a geothermal investigation of 18 bedrock wells in vermont · 2015. 12. 28. · associated with an...

Abstract

At present, the State of Vermont has not completed a thorough survey of the geothermal resources of its shallow subsurface. The lack of investigation is partly because the bedrock of Vermont is generally understood to be old, thick, and tectonically inactive, and thus “cold”. Furthermore, a proper geothermal survey often relies on the existence of many deep, open wells from which Bottom Hole Temperatures (BHTs) can be collected and amassed to create a summary of geothermal potential (Shope et al., 2012). Deep, open wells are often the product of oil and gas exploration, which is practically absent in Vermont and makes such a large-scale survey impossible.

Over the past two years, Jon Kim of the Vermont Geological Survey and Ed Romanowicz of SUNY-Plattsburgh have collected high quality temperature data from 18 bedrock wells in Vermont. Though not as extensive, the Vermont data can be seen as more accurate for each location than BHT data because the entire well is surveyed and analyzed for temperature �uctuations. For each of the 18 wells, temperature and di�erential temperature charts were plotted and analyzed to determine the “true” geothermal gradient at each site. The di�erential temperature log is more sensitive to changes in temperature gradient than the temperature log and provides a more precise reference from which to calculate thermal gradients (Keys, 1989). For each well, transects with a di�erential temperature close to zero were selected to calculate a representative temperature gradient for each site. For 10 of the 18 wells, constant temperature gradients were interrupted by anomalous temperature �uctuations. For these 10 wells, two or three transects of constant gradient were aggregated and a weighted average was calculated.

Data continue to be collected and analyzed in order to gain a greater understanding of the complicated geologic and hydrogeologic setting of Vermont’s shallow subsurface.

Method

Di�erential temperatures were calculated in Microsoft Excel using the equation:

[ dT/dZ = (Tn-Tn-1)/(Zn-Zn-1) ]

Where T = temperature and Z = depth below surface. The precision of the measurements are upwards of 0.1 degrees C and 0.1 meters. A �rst derivative of zero represents a linear relationship between temperature and depth. A linear relationship is expected in areas where there isn’t any signi�cant input from groundwater that would skew the otherwise constant relationship between depth and temperature in the earth’s shallow subsurface. For 10 of the 18 wells, constant temperature gradients were interrupted by anomalous temperature �uctuations – assumedly associated with groundwater in�ltration or some other non-geological control. For these 10 wells, two or three transects of constant temperature gradient were aggregated and a weighted average was calculated to describe the entire well.

Each temperature gradient was calculated by creating a data series in Microsoft Excel consisting of temperature (y-axis) and depth (x-axis) for each transect that was determined to have a di�erential temperature of zero, implying a linear relationship between temperature and depth. For each transect, a trend line was �tted to determine the slope of the line. The coe�cient of the slope could then be scaled to display the temperature gradient in degrees celcius per kilmometer and degrees celcius per 100 feet. The dashed lines on the charts for each well signify where transects were calculated.

A Geothermal Investigation of 18 Bedrock Wells in Vermont

Discussion

Among the 10 wells with multiple transects, dissimliar temperature gradients within the same well are often apparent. These varying temperature gradients may represent changes in lithology as well as changes in the hydrological setting. For these wells, the weighted average may not give the best indication of the geothermal regime at depth. However, if coupled with more geophysical data, conclusions about changes in bedrock and water sources could certainly be reached. For many of these wells. More geophysical data has been collected by Jon Kim and Ed Romanowicz, but not analyzed. Measurements on conductivity (a proxy for dissolved species), gamma radiation (indicator of natural radioactivity), and well diameter (using a caliper tool) have been recorded at all of the sites.

In Hinesburg, research and analysis continue in order to help the town better undestand its water resources. On well H (Hinesburg 7797), note the jump in temperature just below 50 meters. Though relatively deep, this “jump” is associated with an in�ux of warmer water that is presumed to connected to surface water. Well H was logged in late May. If it was logged during the winter the jump may not be apparent and if it was logged in late summer, the jump may be much larger.

“Previously Calculated Gradients”

Jon Kim of VGS calculated preliminary temperature gradients for each of the 18 wells from depths of 50 -100m to the bottom of each well . The last column in the table above displays the percent di�erence between the two methods. Some of the wells showed minimal change between the two methods while others, such as Hinesburg Well 7797 and Berlin Well D, changed signi�cantly.

References

Brophy, Gerald P. “Geothermal Resource Assessment of the New England States.” Rep. no.

DE83 010022. Amherst, MA: Department of Energy, 1982. Print.

Keys, W. S. Borehole Geophysics Applied to Ground-water Investigations. Washington,

D.C.: U.S. Dept. of the Interior, U.S. Geological Survey, 1990. Print.

Railsback, L. B. "Heat Flow, Thermal Conductivity, Geothermal Gradient, and Subsurface

Temperatures." Petroleum Geoscience and Subsurface Geology. University of

Georgia, Athens, GA, Oct. 2011. Web. July 2014.

Ratcli�e, N.M., Stanley, R.S., Gale, M.H., Thompson, P.J., and Walsh, G., 2011, Bedrock

Geologic Map of Vermont: U.S. Geological Survey Scienti�c Investigations Map

3184, 3 sheets, scale 1:100,000.

Renner, J. L., and Tracy L. Vaught. “Geothermal Resource Assessment of the Eastern United

States.” Rep. no. DOE/ET/28373-T2. Arlington, VA: Department of Energy, 1979.

Print.

Shope, Elaina N. "Geothermal Resource Assessment: A Detailed Approach to Low-Grade

Resources in the States of New York and Pennsylvania." 37th Stanford Geothermal

Workshop (2012): n. pag. Print.

Geographic data from the Middlebury College Geography Department

and the Vermont Geological Survey

+$

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CB

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Logged Wells

+$ Previous Reported "Hot Spots"

^ Uninvestigated Sites of Interest

0 10 205 Miles

A. Highgate Springs

G. Place Rd East

B. Jay Peak 12

H. Hinesburg Well 7797

M. Harwood Union N N. Harwood Union O

C. Jay Peak 13

I. Hinesburg Well 8609

D. Spa�ord Water Wells O�ce

J. Hinesburg Well 8608

O. Berlin Well C P. Berlin Well D

E. Champlain College

K. Hinesburg Wainer 1

F. 180 Desarno Rd

L. Norland Well

Q. Berlin Well A R. Edgemont Condos

16.13 C/km0.492 C/100’

8.13 C/km0.248 C/100’

10.38 C/km0.316 C/100’

3.40 C/km0.103 C/100’

13.44 C/km0.410 C/100’

13.13 C/km0.400 C/100’

10.41 C/km0.317 C/100’

11.27 C/km0.344 C/100’

6.36 C/km0.194 C/100’

5.69 C/km0.173 C/100’

9.38 C/km0.286 C/100’

8.63 C/km0.263 C/100’

7.31 C/km0.223 C/100’

13.60 C/km0.415 C/100’

5.01 C/km0.153 C/100’

8.93 C/km0.272 C/100’

12.18 C/km0.371 C/100’

10.30 C/km0.314 C/100’

8 10 12

Temperature Differential Temperature

6 7 8 9


6 7 8


8 9 10 11


7 9 11


5 10 15


8 9 10 11


9 9.5 10 10.5


7 8 9


7 8 9 10


7 9 11


5 10 15


5 10 15


6 8 10


7 7.5 8 8.5


Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Meters

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

Degrees Celsius

8.5 9 9.5


8 9 10


9 9.5 10 10.5


7.25C/km

11.81C/km

16.13C/km

10.41C/km

9.91C/km

12.37C/km

7.31C/km 13.60

C/km

12.38C/km

9.12C/km

1.40C/km

5.67C/km

6.25C/km

6.72C/km

5.69C/km

5.01C/km

6.48C/km6.05

C/km

10.71C/km

13.88C/km

12.92C/km

13.13C/km

10.56C/km

3.43C/km

27.28C/km

6.60C/km

9.50C/km

7.30C/km

11.50C/km

10.30C/km

!!

!

!

!!!!!

!

!

!!

!!!

!

!

ESSEX

WINDSORRUTLAND

ADDISON

WINDHAM

ORANGE

ORLEANSFRANKLIN

CALEDONIA

WASHINGTON

BENNINGTON

CHITTENDEN

LAMOILLE

GRAND ISLE

K

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Weighted Average (°C/km)! 3.39

! 3.40 - 6.00

! 6.01 - 9.00

! 9.01 - 12.00

! 12.01 - 15.00

! 15.01 - 18.00

Hinesburg:

Well Name

Previously Calculated Gradients (°C/km)

Approximate Depth (meters) (°C/km) (C/100')

Percent Change Between Methods

A. Highgate Border 15.16 300 16.127 0.492 6.4B. Jay Peak Well 12 10.82 250 8.132 0.248 24.9C. Jay Peak Well 13 8.94 175 10.406 0.317 16.4D. Spafford Well 10.77 340 11.275 0.344 4.7E. Champlain College 7.02 320 7.308 0.223 4.1F. 180 Desarno Rd. Well 13.81 400 13.600 0.415 1.5G. Place Rd. East 10.54 180 10.377 0.316 1.5H. Hinesburg Well 7797 9.01 90 3.395 0.103 62.3I. Hinesburg Well 8609 7.03 175 6.365 0.194 9.5J. Hinesburg Well 8608 5.64 180 5.689 0.173 0.9K. Hinesburg Wainer 1 N/A 190 5.010 0.153 N/AL. Norland 9.50 95 8.934 0.272 6.0

M. Harwood Union Well N 12.30 180 13.443 0.410 9.3N. Harwood Union Well O 13.88 220 13.132 0.400 5.4O. Berlin Well C 13.69 170 9.383 0.286 31.5P. Berlin Well D 15.36 150 8.630 0.263 43.8Q. Berlin Well A 14.85 185 12.182 0.371 18.0R. Edgemont Condos 10.67 200 10.300 0.314 3.5

Weighted Average

Railsback's Petroleum Geoscience and Subsurface Geology

Geothermal gradient dT/dZ = Q c

Thermal conductivityc Q

D

epth

(z)

Average geothermal gradient

Shale

Sandstone

Halite

Shale

Shale

W/m2 Units:

Petroleum Geoscience (Springer).

Heat Flow and Thermal Conductivity

The graphic below demonstrates how the geothermal gradient of a transect can be used to calculate the thermal conductivity of a rock formation if a constant heat �ow is known. Certain rock types have particular thermal conductivities associated with them. “Kinks” in a temperature log can often be indicative of a change in lithology.

Left: Ed Romanowicz checking tension on the cable at the Altona Flats well �eld near Plattsburgh, New York.Right: Dave Franzi inspecting the full logging set-up.

Cutting Dolostone Formation Hazel Notch Formation Hazel Notch Formation

Bascom Formation Shelburne Marble Shelburne Marble

Stowe Formation Stowe Formation Waits River Formation

Pinnacle Formation Monkton Quartzite Fair�eld Pond Formation

Clarendon Springs FormationDanby Formation Pinnacle Formation

Chittenden Intrusive SuiteWaits River Formation Waits River Formation

Nick Bachman, Dept. of Geology, Middlebury College, Middlebury, VT, 05753, [email protected] Kim, Vermont Geological Survey, 1 National Life Dr., Davis 2, Montpelier VT 05620, [email protected]

Ed Romanowicz, Center for Earth and Env. Science, SUNY at Plattsburgh, 101 Broad Street, Plattsburgh, NY 12901, [email protected]

Map modi�ed from Ratcli�e et al. (2011). Black dashed lines de�ne major tectonic belts.See references for sources for Previous Reported “Hot Spots” and UninvestigatedSites of Interest.

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a geothermal investigation of 18 bedrock wells in vermont · 2015. 12. 28. · associated with an...

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