Linxiao Xing

At medium-deep and deep parts of the Earth's crust lay vast and so far mostly untapped reserves of geothermal energy. This energy source has the potential to meet humanity's needs for energy for thousands of years. However, the large-scale deployment has been hindered by a lack of effective and dependable exploitation technologies. Gravity heat pipes have recently garnered significant attention due to their exceptional heat transfer capability and their ability to harness geothermal heat without water-mining. Nevertheless, it is a persistent challenge to extend gravity heat pipes to depths that are sufficient to exploit deep geothermal energy. In this study, we introduce an innovative ladder-structure inner pipe design to overcome this shortcoming. This design incorporates a series of stepped ladders and small condensate guiding pipes, allowing for the development of super-long gravity heat pipes (SLGHPs). Combined with an optimal working fluid, this novel structure enables kilometers-long gravity heat pipes. As a direct outcome of this innovation, a 4149-meter-long SLGHP was constructed and installed in a geothermal well using ammonia as the working fluid. The SLGHP geothermal system demonstrated the ability of continuous heat output exceeding 1 MW, with a heat flux across the radial section of the SLGHP reaching 4 × 10⁷ W m⁻². Furthermore, an ammonia vapor-driven power generator was developed and integrated with the SLGHP system. During a 72-hour test, this generator successfully produced electricity at a steady rate of approximately 7 kW. The breakthrough design of the SLGHPs holds the potential to unlock Earth's deep geothermal energy reserves, providing a sustainable and reliable energy source for generations to come.

The Jizhong Depression is a typical Mesozoic and Cenozoic fault basin located in the northwestern part of the Bohai Bay Basin that has abundant hydrothermal and geothermal resources and enormous development potential. In this study, hydrochemical and isotopic analyses were conducted on water samples from the southern region of the Jizhong Depression. The formation and evolution processes of the deep geothermal water were analyzed, the circulation process of the deep geothermal water was determined, and the genetic mechanism of the geothermal systems was elucidated. The hydrochemical types of the geothermal fluids in the sandstone reservoirs in the research area are mainly Cl·HCO₃⁻Na type, while the geothermal fluids in the carbonate reservoirs are mainly Cl-Na type and Cl·HCO₃⁻Na type. The ion components in the geothermal water are mainly controlled by the dissolution of the carbonate rocks and the alternate adsorption of cations. The elevation of the geothermal water supply area is 763–1063 m, and the main source is precipitation from the mountainous areas in the western Taihang Mountains. The Na-K-Ca temperature scale and multi-mineral equilibrium method have relatively small errors and are suitable for the southern region of the Jizhong Depression, with average errors of 21.44 °C and 32.64 °C, respectively. The depth of the Jxw thermal storage cycle in the research area is 3033–5187 m, and the depth of the Ng thermal storage cycle is 1360–2862 m. The content of the main ions (Na⁺, K⁺, and Cl⁻) in the water samples of the study area is greater in the Jxw thermal storage than in the Ng thermal storage; the Jxw thermal storage water samples have lower γNa⁺/γCl⁻ values than the Ng thermal storage; and the γSO₄²⁻/γCl⁻ and γCl⁻/(γHCO₃⁻ + CO₃²⁻) values are greater than those of the Ng thermal storage, indicating that the Jxw thermal storage is located in a geological environment with better sealing, longer flow, slower water circulation, more complete leaching, and higher salinity than the Ng thermal storage. Part of the deep thermal storage is transmitted upwards through the rocks via thermal conduction, and part is transmitted upwards along fault channels via thermal convection, forming a convection–conduction-type geothermal system.

The Xiong’an New Area is abundant in geothermal resources due to its unique geological structure. To address whether large-scale deep geothermal exploitation will induce a fault slip, we first determined the initial in situ stress field using shallow (~4000 m) in situ stress measurements from the North China plain. After characterizing the in situ stress field, we analyzed the initial stability of the main active faults in the sedimentary strata of the Rongcheng deep geothermal reservoir based on the Mohr–Coulomb failure criteria. Assuming that this area will be subjected to forty years of continuous fluid injection, we calculated the excess pore pressure in the deep geothermal reservoir and, subsequently, estimated the fault slip potential of the main active faults in this region from 2021 to 2060. Our results indicated that both the in situ stress field in the shallow crust of the Xiong’an New Area and the Middle-Late Pleistocene active faults will initially maintain a stable state. With constant fluid injection for forty years at six geothermal wells in the Rongcheng deep geothermal reservoir, the maximum superposed excess pore pressure at a single well is 18 MPa; this excess pore pressure value impacts the stress state of faults within 8 km of the well location. These pore pressure perturbations heavily impact the F5-10, F5-11, and F9-2 segments of the Rongcheng uplift boundary fault, with FSP values of 92%, 23%, and 47% in 2060, respectively. Porosity exacts little impact on the fault slip potential on the boundary fault segments of F5-10 and F9-2 in the Rongcheng deep geothermal reservoir, while an enhanced permeability can weaken the FSP values for these faults. The predicted maximum moment magnitude of an induced earthquake due to continuous injection of forty years can be up to M_w 5.0 with a 5% fluid loss in the Rongcheng deep geothermal reservoir. Long-term water injection may increase the ambient thermoelastic stress to the point where faults in a critical (or subcritical) stress state become unstable. The results can provide a reference for geothermal development in terms of injection rate and locations of geothermal wells.

The hydrogeochemical characteristics of geothermal fluids can reveal the genesis of geothermal systems and act as important references for developing and using geothermal resources. This study presents hydrogeochemical processes and thermal cycle mechanisms of typical geothermal fields in Western Sichuan. Based on the geological conditions in Western Sichuan, 29 hot springs in three geothermal fields in the Batang and Litang areas were selected for hydrochemical and isotopic (δD and δ¹⁸O) analyses. Furthermore, the temperature of the thermal reservoir was calculated and the upflow cooling process of the hot springs was analyzed. Most of the subterranean hot waters in Batang and Litang are of the HCO₃‐Na hydrochemical type. The ion variation in Batang is primarily affected by water‐rock interactions. There is a strong positive correlation between Na⁺, B⁻, and Cl⁻ in Litang, suggesting that they have the same material source. The Na⁺ and metaboric acid content is relatively high, which indicates that the groundwater runoff in both areas is relatively long‐lasting, with reduced flow velocity; moreover, the metasilicic acid content is relatively high, which supports this conclusion. Both hydrogen and oxygen isotopes plot near the atmospheric precipitation line, indicating that groundwater recharge is functionally obtained from precipitation. The calculated thermal storage temperatures in Batang and Litang were 88–199°C and 96–154°C, respectively. The proportion of cold water mixing in Batang was 64%–67%, while that in Litang was 60%–68%. According to the calculated results, the initial thermal cycle depth of the Batang area (4540–4780 m) was greater than that of the Litang area (3150–3960 m). The enthalpy of the deep parental geothermal fluid in Batang was 1550 J/g with a Cl⁻ concentration of 37 mg/L, while that in Litang was 2100 J/g with a Cl⁻ concentration of 48 mg/L.