Computational methods for estimating precipitation from geothermal brines

Laboratory experiments using Salton Sea Geothermal Field brines at elevated temperatures are costly, time-consuming, and potentially difficult to perform. The LLL Geothermal Program is therefore also attempting to predict equilibria in the SSGF brines by computation. Two approaches to this problem are being taken. Modeling of chemical reactions in the brines is being carried out using the Helgeson-Herrick (HH) code. In addition, the precipitation of many solids is being studied individually using effective activity coefficients which take chloride complexing into account. The results of both methods are consistent with one another in predicting precipitation behavior in the temperature range 100-300 C. For example, results for Sinclair No. 4 brines at 200 C indicate that at low pH, SiO{sub 2}, MnO{sub 2}, and Fe silicates precipitate. As pH increases, Cu and Fe sulfides, Fe silicates and Fe oxides also precipitate. For the San Diego Gas and Electric Magmamax brine at 200 C, the HH code predicts results quite similar to those described above for the Sinclair No. 4 brine with one notable exception, PbS precipitated at pH greater than 4.0. This correlates with observations on the scale examined from the San Diego Gas and Electric test site

Hydrothermal interaction of solid wafers of Topopah Spring Tuff with J-13 water and distilled water at 90, 150, and 250{sup 0}C, using Dickson-type, gold-bag rocking autoclaves

The Nevada Nuclear Waste Storage Investigations Project has conducted experiments to study the hydrothermal interaction of rock and water representative of a potential high-level waste repository at Yucca Mountain, Nevada. The results of these experiments help define the near-field repository environment during and shortly after the thermal period that results from the emplacement of nuclear waste. When considered in conjunction with results contained in companion reports, these results can be used to assess our ability to accelerate tests using the surface area/volume parameter and/or temperature. These rock-water interaction experiments were conducted with solid polished wafers cut from both drillcore and outcrop samples of Topopah tuff, using both a natural ground water and distilled water as the reacting fluid. Pre- and post-test characterization of the reacting materials was extensive. Post-test identification and chemical analysis of secondary phases resulting from the hydrothermal interactions were aided by using monoliths of tuff rather than crushed material. All experiments were run in Dickson-type, gold-bag rocking autoclaves that were periodically sampled at in situ conditions. A total of nine short-term (up to 66-day) experiments were run in this series; these experiments covered the range from 90 to 250{sup 0}C and from 50 to 100 bar. The results obtained from the experiments have been used to evaluate the modeled results produced by calculations using the geochemical reaction process code EQ3/6. 31 refs., 37 figs., 7 tabs

Experimental Studies of Igneous Rock Series: A Zoned Pluton in the Wallowa Batholith, Oregon

The phase relationships of five analyzed granodiorites and tonalites from the zoned Needle Point Pluton of the Wallowa Batholith, Oregon, supplied by W. H. Taubeneck, have been determined in the presence of 15-25 weight per cent H_2O at pressures to 3 kb. Pressure-temperature curves were located for the beginning of melting and for the disappearance of K-feldspar, quartz, plagioclase, biotite, and hornblende. For two specimens, a granodiorite and a tonalite, the melting behavior at 2 kb. pressure was determined in more detail with approximate point counts of crushed fragments. Representative results are: granodiorite (Differentiation Index [D.I.] 76) melting begins at 705° C., K-feldspar disappears at 710° C., quartz disappears at 735° C.; tonalite (D.I. 55) melting begins at 725° C., the trace of K-feldspar disappears at the same temperature, quartz disappears at 755° C. In both rocks, calcic plagioclase, amphibole, and opaque minerals remain at 900° C. The hornblende becomes unstable at a slightly higher temperature and passes into the melt phase. Forty-seven per cent of the granodiorite melts between 705° and 730° C., and 55 per cent melts at 900° C. Twenty-nine per cent of the tonalite melts between 730° and 800° C., and 32 per cent melts at 900° C. At 900° C., the approximate plagioclase content of the granodiorite is 27 per cent, and of the tonalite 40 per cent; the glass in each rock has a refractive index of 1.510 ± 0.002. The experimental conditions (excess H_2O; f_(O_2) not controlled) prohibit detailed comparison with the natural occurrences. However, the persistence of calcic plagioclase and hornblende in abundance at 900° C. at 2 kb. pressure indicates that if these rocks were formed by anatexis, then either high temperatures are required at the base of the crust or the magmas consisted of a eutectic-type granite liquid with suspended crystals. The high liquidus temperatures suggest that gabbroic magma may be involved in the development of magmas of intermediate composition in batholiths

Experimental Studies of Igneous Rock Series: Felsic Body Suite from the Needle Point Pluton, Wallowa Batholith, Oregon

The phase relationships of four analyzed granitic rocks which comprise a late-stage felsic body sequence intruding the Mesozoic granodiorites and tonalites of the Needle Point pluton, Wallowa Batholith, Oregon, have been determined in the presence of 15 wt percent water at pressures to 3 kbar. Pressure-temperature curves were located for the beginning of melting, and for the disappearance of K-feldspar, quartz, plagio-clase, biotite, and hornblende in granodiorite no. 678, quartz monzonites nos. 685 and 774, and granite no. 705. Compositions and structural states of plagioclases from the granitic rocks, basic masses intruding the batholith, and mafic inclusions, as well as the compositions of coexisting feldspars, were determined using the electron microprobe analyzer. Variations in each of these parameters throughout the melting interval of each granitic rock were measured at 2 kbar pressure. Although experimental conditions (excess H_2O present; fO_2 regulated but not controlled) forbid detailed comparison with natural occurrences, the results indicate that, at 2 kbar pressure, temperatures of at least 730° C and 780° C are required to generate liquids of granite and quartz monzonite composition. Several factors suggest that the normal product of partial fusion of many crustal rock types is a H_2O-undersaturated granite liquid; crystal mushes with this type of liquid are probably involved if intermediate magmas are derived from crustal sources

Permeability of Kayenta Sandstone to Hypersaline Brine at 10.3 MPa Confining Pressure and Temperatures to 90{degrees}C

The ability to inject “spent” geothermal brine may be a critical and perhaps limited factor in the development of fluid-dominated geothermal resources. In order to understand and evaluate changes in formation permeability and porosity at depth as a result of injection of brine effluents, experiments were carried out (70°-90°C at 10.3 MPa confining pressure) in conjunction with the ongoing brine chemistry and materials evaluation effort at the Lawrence Livermore Laboratory Field Test Station located in the Salton Sea Geothermal Field, Imperial Valley, California. In summary, the data portrayed in Figure 1 indicate that large permeability losses occurred in Kayenta sandstone (porosity, 20.7 ± 1.66%) when unfiltered, untreated Magmamax brine and filtered, acidified Magmamax brine were the permeating fluids. In the former case, permeability decline was due to the accumulation of a thick filter cake on the top face of the core sample which was composed of amorphous silica and iron. In the latter situation, loss of permeability was caused by the precipitation of amorphous silica and generation of large quantities of calcite particles from the dissolution of the matrix cement. The experimental results thus show that if the Salton Sea Geothermal Field were composed of porous sedimentary formations similar to Kayenta sandstone, long-term injection of unmodified Magmamax brine is not feasible. In the case of acidified brine, most of the permeability decline may result from the mobilization of calcite. Additional experiments will be carried out in the future at lower flow rates to test the possibility of long-term injection of filtered, acidified geothermal brine. 6 refs., 1 tab., 1 fig