Effect of an 860-m thick, cold, freshwater aquifer on geothermal potential along the axis of the eastern Snake River Plain, Idaho

A 1912-m exploration corehole was drilled along the axis of the eastern Snake River Plain, Idaho. Two temperature logs run on the corehole display an obvious inflection point at about 960 m. Such behavior is indicative of downward fluid flow in the wellbore. The geothermal gradient above 935 m is 4.5 °C/km, while the gradient is 72–75 °C/km from 980 to 1440 m. Projecting the higher gradients upward to where they intersect the lower gradient on the temperature logs places the bottom of the cold, freshwater Snake River Plain aquifer, which suppresses the geothermal gradient at this location, at least 860 m below the surface. The average heat flow for the corehole between 983 and 1550 m is 132 mW/m2. Although the maximum bottom-hole temperature extrapolated from a measured time–temperature curve was only 59.3 °C, geothermometers suggest an equilibrium temperature on the order of 125–140 °C based on a single fluid sample from 1070 m. Furthermore, below 960 m the basalt core shows obvious signs of alteration, including a distinct color change, the formation of smectite clay, and the presence of secondary minerals filling vesicles and fracture zones. This alteration boundary could act as an effective cap or seal for a hot-water geothermal system

Evaluation of the Geothermal Potential of the Western Snake River Plain Based on a Deep Corehole on the Mountain Home AFB Near Mountain Home, Idaho

A geothermal exploration corehole was drilled to a total depth of 1821.5 m on the Mountain Home Air Force Base near Mountain Home, Idaho. The corehole was used to collect an unusually large amount of data, including uniaxial compressive stress (UCS) experiments on core samples, to evaluate the geothermal potential of the western Snake River Plain. In addition, unlike many exploration holes in this region, a fluid entry was encountered at 1745.3 m and flowed artesian to the surface. A maximum temperature of 149.4 °C was calculated for the entry. A temperature log run on the corehole from 3 to 1675 m is nearly linear with little variation. The average geothermal gradient is 73 °C/km, and the average heat flow between 200 and 1500 m is 102 ± 15 mW/m2. Chemical analyses of a sample from the fluid entry suggest that a significant proportion of the water is not meteoric. Five geothermometers show equilibrium temperature in the range of 133–157 °C. Furthermore, based on the unconfined UCS experiments on basalt core samples, a brittle unit was found to comprise the fractured reservoir that the geothermal water flows from, while an overlying ductile unit acts as a hydrothermal caprock. This implies that the reservoir/caprock pair may be a target for future exploration wells drilled to delineate the extent of the potential resource and the boundaries of the connected fracture network

Ten years of measurements and modeling of soil temperature changes and their effects on permafrost in Northwestern Alaska

AbstractMultiple studies demonstrate Northwest Alaska and the Alaskan North Slope are warming. Melting permafrost causes surface destabilization and ecological changes. Here, we use thermistors permanently installed in 1996 in a borehole in northwestern Alaska to study past, present, and future ground and subsurface temperature change, and from this, forecast future permafrost degradation in the region. We measure and model Ground Surface Temperature (GST) warming trends for a 10year period using equilibrium Temperature-Depth (TD) measurements from borehole T96-012, located near the Red Dog Mine in northwestern Alaska—part of the Arctic ecosystem where a continuous permafrost layer exists. Temperature measurements from 1996 to 2006 indicate the subsurface has clearly warmed at depths shallower than 70m. Seasonal climate effects are visible in the data to a depth of 30m based on a visible sinusoidal pattern in the TD plots that correlate with season patterns. Using numerical models constrained by thermal conductivity and temperature measurements at the site, we show that steady warming at depths of ~30 to 70m is most likely the direct result of longer term (decadal-scale) surface warming. The analysis indicates the GST in the region is warming at ~0.44±0.05°C/decade, a value consistent with Surface Air Temperature (SAT) warming of ~1.0±0.8°C/decade observed at Red Dog Mine, but with much lower uncertainty. The high annual variability in the SAT signal produces significant uncertainty in SAT trends. The high annual variability is filtered out of the GST signal by the low thermal diffusivity of the subsurface. Comparison of our results to recent permafrost monitoring studies suggests changes in latitude in the polar regions significantly impacts warming rates. North Slope average GST warming is ~0.9±0.5°C/decade, double our observations at RDM, but within error. The RDM warming rate is within the warming variation observed in eastern Alaska, 0.36–0.71°C/decade, which suggests changes in longitude produce a smaller impact but have warming variability likely related to ecosystem, elevation, microclimates, etc. changes. We also forward model future warming by assuming a 1D diffusive heat flow model and incorporating latent heat effects for permafrost melting. Our analysis indicates ~1 to 4m of loss at the upper permafrost boundary, a ~145±100% increase in the active layer thickness by 2055. If warming continues at a constant rate of ~0.44±0.05°C/decade, we estimate the 125m thick zone of permafrost at this site will completely melt by ~2150. Permafrost is expected to melt by ~2200, ~2110, or ~2080, if the rate of warming is altered to 0.25, 0.90, or 2.0°C/decade, respectively, as an array of different climate models suggest. Since our model assumes no advection of heat (a more efficient heat transport mechanism), and no accelerated warming, our current prediction of complete permafrost loss by 2150 may overestimate the residence time of permafrost in this region of Northwest Alaska