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Induced seismicity represents a negative drawback during subsurface exploitation for geothermal energy production. Understanding the triggering mechanisms of induced earthquakes can help implement effective seismic hazard mitigation actions. Among the triggering mechanisms, the pore fluid pressure is of primary importance. Here we provide a static picture of the excess pore fluid pressure at the hypocenters of a seismic sequence induced at the deep geothermal field in St. Gallen, Switzerland, in July 2013. We find that in addition to the Coulomb static stress change, fluids play a key role in promoting the sequence. The estimated excess pore fluid pressure for approximately half of the earthquakes is higher than the injection pressure necessary during the well control phase to fight the unexpected gas kick, that accidently occurred during field operations when a trap of overpressured gas was broken. Plain Language Summary In July 2013, a sequence of more than 340 earthquakes was induced during the exploitation of the subsurface for energy production in St. Gallen geothermal field (Switzerland). To understand the mechanisms underlying the evolution of the sequence, we investigated the role of fluids and elastic stress transfer. The excess pore fluid pressure measurements suggest that the main triggering mechanism is related to high‐pressure fluids. The high values of the pore fluid pressure may be due to an already existing in situ overpressure condition from which a documented unexpected gas kick occurred. Key Points The main earthquake triggering mechanism is the effect of high‐pressure fluids The high pore fluid pressure values may be due to an already existing in situ overpressure condition

The Yangbajain Geothermal Field in Tibet is located in the fault subsidence basin of the central Yadong-Gulu Rift Valley. The spatial distribution of the field is controlled by mountain-front fault zones on the northwestern and southeastern sides of the basin. Geothermal power has been generated in Yangbajain for more than 40 years. However, owing to the lack of three-dimensional (3D) geophysical exploration data, key geological issues related to the partial melt body of the Yangbajain Geothermal Field, such as its location, burial depth, and geometric form, as well as the ascending channel of the geothermal fluid, have for a long time been controversial. In this study, 3D inversion was performed using measured geo-electromagnetic total impedance tensor data from 47 survey points. The extracted horizontal sections at different depths and profiles, and at different lines, reflect the 3D electrical structure model of the geothermal field in the study area. Subsequently, three findings were obtained. First, the partial melt body, located below the China-Nepal Highway extending along the northeast direction, is the heat source of the Yangbajain Geothermal Field. The burial depth range of the molten body was determined to range between approximately 6.2 and 14 km. Moreover, the geothermal fluid ascended a horn-shaped circulation channel with an up-facing opening, located in the northern section of the sulfur ditch area. The study results revealed that deep rock fissures (>2 km) were not well developed and had poor permeability. In addition, no layered heat reservoirs with high water richness were observed in the northern part of the study area. However, the application of enhanced geothermal system (EGS) technology in the northern region would be essential to improving the power generation capacity of the Yangbajain Geothermal Field. In addition, the study found no deep high-temperature heat storage areas in the southern region of the study area.

The circulation depth of geothermal water providing the geothermal system framework significantly dominates the evaluation of renewal capacity and geothermal resources. The traditional evaluation of circulation depth is based on the groundwater temperature variation in the recharge section and the average geothermal heating rate of geothermal system. However, misunderstanding groundwater temperature distribution in geothermal systems will lead to overestimating groundwater circulation depth based on the recharge section. Temperature measurement in a 1000 m geothermal scientific borehole from the Xinzhou geothermal field of south China is discussed as a case study to reassess the circulation depth of geothermal water. For Xinzhou geothermal system, the recharge and discharge temperatures are from 26.2 °C to 32.6 °C and from 67.0 °C to 98.0 °C, respectively. And the heat exchange temperature at the deepest point is from 121 ℃ to 154 ℃. This indicates that the temperature gradient in the recharge section should be greater than that in the discharge section. But the actual observation is opposite that the temperature gradient in the recharge section and in the discharge section is 3.04 ℃/100 m and 4.97 ℃/100 m, respectively. We proposed that the depth of geothermal water circulation evaluated by the temperature change and the geothermal heating rate in the discharge section represents the top depth of convection in the heat exchange zone, and the depth evaluated by the recharge section represents the advection depth of groundwater in the recharge section. The top depth of convection in the heat exchange zone estimated by the discharge Sect. (0.75–1.49 km) is much shallower than the advection depth of groundwater in the recharge Sect. (3.25–4.34 km). In the convective heat exchange zone (between 4.34 km and 1.49 km), the fault zone at a certain depth is the ideal location for geothermal development to extract water and heat.

Advanced Interferometric Synthetic Aperture Radar (InSAR) data has led to an extensive observation of Earth's surface displacements. Whereas the combined use of high‐resolution InSAR, leveling and GPS data may enable highly detailed three‐dimensional deformation models, publicly available modeling and inversion algorithms either seek a single homogeneously deformed source or involve a few thousand modeling elements. We present and release a conjugate‐gradient inversion code that searches for the three‐dimensional distribution of the volumetric strain that predicts simultaneously any observed InSAR, leveling and GPS surface displacement. By applying our algorithm on to leveling and InSAR data of the Cerro Prieto Geothermal area in Mexico for the 2012–2015 period, we find that the volume loss matches the extent and depth of the known geothermal reservoir or recharging aquifer and corresponds to 13% of the reported geothermal fluid extraction volume. We also identify non‐volumetric deformation near the tectonic faults, possibly associated with creep displacement. Plain Language Summary We present a highly detailed image of the volumetric underground deformation around the Cerro Prieto Geothermal Field (CPGF), the world's third biggest geothermal energy producer. To generate this model, we developed a novel 3D joint inversion algorithm for surface displacements and applied it to historical InSAR and leveling survey data. The model shows the interplay of anthropogenic and tectonic processes announcing a volume loss that matches 13% of the 2012 extraction reported for the CPGF for the year 2012 and confirming the existence of non‐volumetric deformation in the major tectonic faults. As an essential part of this contribution, the developed algorithm is also tested on synthetic data and released for its open academic use. Key Points Detailed 3D map of the volumetric origin of subsidence in the Cerro Prieto area Joint InSAR and leveling data inversion algorithm for three‐dimensional sources

Severe calcium carbonate scaling occurs in the wellbores of high‐temperature geothermal wells in the Yangbajain Geothermal Field. Based on sampling and laboratory analysis of the geothermal field, this study investigates the hydrogeochemical characteristics of the geothermal fluids, estimates the reservoir temperature, and reconstructs the bottom‐hole fluid composition. Combined with field discharge tests and scaling characteristics, a prediction model for the flash horizon in the wellbore and a coupled quantitative evaluation model for calcium carbonate scaling are established to forecast the scaling location and amount. The results indicate that the shallow reservoir temperature is 190 ° C ± 20 ° C, with an initial pH between 5.3 and 6.7. The high‐temperature geothermal water is of Na‐Cl type, and the CO 2 in the geothermal fluid primarily originates from high‐temperature decarbonation of deep metamorphic sedimentary rocks and marine carbonates. Higher CO 2 content leads to a greater scaling rate and amount. For the typical geothermal well ZK303, the predicted flash horizon is at a depth of 126.8 m, with the most severe scaling occurring 10–20 m above the flash horizon. The maximum scaling rate is 0.1214 mm/d, with scaling thicknesses of 0.85 mm (7 days) and 3.64 mm (30 days), and scaling amounts of 342.65 kg (7 days) and 1468.52 kg (30 days).

Volatiles transported from the Earth’s interior to the surface through permeable faults provide insights on the gas composition of deep reservoirs, mixing and migration processes, and can also be applied as gas-geothermometer. Here, we present carbon (δ13C), hydrogen (δ2H) and nitrogen (δ15N) isotopic data of CO2, CH4, and N2 from gas samples collected from the Kızıldere and Tekke Hamam geothermal fields, located along the eastern segment of the Büyük Menderes Graben, Turkey. The stable isotopic composition of carbon (δ13C) ranges from +0.30 to +0.99‰ (PDB) for CO2 from Kızıldere and is slightly more variable (−0.95 to +1.3‰) in samples from Tekke Hamam. Carbon isotope data in combination with CO2/3He data reveal that ~97% (Tekke Hamam) to ~99% (Kızıldere) of CO2 derives from limestone sources, with the residual CO2 being magmatic in origin with no evidence for CO2 from organic sources. The slightly higher contribution of limestone-derived CO2 in Kızıldere, compared to Tekke Hamam can be attributed to the higher temperatures of the Kızıldere reservoir and resulting amplified fluid–limestone interaction, as well as helium depletion during phase separation for Kızıldere samples. In contrast to the carbon isotopic composition of CO2, the δ13C values of methane from Kızıldere and Tekke Hamam are clearly distinct and vary between −23.6 and −20.8‰ for Kızıldere and −34.4 and −31.7‰ for Tekke Hamam, respectively. The δ2H-CH4 composition is also distinct, measured as −126.7‰ for Kızıldere and −143.3‰ for Tekke Hamam. CO2-CH4 carbon isotope geothermometry calculations based on the isotopic fractionation of δ13C between the dominant component CO2 and the minor component CH4 reveals temperatures 20–40 °C and 100–160 °C higher than the bottom–hole temperatures measured for Tekke Hamam and Kızıldere, respectively. Based on the CO2-CH4 carbon isotope disequilibrium, unusual high methane concentrations of ~0.3 to 0.4 vol.-% and CH4/3He-δ13C-CH4 relationships we suggest thermal decomposition of late (Tekke Hamam) to over-mature (Kızıldere) organic matter and, to some extent, also abiogenic processes as principal source of methane. The N2/36Ar ratios of most samples reveal the existence of a non–atmospheric nitrogen component within the gas mixture issuing from both fields, in addition to a constant contribution of atmospheric derived nitrogen accompanied into the system via the meteoric recharge of the geothermal system. Based on the δ15N isotopic ratios (varying between −4.44‰ and 4.54‰), the non–atmospheric component seems to be a mixture of both sedimentary (crustal organic) and mantle nitrogen. The thick Pliocene sedimentary sequence covering the metamorphic basement is the likely major source for the thermogenic content of CH4 and crustal N2 gas content in the samples.

We report solid-earth tidal modulation of early aftershocks of the July 2019 Ridgecrest earthquake sequence, which occurred close to the southeastern edge of the Coso geothermal field. We found that the frequency of early aftershocks in the northern part, close to the Coso geothermal field, was modulated by the solid earth tides as they exhibit a strong correlation with the peak shear stress and Coulomb stress imparted by the solid earth tides. However, aftershocks that occurred farther south of the Coso geothermal field in the same sequence exhibit a weak correlation with the solid earth tidal stress. Our analysis implies that the tidal modulation of the earthquake sequence in the northern part is due to its vicinity to the Coso geothermal fields in southern California, which contain high-pressure fluids and are well known for their susceptibility towards tidal triggering.

The Sciacca basin extends in the southwestern part of Sicily and hosts an important geothermal field (the Sciacca Geothermal Field) characterized by hot springs containing mantle gasses. Newly acquired high-resolution seismic profiles (Boomer data) integrated with a multichannel seismic reflection profile in close proximity to the Sciacca Geothermal Field have documented the presence of numerous active and shallow fluid-related features (pipes, bright spots, buried and outcropping mud volcanoes, zones of acoustic blanking, and seafloor fluid seeps) in the nearshore sector between Capo San Marco and Sciacca (NW Sicilian Channel) and revealed its deep tectonic structure. The Sciacca Geothermal Field and the diffuse submarine fluid-related features probably form a single onshore–offshore field covering an area of at least 70 km2. This field has developed in a tectonically active zone dominated by a left-lateral transpressive regime associated with the lithospheric, NNE-striking Sciacca Fault System. This structure probably favored the rising of magma and fluids from the mantle in the offshore area, leading to the formation of a geothermal resource hosted in the Triassic carbonate succession that outcrops onshore at Monte San Calogero. This field has been active since the lower Pleistocene, when fluid emissions were likely greater than today and were associated with greater tectonic activity along the Sciacca Fault System.

The Pauzhetka geothermal field was surveyed to study the mineral deposits that were formed as thermal water was discharged from well separators. We studied the compositions, structure, and geochemical properties of these sediments along the flow and in vertical cross sections of manmade “sinters”. It was found that, at the beginning of thermal water discharges, the sediments were composed of X-ray-amorphous mordenite–opal mixtures; later on, the sediments became wholly siliceous. The zeolite component of the mineral deposits determines their high sorption properties in relation to Au, Ag, Hg, As, Rb, Sr, Ba, Cs and other elements; the mordenite matrix receives sulfides of iron, silver, and copper. It is shown that the mineral deposits which were formed at the ground surface of the Pauzhetka geothermal field constitute an indicator of alkaline mineral-forming and ore-forming environment in the lower horizons of the Pauzhetka geothermal system.

Zhangzhou geothermal field is one of the highest temperature in southeastern coastal areas in China. Zhangzhou geothermal field is an uplift-fracture type geothermal resource, and there are several deep fractures in the geothermal field, controlling water flow and heat transfer. Presently there is no systematic study of the characteristics of the temperature field, pressure field and water density distribution in the geothermal field, and there is no systematic analysis of the main factors affecting the temperature field. In this work, geological features of the fracture system are considered, and a conceptual model of the fracture system is established. Based on these, the distribution of the temperature field, pressure field and water density field at Zhangzhou geothermal field are numerically studied, the controlling effect of the fracture system on the temperature field is analyzed, and the main factors affecting the temperature field, pressure field and water density field are discussed. The results indicate that the temperature field and water density field at Zhangzhou geothermal field are strongly controlled by the fracture system, and the zone of high-temperature and low-density is confined within the fracture system. Main factors affecting the temperature field and water density field include the permeability of the fracture zone, the thermal conductivity of rocks and the water recharge rate. Higher fracture zone permeability will reduce the temperature and increase the water density in the center of the fracture system. Higher water recharge rate will increase the temperature and reduce the water density in the center of the fracture system.