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

Geothermal energy is the natural heat based within the earth’s crust. The energy is manifested on earth’s surface in form of fumaroles, hot springs and hot altered ground. To extract the energy, wells are drilled to tap the steam and water at high temperature (250 -350 °C) and pressures of (600 – 1200 psi) at depths of 1 to 3 kilometers. Geothermal power is uniquely ideal because its base load, it is a clean renewable fuel and ranks among the most affordable forms of renewable energy. Geothermal energy has been exploited world over credit to government policies and global shift towards clean energy sources. In the USA, the Hawthorne geothermal area located in the Walker Lake basin contains several blind geothermal systems, with no surface thermal manifestations. This area has been the focus of geothermal investigations for over 40 years, with initial discovery of blind resources via anomalously-warm water wells. Subsequent studies and drilling of temperature gradient holes and geothermal wells identified three separate blind geothermal prospects (A, B and C) in the Hawthorne part of the Walker Lake basin. Downhole temperature logs indicate that these three systems are low temperature resources (< 120 °C), and at least two of the three have shallow outflow plumes as observed in temperature logs.

Recent government initiatives, particularly by injection of funds, have enabled development of new resource conceptual models at different levels of certainty. These models provide an estimate of the electrical power generation potential of the resource, by applying the power density approach. Nonetheless, much research is needed for the three identified separate blind geothermal prospects in the Hawthorne area. In this regard, University of Nevada researchers: Professor Bridget Ayling and Dr. Nicholas Hinz, conducted a detailed expert review of all existing geoscience data acquired at the site to date to develop a quantitative estimate of geothermal resource potential for one of the Hawthorne geothermal prospects (prospect A — along the southwest side of the basin). Their works is currently published in the research journal, Geothermics.

In their approach, they focused on evaluating the potential resource capacity of prospect A in terms of temperature, fluid production rates and reservoir size/volume, through review and integration of the existing geoscientific data for the area, and development of new resource conceptual models at different levels of certainty (but always honoring the data). To be specific, their work entailed review of substantial well data from water wells and geothermal exploration wells (downhole temperature logs, lithology, water chemistry, borehole televiewer, and alteration mineralogy), detailed geological and structural mapping information, geophysical data (gravity, magnetic, and seismic reflection), 2-meter temperature data, and an existing 3D geological model of the basin.

The authors reported that the thermal anomalies associated with prospect A reflect the influence of two geothermal fluids in close proximity that are chemically-distinct, with different temperatures and spatial extent (lateral and vertical). Further, it was seen that one fluid represents a deeper resource, hosted in altered, fractured Mesozoic granitic basement along a segment of the Wassuk Range-front fault system, and characterized by equilibrated, alkali-chloride fluids, with ∼4000 ppm total dissolved solids and a maximum measured temperature of ∼115 °C at ∼1500 m depth. A second fluid was noted to be hosted in Neogene basinal sediments at < 400 m depth, with maximum measured temperatures of ∼100 °C, total dissolved solids of ∼1000 ppm, and a sodium-sulfate fluid chemistry. Moreover, the researchers established the outflow of this shallow resource can be tracked down gradient into the basin using well temperature data, which map a vertically-constrained plume that cools with distance from the inferred up-flow location.

In summary, the Ayling-Hinz study presented an in-depth re-evaluation and synthesis of available sub-surface data to develop a new conceptual model of prospect A, which is located at the base of the Wassuk Range, west of the town of Hawthorne. The data suggest that the deeper resource is conductively transferring heat to the shallow resource, and structural and/or stratigraphic compartmentalization is preventing direct interaction and fluid mixing. In a statement to Advances in Engineering, Professor Bridget Ayling explained that by applying the newly developed conceptual model of prospect A (with P10, P50, and P90 scenarios), and application of the power density method, they were able to estimate that deep and shallow systems may have resource potential of 7 MWe (P50) and 1.6 MWe (P50) respectively.