Lithium Evolution and Reservoir Sustainability in the Salton Sea Geothermal Reservoir from Reactive-Transport Modeling

Abstract

Assessing the extent and sustainability of the lithium (Li) resource in the Salton Sea Geothermal Field (SSGF) is aided by an understanding of the processes that led to the development of lithium-rich brines. The purpose of this work is to examine the rates and extent of water-rock reactions involving Li in the SSGF, the rates of Li-rich brine replenishment into the reservoir, and the effects of water-rock reactions on Li-depleted brine (+/- condensate) that are re-injected into the reservoir. Simulations were performed using an updated version of the reactive-transport simulator TOUGHREACT V4.13-OMP (Sonnenthal et al., 2021). A surface evaporation model, with reduced atmospheric relative humidity leads to hypersaline brine formation from Salton Sea water, with an increase in Li to 120 ppm after about 44x evaporative concentration. This forms the observed low-temperature phases of gypsum, barite, halite, glauberite, bloedite, carbonates, and the Li-rich clay hectorite, consistent with observed salt and clay mineral assemblages. Three-dimensional reactive-transport simulations over 4000 years using the SSGF reservoir model, considering hypersaline brine convection in unaltered reservoir rocks, results in alteration to hectorite at lower temperatures and primarily cookeite (Li-chlorite) at temperatures over 300 C. The high initial Li concentration in the hypersaline brine leads to small increases/decreases in reservoir bulk rock Li concentrations, and little change in brine Li concentrations (except at the top of the reservoir, where more Li is removed from the brine). The primary replenishment mechanism for Li is the upward flux of convecting Li-rich brine from below the producing reservoir, and unexploited brines in the reservoir. Assuming the calibrated upward basal fluxes in the reservoir model, and Li concentrations in the upwelling brine of about 200 ppm, Li enrichment of depleted brines at 2750 m depth is roughly 10 ppm per 100 years. At the bottomhole depths of deep production wells (~2000-2700 m), with brine temperatures exceeding 300°C, reactions of relatively stable Li-bearing metamorphic minerals (primarily cookeite-chlorite, feldspars, and micas) are slow, and thus injection of Li-depleted brine or condensate is not enriched by mineral-water reactions over time periods of hundreds of years.