Elsevier

Geothermics

Volume 37, Issue 6, December 2008, Pages 565-585
Geothermics

Understanding the Chena Hot Springs, Alaska, geothermal system using temperature and pressure data from exploration boreholes

https://doi.org/10.1016/j.geothermics.2008.09.001Get rights and content

Abstract

Chena Hot Springs is a small, moderate temperature, deep circulating geothermal system, apparently typical of those associated to hot springs of interior Alaska. Multi-stage drilling was used in some exploration boreholes and was found to be useful for understanding subsurface flow characteristics and developing a conceptual model of the system. The results illustrate how temperature profiles illuminate varying pressure versus depth characteristics and can be used alone in cases where staged drilling is not practical. The extensive exploration activities helped define optimal fluid production and injection areas, and showed that the system could provide sufficient hot fluids (∼57 °C) to run a 400-kWe binary power plant, which came on line in 2006.

Introduction

Chena Hot Springs (CHS) is located 96 km northeast of Fairbanks, Alaska (Fig. 1, insert). It is one of approximately 30 low-to-moderate temperature hot springs in central Alaska (Waring, 1917) lying along a band extending from the Seward Peninsula in the west into the Yukon Territory of Canada in the east. They are generally associated with Mesozoic to Early Tertiary granitic bodies within a matrix of the pelitic metamorphic rocks (Newberry et al., 1996, Wilson et al., 1998). The hot springs discharge zone at CHS is about 1000 m long and 100 m wide. The maximum temperature of the thermal waters reaching the surface naturally or being produced by shallow wells is 74 °C. The hot springs at CHS and the associated spa resort are privately owned and have been under commercial exploitation since 1907. At present, the thermal waters heat pools, buildings, and greenhouses, and are also used to keep an ice hotel frozen during the summer months utilizing an ammonia absorption chiller (Erickson et al., 2005).

The assessment of the CHS geothermal resource started in 1973 as an M.S. thesis project through the University of Alaska-Fairbanks (Biggar, 1973). Before this study, the only data available were some chemical analyses of the hot spring waters by the US Geological Survey (Waring, 1917). These early analyses and more recent chemical studies (see below) indicate that the deep hot water source at Chena has temperatures as high as 127 ± 5 °C.

In 1980, a regional study of helium and mercury soil concentrations was conducted by collecting 50 samples around the Chena pluton (Wescott and Turner, 1981). All helium concentrations outside the CHS area reflected normal atmospheric concentrations (i.e. within 4% of 5.2 ppm). However, a very sharp and narrow anomaly exceeding 750 ppm was found northwest of the main CHS area, near the site of borehole TG-8 (see Fig. 1). The very high He concentrations were interpreted to be related to a deep thermal source (Wescott and Turner, 1981). Toward the southeastern end of the hot springs zone, anomalous He concentrations were relatively lower (∼200 ppm), and dispersed over a wider zone.

A number of geophysical studies were conducted in 1979 (Wescott and Turner, 1981). Both shallow electrical conductivity (10 m) and shallow temperature (0.5 m depth) surveys outlined the main shallow permeability zone as a narrow NW-SE trending anomaly (solid gray curve in Fig. 1) along Spring Creek. Seismic refraction experiments along the central valley revealed that the thickness of the sedimentary fill is about 40 m at the center, thinning to 2–15 m near the valley edge, including the area where the hot springs are located.

In 2005, extensive exploration activities were started to complement plans to install a small binary geothermal power plant (Holdmann et al., 2007). The objectives of the activities were twofold. The primary goal was to augment existing development of the shallow geothermal reservoir in order to provide hot water to the geothermal power plant without negatively impacting the shallow reservoir that feeds the hot springs and draws tourists to the area. A secondary objective was to characterize an inferred hotter (and presumably deeper) geothermal reservoir implied by the geochemical temperatures (see Section 2) to determine if larger scale generation is possible, and to develop a conceptual model for interior Alaskan geothermal systems.

The exploration studies included a detailed geologic analysis of the immediate CHS area (Kolker et al., 2007), an audio-magneto-telluric (AMT) resistivity survey, and a suite of airborne geophysical measurements. A total of 18 boreholes with depths ranging from 60 to 311 m have been drilled to date, including six drilled prior to the 2005 program. In combination, these activities yielded substantial information about the geothermal system. The first power plant of 0.2 MWe commenced operation in August 2006, and a second one with the same generation capacity was added in December 2006.

In this paper, we analyze the geophysical data collected during the exploration activities and include a detailed discussion of the temperature–depth curves and pressure measurements in the boreholes. We further propose a conceptual model based on the analysis of the available temperature and pressure data.

Once a hole is drilled the natural-state pressure distribution with depth is essentially unrecoverable (Grant et al., 1982). One of the best ways to mitigate this effect is to use multi-stage drilling (White et al., 1975, Grant et al., 1982). This type of drilling was applied at Chena and its usefulness in understanding the natural flow regimes is demonstrated. Here, we illustrate how high-quality equilibrium temperature logs can often be used to identify permeable fractures. The independent interpretations of flow regimes based on temperature–depth curves and the relative pressure distribution compared to direct pressure measurements show an excellent correlation and allow us to generate a conceptual model of the geothermal system.

Section snippets

Geologic and structural setting

Chena Hot Springs is located in the Yukon-Tanana upland in east-central Alaska. The regional geology around CHS is composed of a matrix of Paleozoic to Precambrian metamorphic rocks, mainly greenschist facies pelitic rocks. This metamorphic unit is disrupted by plutonic bodies of mainly Mesozoic age. Lithologic units within the plutons include granite, tonalite, granodiorite, diorite and mafic dikes. CHS is located within a 40 km by 5 km pluton (Chena pluton) chiefly composed of coarse-grained

Airborne geophysics

To achieve a better understanding of the local geology, a suite of airborne geophysical measurements including radioactivity, magnetic, and resistivity were collected by helicopter over an area of 20 km × 20 km (Pritchard, 2005). The results of these studies are summarized in three panels in Fig. 2; a fourth panel presents the local portion of the regional geological map (Wilson et al., 1998) for comparison purposes. In general, the patterns of the geophysical data reflect the lithologies shown in

Thermal gradient and exploratory drilling

Temperature–depth measurements at CHS started in 1979 with eighty 0.5 m deep holes (Wescott and Turner, 1981). The resulting 0.5 m isotherm map shows an elongated thermal anomaly around the hot springs area with main boundaries about 1000 m long and 100 m wide and a maximum temperature of 48 °C (Fig. 1; solid gray loop). Deeper geothermal exploration and precision temperature logging was commenced in late 2005 by logging temperatures in five boreholes that were previously drilled by the resort

Fluid geochemistry

Table 2 summarizes the results of the geochemical analyses of waters from most of the CHS wells and the calculated geochemical temperatures. Concentrations of dissolved solids from surface samples and from the borehole waters generally show low values of 280–340 mg/l. The two end member compositions are the more saline geothermal waters of the western portion of the field, and the very dilute meteoric waters from the Monument and Spring Creeks. All other waters are mixtures of these two end

Static borehole pressures

Benoit et al. (2007) presented a detailed history of borehole pressure testing at CHS. The discussion here outlines the natural-state pressure regime at Chena. In a geothermal system a borehole immediately collapses the natural-state (initial) pressure regime in its vicinity as it acts as a short circuit between different reservoir pressure zones (Grant et al., 1982). One way to obtain natural-state pressure information is to use multi-stage drilling strategy and measure the pressure at each

Geothermal system heat loss and the production potential

The natural system heat loss at CHS was calculated using the shallow thermal gradients tabulated in Table 1. A thermal conductivity of 2.7 W/(m K) was assumed for the granitic rocks that are characteristic of the area. The heat loss was calculated based on surface temperatures of 0 °C and −2.2 °C. The gradients in the first 20 m were used in the heat loss calculation. Only the area with thermal data was included in the calculation so the calculated values should be conservative.

The natural heat loss

Temperature disturbances due to production

Electricity production in Chena started in July 2006 with the installation of a 200-kWe binary unit. Initially a single well (W-7) with a temperature of 73 °C and 2000 l/min flow rate was used for running the power plant. For a period of 3000 h between July 2006 and April 2007, the average production rate was 192 kWe. At the present, the electricity generation is 400 kW from two binary units using 3000 l/min of hot water from W-7 and W-5. Cooled (waste) geothermal waters are injected into wells TG-7

Conceptual model for the geothermal system

The above discussion on the static temperature logs and measured natural-state pressure regimes allow us to build a conceptual model for the CHS geothermal system; see Fig. 8. In that figure the subsurface temperatures and the inferred flow paths (open arrows) are illustrated; the widths of the arrows are proportional to the total amount of flow along those paths.

The currently explored portion of the CHS system (its upper 300 m) is relatively well understood. The shallow (upper 1000 m) model here

Conclusions

Stable isotope analyses show that thermal waters at Chena Hot Springs are meteoric in origin. A Carbon-14 analysis indicates that the age of the spring waters is less than 3000 years. The minimum depth of circulation must be about 3500 m in order to reach the geochemical temperatures of 100–130 °C based on a background temperature gradient of 35 °C/km. High background heat flow, deep circulation of thermal waters along fractures and/or faults, relatively recent age of the circulating waters,

Acknowledgements

This study is supported by of the U.S. Department of Energy Geothermal Resource Exploration and Definition (DOE-GRED) Program (Award number DE-FC36-04GO14347). Bernie Karl of Chena Hot Springs supported some of the work and was extremely cooperative in the collection and dissemination of the results of the project. We thank J. Combs, S. Garg, M. Lippmann and an anonymous reviewer for the improvement of the manuscript. We also wish to thank P. Stepp for helping in an early version of the

References (21)

  • K.W. Wisian et al.

    Numerical modeling of basin and range geothermal systems

    Geothermics

    (2004)
  • D. Benoit et al.

    Low cost exploration, testing, and development of the Chena geothermal system

    Geothermal Resource Council Transactions

    (2007)
  • Biggar, N., 1973. A Geological and Geophysical Study of Chena Hot Springs, Alaska. M.S. Thesis, University of Alaska,...
  • F. Birch

    Temperature and heat flow in a well near Colorado Springs

    American Journal of Science

    (1947)
  • C.A. Brott et al.

    Continuation of heat flow data, a method to construct isotherms in geothermal areas

    Geophysics

    (1981)
  • D.C. Erickson et al.

    Geothermal powered absorption chiller for Alaska Ice Hotel

    Geothermal Resources Council Transactions

    (2005)
  • M.A. Grant et al.

    Geothermal Reservoir Engineering

    (1982)
  • Holdmann, G., Benoit, D., Blackwell, D., 2007, Phase I final report, integrated geoscience investigation and geothermal...
  • A. Kolker et al.

    Geologic setting of the Chena Hot Springs Geothermal System, Alaska

  • Newberry, R.J., Bundtzen, T.K., Clautice, K.H., Combellick, R.A., Douglas, T., Laird, G.M., Liss, S.A., Pinney, D.S.,...
There are more references available in the full text version of this article.

Cited by (32)

  • Developing a conceptual model and power capacity estimates for a low-temperature geothermal prospect with two chemically and thermally distinct reservoir compartments, Hawthorne, Nevada, USA

    2020, Geothermics
    Citation Excerpt :

    Amedee is associated with upflow along N-NE-trending faults, and is produced from the systems outflow into basin sediments (Juncal and Bohm, 1987). Chena Hot Springs is produced from fracture zones within a weathered, Late Cretaceous granite pluton, which may be associated with a fault zone (Erkan et al., 2008). At Hawthorne, the presence of conglomerate buried in the basin sediments could yield high permeability, on par with the best fault zones.

  • Thermoeconomic cost analysis and comparison of methodologies for Dora II binary geothermal power plant

    2018, Geothermics
    Citation Excerpt :

    Binary systems that are developed with working fluids at low boiling points make it possible to produce electricity from low-temperature geothermal water. A binary plant in Alaska uses a geothermal resource at 57 °C (Erkan et al., 2008). These plants operate on a Rankine cycle with a binary working fluid (isobutene, pentane, isopentane, R-114, etc.) that has a low boiling point.

View all citing articles on Scopus
1

Current address: Division of Geodesy and Geospatial Science, School of Earth Sciences, The Ohio State University, Columbus, OH, USA.

View full text