location of a hotter outflow resource of 360 -392 °F (180-
200 °C), for which there is a substantial amount of geochemical evidence. Therefore, an alternative
model is proposed called ‘CM2’ (Figs. 12 and 13). In CM2, the shallow outflow path takes a northerly
trajectory from the fumarole to the ENE towards the hot springs, circumventing HSBV altogether. This
model appears more likely based on several lines of reasoning: 1) the temperature profile for TG-4
shows no evidence for being along an outflow path, implying that outflow feeding the hot springs is
laterally distal; 2) a low-resistivity clay cap appears to form a dome pattern around the northerly outflow
path, which is consistent with the interpretation that the HSBV is near, but not in, the main outflow path
of geothermal fluids (Figs. 10 and 12); and 3) the isotherm contours on the CM2 profile (Fig. 13) are
slightly more typical of an outflowing geothermal system.
Akutan Geothermal Resource Assessment
24
Figure 13. Cross section of profile line “CM2,” as shown in Fig. 12. This model shows outflow beneath the mountain
north of HSBV. Isotherm contours based on downhole temperature data and chemical geothermometry, MT
resistivity data, hot springs and fumarole locations, and fault lines.
Both models suggest that producing the outflow resource would be very risky, both because of the
generally low permeability expected based on several lines of reasoning and also because there is no
well-developed clay cap to indicate that a large-very permeable reservoir volume at ~360-390 °F (180-
220 °C) exists under HSBV. The lack of widespread surface alteration, geochemical, and ground
temperature anomalies (Kolker et al., 2009) in HSBV are consistent with this interpretation. Additionally,
the chemical composition of the hot springs fluids suggests that outflow fluids become extensively
mixed with cooler meteoric waters near the surface, raising concerns about cold water influx into the
outflow system with production.
While the conceptual models of the outflow resource have downgraded its potential for development,
geochemical data from the fumaroles significantly upgrades the fumarolic area as a drilling target. The
fumarole data suggest that the flank fumarole field lies in fairly close proximity to an upflow zone from
the reservoir, that a steam cap may overlie the upflow, and that reservoir temperatures could approach
570 °F (300 °C) within the upflow. The deep reservoir probably consists of a brine liquid capped by a
small two-phase region (steam cap) (Rohrs, 2011). Resistivity data suggest that the upflow reservoir is
situated in brittle rocks, implying propylitic alteration regime and a good possibility of high permeability.
Akutan Geothermal Resource Assessment
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Future Drilling Targets
The two types of resource targets represent two development options for the AGP:
1) The upflow resource has a high exploration risk, but potentially low development risk and high
output capacity.
2) The outflow resource has potentially easier access but higher exploration risk, and presents
several development risks, including a lower output capacity per well.
The highest priority target for future drilling is at the upflow resource, due to the following factors:
(1) Evidence of commercial-grade permeability due to the presence of fumaroles that are
chemically connected to a neutral-chloride reservoir with a steam cap.
(2) The fumarole gas geothermometry indicates that the source fluids are likely to be being
equilibrated to a temperature of >572 °F (>270 °C). These temperatures could exist directly
beneath the fumaroles. Alternatively, if the upflow originates further west, the fumaroles may
mark the location where the outflow first encounters boiling conditions. In this case,
temperatures beneath the fumaroles could be in the range of 428-464 °C (220—240 °C).
(3) As the highest-enthalpy target, the fumarole area could be expanded to meet additional
power demand if it should ever present itself (e.g., new industrial capacity, larger scale
secondary use, etc.)
The exploration data suggests that the likely upflow location is in the general vicinity of the fumarole.
However, the fumarole is located ~1150’ (~350 m) up a very steep hillside, posing access limitations.
Hence, an important issue is the trade-off between the cost and practicality of constructing a pad closer
to the fumarole and drilling further directionally. A small rig may only be able to achieve a very modest
directional offset, but a rig capable of greater directional offset would be several times more expensive.
Two alternative surface locations have been sited to target the high priority upflow zone. Regardless of
the pad location, the well should be targeted to at least 4500 ’ (1350m) MD and preferably to 6000’
(1800 m) MD.
The first, preferred alternative “A” is located near the fumarole field (Fig. 14). This is closest to the high
temperature upflow zone. Access to this location could be via a road running east-west which skirts the
mountain to the north of HSBV. Such a road appears to be buildable at less than 3 miles (6 km) from Hot
Springs Bay access, but would require dock facilities to be built at the beach. However, this access
option raises the question of transmission to Trident and Akutan Village. It also raises questions about
volcanic hazards (see ‘Risks’ section, below).
The second, less preferable alternative is located in the Fumarole Valley >2/3 mi (~1 km) southeast of
the fumarole field (Fig. 14). A directional well drilled from this location (called “Well -1” in past reports)
beneath the fumarolic features or to the north beneath the local resistivity high may intersect the
upflow zone. From pad location “B”, the margins of the upflow resource would be targeted via
directional drilling and the outflow resource could be targeted via vertical drilling. However, limitations
Akutan Geothermal Resource Assessment
26
on directional drilling may not allow the target to be reached from pad location “B.” Hence, this wellpad
location is riskier than alternative “A.” A determination on this issue should be solicited from a qualified
geothermal drilling engineer before a final decision is made.
Figure 14. Map showing possible wellpad locations to target the Akutan upflow resource (blue triangles). Each pad
could host two wells – a directional well aimed towards the fumarole field (knobbed black line), and a vertical well.
Possible road alignments to the wellpads are shown in red. Also shown are the three sections recently selected by
the Akutan Corporation for subsurface ownership rights in a land swap agreement with the Aleut Corporation.
Should drilling at sites A and B fail, or if developing the upflow resource is not possible, subsequent pad
locations could be sited to target other parts of the outflow zone. A 380 to 428 °F (180 to 220 °C)
outflow resource target about 2200’ (800 m) to the northwest of TG-2 might be preferred if its higher
targeting risk and lower generation per well due to its lower temperature were sufficiently offset by
lower drilling and access cost.
Prior to generating outflow targets, additional exploration activities should be undertaken to target the
hottest and most permeable part of the outflow resource. This would likely require additional
subsurface imaging work using one or several geophysical techniques. It would also probably require
additional slimhole drilling and/or deepening of TG-2. Since the casing in TG-2 was not cemented in
place, it could be retrieved it and the well deepened by 1000’ (300m) or more.
Akutan
Corporation
Subsurface
Selections
1
36
12
A
B
Akutan Geothermal Resource Assessment
27
Capacity Assessment
As summarized by Glassley (2008), different approaches and methodologies to geothermal resource
capacity assessment have given rise to a broad range of results that are not directly comparable. Hence,
the outcomes of resource assessments are sensitive to the methodology and assumptions employed in
the analysis, and different studies often produce widely different estimates of a resource. The
assessment of reserves by analogy used in previous Akutan reports is updated here based on the results
of the wells and the fumarole gas geochemistry. The approach applied here is to use analogies to a few
published examples in order to highlight important similarities and differences with respect to Akutan.
Resource Existence and Size
Resource risk assessment approaches commonly divide the assessment into two parts; 1) an assessment
of confidence in the existence of a resource as a percent probability, and, 2) assuming the resource
exists, an assessment of its size, usually as a statistical distribution (e.g. Newendorp and Schuyler, 2000).
The probability of existence is sometimes restated as the probability of exploration success; i.e., the
probability that an exploration drilling program would discover at least one economically productive
well. In many published geothermal resource assessments, the assessment of existence is often not
explicitly evaluated but nominally included in the size distribution, for example, in the Western
Governors’ Association (2006), Clean and Diversified Energy Initiative Geothermal Task Force Report. In
this report, many geothermal prospects in the western USA with poorer indications of temperature over
220 °C and much lower surface heat flow than Akutan are assessed as having over 20 MW potential.
Confidence in Resource Existence
The most common method of estimating the probability of existence for a resource is to have a group of
experts review the available data and, based on analogous experience with other geothermal prospect
areas, estimate the confidence (as a probability) that the necessary components of a resource exist
together. For volcanic prospects that have hot springs with cation geothermometry similar to Akutan’s
and a non-magmatic fumarole, few failure cases exist in which the most attractive target was drilled.
With respect to the earlier assessments of Akutan, the very minor magmatic indications and excellent
gas geothermometer estimates of resource temperature have increased confidence in the existence of a
high enthalpy resource.
The numerous geothermal success cases differ in detail, particularly with respect to the geology and
very dilute chemistry characteristic of Akutan. For example, in the Americas there are several developed
geothermal fields in volcanic systems with different geologic settings but broadly similar liquid and/or
gas geochemistry. The 572 °F (270 °C) San Jacinto Field in Nicaragua has 10 MW installed and 72 MW
under development. It has roughly analogous fumarole gas geochemistry and a similar area of intense
alteration, although the resistivity of its clay cap is much lower, more like a conventional geothermal
field. The 320 to 350 °F (160 to 175 °C), ~40 MW Casa Diablo field at Long Valley (Sorey et al., 1991) and
the 320 to 360 °F (160 to 180 °C), 45 MW Steamboat Springs Field near Reno (Mariner and Janik, 1995)
have liquid chemistry similar to Akutan, but again, a lower resistivity clay cap. At Akutan, the
combination of a non-magmatic flank fumarole with excellent gas geothermometry over 518 °F (275 °C),
a trend in cation geothermometry to >428 °F (>220 °C), silica geothermometry over 320 °F (160 °C) with
Akutan Geothermal Resource Assessment
28
sinter deposition proven to exist in the subsurface by a well support the existence of a convecting
geothermal resource on Akutan with a high confidence of 80%.
Probable Resource Capacity
The capacity of the geothermal resource at Akutan in terms of electrical power can be assessed using
analogies, both the rough comparisons to the prospect estimates provided in the Western Governors’
Association report and the analogs to the 20 to 72 MW San Jacinto development and the 40 to 45 MW
Casa Diablo and Steamboat Springs developments. Because of the dilute outflow chemistry and low
permeability relict alteration at Akutan, handicapping the Akutan likely 320 to 358 °F (160 to 180 °C)
outflow resource by 75% relative to these developed reservoirs would be reasonable, giving an
analogous low temperature resource capacity estimate of 15 MW with a 66% probability. Because a
high temperature resource very likely exists, a more optimistic capacity estimate for the entire system
would be like San Jacinto, 10 to 72 MW with a 66% handicap because of its high clay cap resistivity and
difficult access, this results in a risk weighted estimate of about 20 MW. Using the Western Governors’
Association report assessments as analogs, an assessment as high as 100 MW seems reasonable.
An alternative approach
An output capacity for the 359 °F (182 °C) fluids produced by TG-2 was estimated in 2010 based on the
flowing temperature of the well and assumptions about flow rate (Kolker et al., 2011). Based on then-
available information, it was estimated that a production well drilled at or near the TG-2 site could
produce 1.34 MW up to a maximum of 2.38 MW. However, recent data and analyses including the
stabilized temperature curves, alteration mineralogy from cored rocks, have supported revisions of the
earlier assumptions used for estimating wellhead flow capacity were optimistic.
Monte Carlo Heat-in-Place Option
In previous reports on Akutan (Kolker, 2010), the heat-in-place method has been outlined but it has not
been formally applied. Initially developed by the USGS for rough regional estimates (Muffler, 1979),
more elaborate Monte Carlo versions of the method have recently been adopted by stock exchange
regulators in Australia and Canada as a standard for publishing geothermal reserves (Lawless, J., 2010).
Despite its common use by geothermal investors, as detailed by Garg and Combs (2010) and more
generally considered in the context of other methods by Grant and Bixley (2011), Monte Carlo heat-in-
place approaches are commonly misleading and difficult to validate. If such an analysis is needed to
meet a reviewer’s request, the City of Akutan could consider employing a large consultancy that
routinely provides such analyses to meet regulatory needs, like GeothermEx or SKM.
Akutan Geothermal Resource Assessment
29
Resource Risks
The major volcanic hazard posed to a geothermal development on Akutan is ash fall. The modern
volcanic complex forms the western half of the island, and future eruptions are unlikely to affect the
eastern portions of the island (Ancestral Akutan), including HSBV. Destabilization of the fumarole area,
at an elevation of ~1150’ (350 m; see Fig. 1), may generate debris flows, and such deposits are seen in
the valley floor. According to the hazards map of Akutan, it is possible that the entire HSBV could be
inundated by cohesive lahars associated with small-scale slope failure(s), but not likely. Another possible
but unlikely hazard is a pyroclastic flow near the fumarole field (Waythomas et. al., 1998).
Re-injection beneath the surface is the most environmentally responsible means of disposing of the
produced fluids. Re-injection also supports reservoir pressure. Normally the fluids would be injected
back into the reservoir because this is where adequate permeability exists. Because of the possibility
that HSBV is fault-controlled, reinjecting the fluids into the shallow aquifer incurs a high risk of pre-
mature thermal breakthrough to the producing wells. This risk can be investigated through pressure-
interference and possibly tracer testing as additional delineation wells are drilled.
Calcite scaling in the production wells and silica scaling of the production pipeline system and injection
wells are both possibilities for the Akutan system. However, gas levels are likely to be moderate and
calcium and silica concentrations are low, suggesting that scaling risks should be low. Silica is unlikely to
achieve a significant level of supersaturation should the fluids be produced to a binary power plant.
Also, the risks of silica precipitation can be mitigated through pH modification of the produced waters.
In addition, the potential for producing acid corrosive fluids at Akutan is very low. The hot springs and
discharge waters demonstrate that the reservoir hosts a near neutral chloride reservoir.
Further assessment of the risks would require the acquisition of additional well performance data, such
as interference testing, and additional geochemical samples from the production and injection zones.
Upflow Development Risks
Two key questions remain unresolved concerning the deep high temperature (“upflow”) resource. The
first is the location of the upflow to the system. The second is the volume of the high temperature
resource. Resolution of these questions would require additional drilling and possibly the acquisition of
additional resistivity data near and to the west of the fumaroles. A reasonable minimum size at a 10%
confidence level would be the area covered by the fumaroles and gassy alteration. If this is taken to be
roughly 1500 ft2 (0.5 km2) then using the base case numbers for power density of 15 MW/km 2, a
reasonable minimum for expected capacity of the upflow is 8 MW after Grant and Bixley (2011).
The risk of volcanic hazards should be carefully investigated if the wellpad were to be sited near the
fumarole field on the flank of Akutan volcano, as that location lies on a possible path of pyroclastic flows
from Akutan volcano (Waythomas et al, 1998).
Fumarole gas contents are 3.5 to 4 wt. %, which do not present an obstacle to development. A well
drilled beneath the fumarolic complex is also at a low risk of encountering acid fluids, because the gas
chemistry is indicative of a neutral chloride upflow to the geothermal system.
Akutan Geothermal Resource Assessment
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Outflow Development Risks
Other than the general risks mentioned above (injection breakthrough, scaling/corrosion, etc), the most
important development risk of the outflow resource discovered by core hole TG-2 is permeability and/or
volume limitations. Neither of the coreholes encountered clay alteration characteristic of a well-
developed smectite-rich argillic caprock. Perhaps the argillic alteration did not develop because of
dense lavas, or higher rank alteration that does not retrograde, or it never became well-developed, or
perhaps it was eroded off. Resistivity profiles of the HSBV also suggest only a very thin conventional clay
has formed over the outflow system, close to the hot springs (Kolker et al., 2009). The rocks in general
appear to be only weakly altered. This implies that HSBV does not host a substantial volume of hot fluid,
making it a risky development target.
Mineralogical studies of TG-2 core rocks suggest that secondary mineralization of permeable fractures
has “sealed” the outflow area, restricting flow. Analysis of the temperature profiles and flow behavior of
TG-2 suggests that the produced fluid of 359 °F (182 °C) was “pulled” into the system from elsewhere.
The subsurface source of the 359 °F (182 °C) fluid is unknown and therefore targeting this resource is
highly risky.
Additionally, there is a high risk that exploitation of the shallow reservoir could result in rapid enthalpy
declines during exploitation. The risk arises from any of the following: recharge of the reservoir by sea
water; cold meteoric water influx from near surface aquifers; and breakthrough from injection wells.
Groundwater influx probably poses the most significant risk. There are also a large number of
connection points between the shallow thermal aquifer and the surface along the outflow path. Any
pressure decline as a result of exploitation would likely allow these colder waters to descend into the
reservoir and cool the production wells.
Conclusions
The Akutan geothermal resource can be divided into an upflow zone and one or more outflow zones.
Studies of alteration minerals in the core suggest that the outflow resource discovered by TG-2 is likely
to have significant permeability limitations (Stelling and Kent, 2011). The outflow resource of 359 °F (182
°C) discovered by slimhole exploratory drilling in 2010 appears to have migrated from a more distal
source and may not be commercially developable. A temperature reversal at the bottom of the
stabilized TG-2 profile reduces the possibility that a hotter or more voluminous reservoir would be
encountered by drilling deeper at that location. These conclusions indicate that earlier estimates of
production capacity of the outflow resource discovered by slimholes are inaccurate, because the flow
assumptions for this estimate appear to have been overly optimistic.
The hottest modern zone in the TG-2 core is at 585-590’ (178-180 m), with a static measured
temperature of 338 °F (165 °C). The occurrence of wairakite, epidote and prehnite suggest that this
zone was permeable during the earlier higher temperature alteration event. The outflow zone
penetrated by the exploratory slimholes shows evidence of “self-sealing” through mineralization of
primary and secondary permeability channels. The chlorite- and zeolite-dominated hydrothermal
mineralogy of wells TG-2 and TG-4 indicate that a lower temperature alteration assemblage has been
overprinted on a higher temperature assemblage. The higher temperature alteration assemblage
contains illite, epidote, prehnite, and adularia. The mixed layer clays, illite-smectite and chlorite-
Akutan Geothermal Resource Assessment
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smectite, and zeolites are part of the lower temperature retrograde assemblage corresponding to
temperatures of 300 – 430 °F (150-220 °C). The overprinting observations can be explained by a
vertically-limited outflow system in a waning phase after attaining higher temperatures, which suggests
that a deeper well drilled at or near the location of TG-2 is not likely to encounter the hotter fluids
predicted by chemical geothermometry. Since those fluids appear to be flowing laterally along the water
table towards Hot Springs Bay, the source fluids are probably westward up the valley towards the
fumaroles and/or southwest to the valley junction and then northwest (Figs 10 and 12).
Future drilling should target the upflow resource as the highest-grade, lowest-risk part of the system.
The upflow source fluids are likely to be within the range of 428-572 °F (220-300 °C), and are chemically
benign. The estimated output capacity of the upflow target is 15-100 MW by analog analysis, with a
minimum output of 8 MW based on pessimistic volume considerations.
If developing the upflow resource is not possible, the hottest part of the lower-grade outflow zone could
be targeted but with greater risk.
Recommendations
The unresolved resource properties and risks can only be addressed by additional characterization of the
resource through drilling. An evaluation of the access, drilling and financial issues involved in targeting a
well on the upflow resource below the fumarole should be top prioirity at this time. Future well(s)
should be directed west beneath the fumaroles or north to a postulated upflow beneath a local
resistivity high. The well should attain a minimum depth of 4500’ (1350m) to insure that it penetrates
through the reservoir top, although a target depth of 6000’ (1800m) would be preferable in order to
better establish reservoir thickness. As evaluation of the resource potential continues, obtaining
samples of separated steam and water from the production wells will be valuable for further assessing
the geochemical risks related to scaling and cold water influx. Pressure-interference and possibly tracer
testing will need to be conducted as additional wells are drilled.
The risks associated with drilling the upflow target could be mitigated by additional exploration work.
One focus of additional exploration work could be the identification of controlling structures (likely
faults). Since hot fluids are constantly plugging up the "plumbing" channels by depositing minerals in
open fractures, large-scale activity on faults is required to keep the system permeable. These faults must
exist, but none have been conclusively identified in HSBV or near the fumaroles. Hence, mapping large-
scale active structures controlling permeability in the Akutan geothermal field could reduce well
targeting risks. Aerial photography survey planned for summer 2011 may provide useful data.
Additional studies (LIDAR, seismic, or other geophysical methods) could supplement this investigation
after the initial review of the aerial photography.
Another exploration activity that could mitigate the upflow target drilling risk is extending coverage of
the MT survey further to the north and west of HSBV. This could be done with very limited additional
stations (possibly 10-20) with or without a helicopter, limited to the area in the direct vicinity of the
fumaroles. However, this additional data may not have a significant effect on the project risk assessment
of the outflow, especially along the north rim of the outflow. Because it is likely to be relatively
expensive and prone to severe wind noise, this activity is not top priority at this time.
Akutan Geothermal Resource Assessment
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If the shallow outflow is deemed to be more suitable for geothermal development, an important risk
mitigation measure would be analog studies of more shallow outflows that have been developed for
power generation and/or space heating. Analog studies of similar geologic environments in Iceland
could be particularly useful for assessing risks associated with cold water influx and injection
breakthrough. Additionally, several low-cost additional studies could help characterize the outflow
resource. These include: (1) Fluid inclusion analysis of hydrothermal minerals in deposited in fractures in
core rocks; (2) Analysis of the fracture orientation within well cores; (3) Detailed clay analysis identifying
the percentages of illite in illite-smectite and chlorite-smectite. As with the upflow target, pressure-
interference and possibly tracer testing should be conducted as additional wells are drilled.
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