Analysis of Convective Temperature Overturns near the East Rincon Hills Fault Zone using Semi-Analytical Models

The Rincon geothermal system (RGS) is one of the more promising intermediate temperature (~150^°C) geothermal prospects in New Mexico; one well has a bottom hole temperature of 99°C (SLH-1). This well is located about 50 m east of the East Rincon Hill Fault (ERHF) zone in the Rincon Hills in south-central New Mexico. Brackish (~ 1900 mg/l) geothermal fluids have migrated up the ERHF and flow eastwards in water-table aquifer. This blind geothermal system has no surface expression other than a series of stacked opal deposits. The temperature-depth profile measured in 1993 in SLH-1 was overturned, suggesting transient geothermal behavior. SLH-1 in 1993 had a temperature of 83.9°C just below the water table (depth of 100 m). Below the water table temperatures declined in the borehole to 70.6°C at 176-m depth before increasing. The highest temperature was 99°C at the bottom of the borehole at a depth of 371 m. We remeasured the temperature profile in SLH-1 in September 2018 and found it to be nearly unchanged in the intervening 25 years. The temperature at 100 m in 2018 was slightly warmer compared to 1993 (84.8°C) and was warmer at the minimum at 173 m (73.7°C), but otherwise the two profiles overlap perfectly. This suggests steady-state hydrothermal conditions. As a consequence of these observations, we developed and applied a semi-analytical, steady-state model describing conductive and convective heat transfer resulting from three-dimensional flow groundwater flow. The solution is based on the assumption of fault perpendicular groundwater flow (qh) within an unconfined hot water-table aquifer and regional fault-parallel flow within an underlying cooler confined aquifer (qc). Vertical conductive heat transfer is assumed to dominate above, below, and in between these stacked aquifers. The three-dimensional flow rates needed to produce temperature overturns were evaluated using a sensitivity study. This model produces steady-state temperature overturns similar to the overturn measured in SLH-1 provided that convective heat transfer dominates within both the hot and the cooler aquifers and that qh/qc is about 2.