Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Tempel, Et Al., 2011)

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Exploration Activity: Modeling-Computer Simulations At Long Valley Caldera Geothermal Area (Tempel, Et Al., 2011)

Exploration Activity Details
Location Long Valley Caldera Geothermal Area
Exploration Technique Modeling-Computer Simulations
Activity Date - 2011

Usefulness useful
DOE-funding Unknown

Exploration Basis
The study investigates the hypothesis that hydrothermal fluids upwell beneath the West moat area from a source reservoir with estimated temperatures ranging from 240° to 273°C, then flow laterally to the east and mix with cool groundwater that infiltrate and recharge the system along ring fractures and faults (an interpretation supported by Sorey, 1985; Fishcher et al., 2003; and Pribnow et al., 2003).
The simulation integrates published hydrologic and geochemical data from exploration wells and hot springs, and refers to previous petrographic studies in order to provide a realistic assessment of aquifer mineral reactions along the flow path. The extent of water-rock interactions were also constrained by mass balance calculations and thermodynamic modeling based on these data. These considerations allow for a more quantitative examination of hydrologic and geochemical processes that occur in the shallow geothermal aquifer between Casa Diablo and Shady Rest. Mixing of shallow thermal fluids (with a composition similar to those sampled from a well at Shady Rest) with cold groundwater (with a composition similar to fluids sampled at Laurel Spring) and reaction of those fluids with aquifer minerals along their flow path around the southern side of the resurgent dome towards Casa Diablo was modeled using the geochemical reaction path code EQ3/6 (Wolery and Daveler, 1992). The mixing/water-rock reaction simulation yielded a fluid with a composition similar to that measured in geothermal well MBP-4 at Casa Diablo. The authors concluded that: (1) secondary mineral assemblages encountered within the shallow Long Valley caldera reservoirs are best represented in the model under open system conditions, (2) the chemical composition of the Casa Diablo well fluids is most closely reproduced through conservative mixing of 82% Shady Rest and 18% Laurel Spring waters, and (3) saturation of chemical components that leads to the precipitation of quartz, pyrite, smectite, and hematite in the system appears to relate strongly to water-rock interactions affecting the composition of the fluid phase�these modeling results are consistent with previous studies that inferred that high flow rates of 100�200 m/yr in the Long Valley system are indicative of an open system. Similarities between the water compositions produced by the reaction path model of water�rock interactions, conservative water mixing model, and actual water compositions measured in the Casa Diablo well suggests that dissolution of feldspars (the dominant reactive mineral phase in the system), provides the chemical components needed for the conservative formation of clay minerals and calcite in the reservoir.

Additional References