Session 1F: Geochemistry

Recent work in the Great Basin region of the western United States has made it possible to predict the depth of hydrothermal reservoirs (i.e., the depth at which heat is accumulated prior to ascent via hydrothermal upflow) identified through geochemistry and to contextualize the spatial patterns of these reservoir depths. Chemical geothermometers use the chemical and mineral constituents of hydrothermal fluids to predict the temperature at which fluids equilibrated with the host rocks at depth. Assuming that most of the Great Basin is dominated by conductive conditions until a vertically connected hydrothermal flow path is created (e.g., by faulting), geothermometers reflect the chemical and thermal conditions at the depth interval that the fluid has conductively equilibrated over a long period before a vertical conduit allows convective upflow. By pairing geothermometer temperature estimates with our recent three-dimensional temperature model of conductive heat flow in the Great Basin, we estimate the corresponding reservoir depths and construct a map of circulation depths.

The predicted depths from geothermometers have spatial patterns across the Great Basin that relate to patterns seen in other geologic and geophysical data, allowing broad physical explanations for spatial variations of geothermometer depths. Deeper springs generally occur disproportionately in areas with higher strain rates and in basins. We posit that current elevated strain rates reflect patterns of historic deformation where ongoing tectonic activity maintains permeable pathways to deeper reservoirs, some of which are estimated to exceed 6 km depth. Basins, not surprisingly, contain a disproportionate number of these deep systems, because the underlying aquifers are closer to the surface in basins, thus requiring less water pressure to reach the surface than in mountain ranges. Most springs estimated to have their source in a deep reservoir occur at places known to host a hydrothermal system; these refined depth estimates of the source reservoir can help to better constrain the source depth for many known hydrothermal systems across the Great Basin.

In geothermal power plants, reduction of power generation due to silica scale is one of the serious issues. Once silica scale adheres to the inner surface of the pipes, valves or other parts on the brine line, such silica scaling results in flow rate reduction or improper performance. To preserve the power generation capacity, it is necessary to take measures to prevent silica adhesion. One of the issues is to improve the surface properties of the components by surface coating or applying alternative materials to metal. As a solution to this issue, a study on searching for new materials with low adhesion to silica would be necessary.

There are diverse resin materials in the world with various characteristics and the studies about the performance design and synthesis of the resin materials have been widely discussed. It is considered that some new alternative materials which are easy-to-install on geothermal field with high durability and low silica adhesion could be worked out. However, it is said to be such a tough problem because the experiments which take a lot of time and effort are required to verify the silica adhesion properties for the large number of the resin materials.

In this work, a new method for predicting the silica adhesion properties of the resin materials has been developed based on quantum chemical calculations. First, the models for the calculations have been built up and structural optimizations were performed for seven kinds of resin materials: polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polybutylene terephthalate (PBT), polyethylene terephthalate (PET), sheet molding compound (SMC), polymethyl methacrylate (PMMA) and polycaprolactam (nylon 6). Then the ab initio calculations were performed by using the molecular orbital method for each resin molecules. The orbital energies were estimated to evaluate the reactivity between candidate resin materials and silica based on the famous frontier orbital theory. As a result, the high silica adhesion group: PBT, SMC, PET and the low silica adhesion group: PTFE, PVC, PMMA, nylon 6 were obtained.

Filed tests for verification were performed with the same resin materials at one geothermal power plant in the northeast part of Japan. The low silica adhesion group and the high silica adhesion were obtained as well. It is revealed that the results of the field tests were in excellent agreement with the calculation results. Thus, the accuracy of the calculations has been verified.

According to our results, ab initio calculation is considered to be a powerful research methodology for predicting the reactivity between resin molecules and silica. Furthermore, an efficient and effective screening method for low silica adhesion resin materials has been developed in this work.

Amorphous aluminosilicate scales are a common occurrence within infrastructure at many geothermal fields around the world, often resulting in the unexpected fouling of surface infrastructure and injectivity decline in geothermal reinjection wells. Amorphous aluminosilicate scales often form under conditions at which pure amorphous silica scales would not be expected. Such conditions include temperatures significantly higher than that predicted for amorphous silica polymerisation and extreme low pH, such as the formation of blockages in sulfuric acid injection quills. Additionally, the formation of aluminosilicate scale does not share the kinetic limitations displayed by its pure amorphous silica counterpart, having been observed to nucleate quasi-instantaneously both in the laboratory and in the field. Given these unique characteristics, a novel formation mechanism triggered by the precipitation of aluminium (oxy)hydroxide was tested in laboratory experiments, leading to a simple predictive tool for the occurrence of aluminosilicate scales. These results and tool are then compared to examples of and formation conditions of aluminosilicate scales within geothermal power stations at 3 separate geothermal fields. These examples include a rapidly formed aluminosilicate material deposited en masse during a binary plant excursion, leading to the first reported observation of the aluminium (oxy)hydroxide mineral, boehmite, in a geothermal scale,

The high temperature reservoir (HTR) in the Northwest Geysers inherently produces steam of varying quality, for example, higher volatile chlorides than the normal temperature reservoir (NTR) in the central and southeast Geysers. HTR rocks from the northwest central Prati area (formerly Central California Power Agency- have distinct δ 18-Oxygen isotopic compositions compared to rocks of the normal temperature reservoir, but HTR rocks from the nearby Aidlin and High Valley areas have isotope signatures similar to the rocks of the NTR. Reservoir metagraywackes of the NTR have silicate δ18-O values as low as +3 ‰, while anomalously high values in metagraywackes in productive portions of the central northwest Prati HTR typically range from +8-10 ‰. This 18-O anomaly in the Prati portion of the NW Geysers reservoir is particularly striking given that is bounded to the SW (Aidlin) and the NE (High Valley) blocks which produce steam of comparable pressure and temperature from similar depths, suggesting that the deep Prati reservoir is not hydrologically isolated even if it is isotopically distinct.

Many previous studies have concluded that the primary cause of isotopically “light” rock compositions in the main Geysers field was pervasive flushing of the permeable reservoir rock by meteoric waters with an average value of -8‰, while permeable reservoir rock in the Prati HTR has not been pervasively flushed and is therefore producing steam from boiled connate and / or magmatic waters with values of δ18-O = 0. Some studies have suggested mechanisms by which water flushing of The Geysers reservoir could have reached far northwest into the high temperature reservoir sections of Aidlin and High Valley without impacting the HTR in the Prati area. One key line of evidence for such models is the overall lower NCG content of steam produced from Aidlin and High Valley vs. Prati high temperature reservoir sections. Although stable isotope systematics have always been considered as supporting evidence in such studies, detailed software-based mapping of rock 18-O data has not previously been done at The Geysers.

Whole rock and steam condensate samples have been collected continuously during drilling and flow testing of new geothermal wells over the past six years and added to the existing legacy data set of >1,000 silicate 18-O analyses. Sampling from a new step out drilling program in the northwest Geysers has also significantly increased data density in both the NTR and HTR sections in this area. The entire Geysers δ 18-Oxygen whole rock isotope catalogue (now ~1,400 δ 18-O analyses) has been turned into an interpolated 3D model using Leapfrog Energy radial basis function. Integration of this new isotope model with fault mapping, micro earthquake (MEQ) hypocenter alignments and lithology data has allowed for new insights into the possible fault controls for meteoric water flushing in the Northwest Geysers and its impact on HTR steam quality. This isotope map has also been used to assist in the planning and targeting of new geothermal wells at The Geysers.

Thermal energy storage at large scale has significant potential for large scale clean energy deployment. However, it is necessary to understand and address the challenges (Dobson et al., 2023) associated with high temperature reservoir thermal energy storage (HT-RTES). Lessons learned from the previous demonstrations identify insufficient site characterization, thermal short-circuiting, lack of available heat, scaling, corrosion, and biofouling as key factors affecting the performance of HT-RTES. The objective of this paper is to develop reactive transport modeling strategies to understand the reactive-transport processes associated with HT-RTES in high-saline reservoirs by evaluating the significance of changes in temperature, pressure, mineralogy, and porosity of the formation during the HT-RTES operation. The results from the model will also evaluate the retrograde solubility of minerals, changes in permeability, and changes in redox conditions during the HT-RTES operation.

For this study, an isolated injection-production well doublet is used for injecting hot and cold fluids during the seasonal cycle. During summer, brine at 75 °C is surface heated to 140 °C and injected into the reservoir with 15% porosity. Produced brine from the heat-exchanger at 60 °C is injected back into the cold well during winter. Reactive transport simulations are carried out using TOUGHREACT-EOS7(Dobson et al., 2004; Sonnenthal et al., 2021) for 5 years of cyclic RTES operation. Representative geochemical data were obtained from the depleted Leopoldshafen oil field of Leopoldshafen around the DeepStor site (Banks et al., 2021).

In agreement with findings of Banks et al., we observe an increase in the porosity, in our case of 1.5%, near the hot wells after 5 years of operation. After five years, there has been a marginal decrease in porosity, approximately 1%, near the cold well. This suggests that mineral dissolution is more prevalent in the vicinity of the hot wells, likely a result of the injection of hot, somewhat acidic brine, whereas mineral precipitation is predicted around the cold well as a result of the temperature decrease. Iron minerals such as goethite show dissolution near the hot wells and precipitation in the relatively colder brine slightly away from the hot well. Also, changes in permeability have been evaluated using a cubic law of porosity-permeability correlation. There is no significant interference of hot and cold plumes, which indicates that thermal short-circuiting has not occurred under the simulated operating conditions. Future work will include a modeling scenario under strong oxidizing conditions such as the presence of dissolved oxygen in the injection brine, which can better quantify the possibility of corrosion and scaling due to air intrusion.

The Upper Rhine Graben (URG) benefits from extensive geothermal experience with several industrial projects on stream since the beginning of the 21st century. Several deep wells have demonstrated that hydrothermal brines circulate inside a complex network of natural fractures developed in the Triassic sandstones and the altered granitic basement with temperatures up to about 210°C and Li concentrations up to 210 mg/L. More recently, scientific studies have revealed that geothermal lithium could be extracted from this brine. The opportunity for lithium extraction combined with a low CO2 emission heat gave a second life to the geothermal exploration in the URG. A better knowledge of lithium sources will be based on mineralogical and geochemical characterization of the potential reservoir rocks. Representative samples were collected, i.e., several facies from the base to the top of the Triassic and Permian sandstones and granites of Hercynian basement from quarries in Vosges mountains as well as from deep wells. Micro-analytical techniques were correlated to petrological and geochemical data to discuss the Li resource circulations at the regional scale. Further investigations in other samples of the URG will be compared to attempt to upscale the Li rich rocks assessment at the regional scale and elucidate the Li enrichment in the brines.