High-temperature Geothermal Systems Research Articles

Heat flow, thermal anomalies, tectonic regimes and high-temperature geothermal systems in fault zones

Heat flow, thermal anomalies, tectonic regimes and high-temperature geothermal systems in fault zones

Representative High-Temperature Hydrothermal Activities in the Himalaya Geothermal Belt (HGB): A Review and Future Perspectives

Southern Tibet and western Yunnan are areas with an intensive distribution of high-temperature geothermal systems in China, as an important part of the Himalayan Geothermal Belt (HGB). In recent decades, China has conducted systematic research on high-temperature geothermal fields such as Yangbajing, Gudui, and Rehai. However, a comprehensive understanding has not yet been formed. The objective of this study was to enhance comprehension of the high-temperature geothermal system in the HGB and to elucidate the hydrogeochemical characteristics of geothermal fluids. This will facilitate the subsequent sustainable development and exploitation of domestic high-temperature hydrothermal geothermal resources. To this end, this study analysed geothermal spring and borehole data from the Yangbajing, Gudui, and Rehai geothermal fields. Based on previous research results, the source, evolution, and reservoir temperature characteristics of geothermal fluids are compared and summarised. The main high-temperature geothermal water in the geothermal field is derived from the deep Cl-Na geothermal fluid. Yangbajing’s and Gudui’s geothermal waters are primarily recharged by snow-melt water, while Rehai’s geothermal water is mainly recharged by local meteoric water. The average mixing ratios of magmatic water in the Yangbajing, Gudui, and Rehai geothermal fields are 17%, 21%, and 22%, respectively. The Yangbajing and Gudui geothermal fields have a relatively closed geological environment, resulting in a stronger water–rock interaction compared to the Rehai geothermal field. As geothermal water rises, it mixes with shallow cold water infiltration. The mixing ratios of cold water in the Yangbajing, Gudui, and Rehai geothermal fields are 60–70%, 40–50%, and 20–40%, respectively. Based on the solute geothermometer calculations, the maximum geothermal reservoir temperatures for Yangbajing, Gudui, and Rehai are 237 °C, 266 °C, and 282 °C, respectively. This study summarises and compares the hydrogeochemical characteristics of three typical high-temperature geothermal fields. The findings provide an important theoretical basis for the development of high-temperature geothermal resources in the Himalayan Geothermal Belt.

Soil CO2 fluxes measured in the Acoculco Geothermal System, Mexico: Baseline emissions from a long-term prospection programme

The Acoculco Caldera Complex is considered a promissory hidden high-temperature geothermal system in Mexico. To support the geothermal prospection of this anomalous area, a comprehensive programme of soil CO2 flux measurements was performed. A long-term measurement programme was conducted to determine the baseline of natural soil CO2 effluxes. Significant efforts were devoted both to measuring the CO2 fluxes between 2015 and 2022 and interpreting their origin. Eighteen soil gas surveys of CO2 were carried out by using the accumulation chamber method. >1200 diffuse CO2 fluxes were measured in six different areas of the Acoculco Caldera. Two areas (Los Azufres and Alcaparrosa) exhibited cold degassing sites, acid-sulphate springs, and gas bubbling in surface water bodies. The soil CO2 fluxes ranged from 1 to 26,000 g m−2 d−1, whereas lower fluxes <29 g m−2 d−1 were determined as the degassing baseline. A total CO2 output of 492 t d−1 km−1 was estimated using an integrated SGS-GSA approach, where the highest total soil CO2 fluxes were obtained for Alcaparrosa (299 t d−1 km−2) compared with Los Azufres (164 t d−1 km−2), and Surroundings (29 t d−1 km−2). Such results agree well with those values measured in other worldwide volcanic and active geothermal ecosystems. The range of CO2 isotopic composition values from −28.83 ‰ to −3.11 ‰, together with their statistical distribution, suggests multiple CO2 production sources feeding soil degassing. The combined interpretation of flux and isotopic data allowed us to identify two distinct gas sources: endogenous and biogenic. The present study highlights the importance of using soil CO2 monitoring to determine baseline emissions at the early exploration stage of geothermal systems.

Geophysical imaging of the active magmatic intrusion and geothermal reservoir formation beneath the Corbetti prospect, Main Ethiopian Rift

SUMMARY Silicic volcanic complexes in the Main Ethiopian Rift (MER) system host long-lived shallow magma reservoirs that provide heat needed to drive geothermal systems. Some of these geothermal systems in Ethiopia appear to be suitable for green and sustainable electricity generation. One such prospect is located at the Corbetti volcanic complex near the city of Awassa. High-resolution imaging of the subsurface below Corbetti is of imminent importance, not only because of its geothermal potential, but also due to reported evidence for an ongoing magmatic intrusion. In this study, we present a new subsurface 3-D electrical conductivity model of Corbetti obtained through the inversion of 120 magnetotelluric stations. The model elucidates a magmatic system under Corbetti and reveals that it is linked to a magma ponding zone in the lower crust. Magma is transported through the crust and accumulates in a shallow reservoir in form of a magmatic mush at a depth of $\gtrapprox 4\, {\rm km b.s.l.}$ below the caldera. The imaged extent and depth of the shallow magma reservoir is in agreement with previous geodetic and gravimetric studies that proposed an ongoing magmatic intrusion. Interpreting our model with laboratory-based conductivity models for basaltic and rhyolitic melt compositions suggests that Corbetti is seemingly in a non-eruptible state with ∼6–16 vol. per cent basaltic melt in the lower crust and ∼20–35 vol. per cent rhyolitic melt in the upper crust. With these observations, Corbetti’s magmatic system shares common characteristics with volcanic complexes found in the central MER. Specifically, these volcanic complexes are transcrustal two-stage magmatic systems with magma storage in the lower and upper crust that supply heat for volcano-hosted high-temperature geothermal systems above them. According to the presented subsurface model, a cross-rift volcano-tectonic lineament exerts first-order controls on the magma emplacement and hydrothermal convection at Corbetti. Our study depicts hydrothermal convection pathways in unprecedented detail for this system and helps identify prospective regions for future geothermal exploration. 3-D imaging of both the Corbetti’s magmatic and associated geothermal systems provides key information for the quantitative evaluation of Corbetti’s geothermal energy potential and for the assessment of potential volcanic risks.

Lithology controls on the mixing behavior and discharge regime of thermal groundwater in the Bogexi geothermal field on Tibetan Plateau

Geothermal springs within the same geothermal system are naturally considered with similar output manifestation, but usually distinct. A low temperature geothermal spring group (Bogexi) on eastern Tibetan Plateau was investigated and compared with the boiling geothermal spring group (Rekeng) in the same fault zone to get insights into the reasons behind. The Bogexi geothermal springs have quite lower output temperature (25.0–65.3℃) than the boiling Rekeng geothermal springs (84.5–90.0℃). Although all these two geothermal spring groups have the same hydrochemical facies of HCO3-Na, the Bogexi geothermal springs present lower concentration of Na+ and trace elements. The geothermal waters of Bogexi were originated from the precipitation and glacier/snow meltwater of the eastern Haizi mountains with the recharge elevation of 4,400–4,700 m. These recharged waters infiltrate into the geothermal system that with the reservoir temperature of 160-180℃ and are heated by tectonic deformation heat in the depth of 3,100–3,400 m. Lithology determines the fissures development in the shallow and furtherly controls the mixing behaviors and discharge characteristics of geothermal springs in the discharge area. As a result, the thermal groundwaters of Bogexi are mixed with great quantity of shallow cycling cold water with the mixing proportion averaging at 73–76 % during the ascending process, leading to their lower output temperatures and distinct hydrochemical and isotopic features from the boiling Rekeng geothermal springs. A conceptual model was proposed to portray the genesis of low temperature geothermal springs in high temperature geothermal system. The recharged water percolates downward and is heated in the deep thermal reservoir. Heated waters are enriched of chemical elements during this process and then flow upwards under the driving of hydraulic pressure. While the lithology determines the circulation of cold water in the discharge area. Karst lithology, usually with developed fissures and channels, can provide more favorable conditions for cold water cycling in the shallow part than other lithology. This leads to considerable cold water mixed into the upward-flowing geothermal groundwater, and differs these geothermal springs from other ones in the same fault zone in output temperature and hydrochemcial features. This research can improve the understanding of formation mechanism of output discrepancies of various springs within one geothermal system.

Environmental impact and thermodynamic comparative optimization of CO2-based multi-energy systems powered with geothermal energy

The continuous increase in global population and energy demands has enhanced the use of fossil fuels. The evident climate change effect in recent years has brought about the necessity to reduce fossil fuels. Hence, there exists an aggregate problem of meeting energy demands and reducing carbon emissions. Renewable energy sources have been proffered as the probable solution; however, multi-energy systems are effective options/alternatives to solving this problem. In recent literature, geothermal energy has been proposed as a renewable energy source that can continuously meet energy demands. However, there exists a significant gap in literature about the most viable temperature range for geothermal energy applications in multi-energy systems. In this study, two innovative CO2-based systems namely, high-temperature geothermal multi-energy system (HTGMES) and low-temperature geothermal multi-energy system (LTGMES) are designed, modelled, analyzed, and compared using a thermodynamic approach. While the HTGMES is modelled to predominantly use CO2 as a working fluid, a novel modified absorption system is integrated with the LTGMES. The two systems are modelled to produce electricity, cooling/refrigeration effect, space heating, hydrogen, and hot water. The energy, exergy environmental, exergy destruction, and exergoeconomic methodology is used to evaluate the performance of the innovative GMESs. Also, multi-objective optimization of the HTGMES and the LTGMES is carried out in a bid to minimize the total product unit cost and maximize overall exergy efficiency. The environmental impact analysis of the proposed system is presented considering CO2 mitigation. The results from analyses showed that the overall energetic and exergetic efficiencies in steady state are 44. 22 % and 33.5 % for the HTGMES and 45.40% and 32.9 % for the LTGMES. The optimized LCOE, LCOC, and LCOH based on the total unit cost are 0.0573 $/kWh, 0.1833 $/kWh, and 11.59 $/kg for the HTGMES; 0.0323 $/kWh, 0.0032 $/kWh, and 11.9$/kg for the LTGMES. Furthermore, the optimized exergetic performance showed that the LTGMES can achieve as high as 64.54 % and 40.96 % energy and exergy efficiency.

Unraveling the Genesis of the Geothermal System at the Northeastern Edge of the Pamir Plateau

High-temperature geothermal systems hold promise for sustainable and environmentally friendly power generation. However, China’s geothermal power capacity significantly underutilizes its abundant resources. This study focuses on the geothermal potential of the Pamir Plateau, particularly its northeastern edge, where complex tectonic forces converge. We aim to unveil the mechanisms driving the emergence of high-temperature geothermal reservoirs in this unique geological setting. Hydrogeochemical analysis reveals diverse profiles in geothermal water, primarily derived from atmospheric precipitation. Estimation of reservoir temperatures and simulation of geotherms unveil distinct geothermal systems. Kongur exhibits a medium–low-temperature hydrothermal system and Tashkurgan demonstrates high-temperature hydrothermal system characteristics, while the Pamir’s northeastern edge hints at a potential high-temperature dry geothermal system where there might not be a fault. These findings have important implications for sustainable energy development and future geothermal exploration.

3D inversion of multifrequency controlled-source electromagnetic data and its application to geothermal exploration in the Tianzhen region of the northern Datong Basin, China

Owing to its sensitivity to hydrothermal alteration and geothermal fluids, the distribution of subsurface electrical conductivity provides critical information for characterizing geothermal systems. For geothermal exploration, the controlled-source electromagnetic (CSEM) technique provides a crucial tool to image the subsurface resistivity structures, especially in areas with strong cultural noise. Usually, land-based CSEM surveys are carried out with dozens of operating frequencies to enhance spatial resolution. However, the 3D inversion of multifrequency CSEM data using the commonly used direct solver is challenging with limited computational resources. In this study, we implement a practical inversion strategy for interpreting 3D multifrequency CSEM data. By combining a hybrid direct-iterative solver with the inexact Gauss-Newton optimization, 3D inversion of CSEM data involving multiple frequencies can be performed effectively on typical workstations. First, we test the effectiveness of the developed approach using synthetic multifrequency 3D CSEM data generated for a simplified geothermal model. The inversion strategy is then applied to the multifrequency CSEM field data collected for the geothermal exploration at Tianzhen region in Shanxi Province, China. The comparison between the inversion results using the sparse data set (six frequencies) and the dense data set (12 frequencies) highlights the necessity of using data from more frequencies in the inversion to improve the resolution. The resulting 3D resistivity model clearly delineates the clay alteration layers and shallow thermal reservoirs of the potential high-temperature geothermal system within the study area, while its deep heat source is not revealed owing to the limited investigation depth of the survey.

ღრმად განლაგებული სითხის გეოთერმული პირობები კოლხეთის არტეზიულ აუზში: წყლის ქიმიური შედგენილობა და გეოთერმომეტრია

Among the most important issues to be characterized is the temperature of the aquifer in any geothermal system. Fluids located deep within a hydrothermal system provide heat, and they control thermal groundwater temperature changes in aquifers. Geothermometers based on water chemistry have been used to estimate groundwater temperatures in various complexes. However, the average temperature measured by hydrochemical geothermometers is often variable, and it is difficult to assign a corresponding average geothermal fluid temperature because temperature and pressure change as the geothermal fluid moves upward. Different geothermometers cover different temperature ranges. Some hydrochemical geothermometers are suitable for high temperature geothermal systems, while others are suitable for medium and low temperature. Based on these geothermometers, it is difficult to assign a temperature that is close to the actual temperature of the geothermal system, because the influence of atmospheric precipitation, which causes a decrease in the concentration of chemical components and dilution due to mixing with shallow groundwater.

Electrical structure of Gulu geothermal field in Southern Tibet and its implication for the high-temperature geothermal system

The Yadong-Gulu rift (YGR) South Tibet is a Cenozoic active rift, which is endowed with abundant geothermal resources. The Gulu geothermal field (GGF) is located in the Northern section of the rift, where a large number of high-temperature hot springs develop, but its geothermal system is mysterious. In this study, the three-dimensional (3D) electrical structure of GGF is revealed by broad magnetotelluric (MT) and audio magnetotelluric (AMT). MT reveals that middle and upper crust conductors are developed in the subsurface of GGF. The conductors may originate from the partial melting that drives the geothermal system. AMT reveals that the electrical structure of GGF is conductive alternation cap overlying more resistive reservoir, which is consistent with the classical electrical structure of geothermal systems in worldwide active tectonic zones. According to the geothermal system model, cold fluids may converge from the periphery of GGF to the middle, wherein fault F1 (the Western branch of Jiulazi-Sanxung fault) may be the main channel for cold fluids to migrate downward. The fluids are heated by partial melting in the middle and upper crust, and may migrate upward along fault F2 (the middle branch of Jiulazi-Sanxung fault) and develop into heat reservoirs.

Factors controlling the distribution of granite reservoirs of hydrothermal system type in South China: A case study of Huangshadong geothermal field in Yuezhong Depression, China

The granitoids widely distributed in South China are characterized by multi-stage evolution via episodic intrusions, in a complex geodynamic setting. Since granites have high radioactive heat generation and excellent thermal conductivity, a deep moderate- to high-temperature geothermal system can be formed in the presence of high-quality, fissured granite geothermal reservoirs and thermal insulation with appropriate cap rocks. The key to exploring deep geothermal resources is to identify high-quality fissured granite geothermal reservoirs of a certain scale in a thermal anomaly zone with high background heat-flow values. To determine the controlling effects of the distribution and development characteristics of granite geothermal reservoirs on the generation and enrichment of deep geothermal resources, this study analyzed the characteristics of the geothermal reservoirs in the Huangshadong geothermal field in the Yuezhong Depression, Guangdong Province, and their controlling effects on the formation of geothermal resources. The results are as follows. The hydrothermal system in the Huangshadong geothermal field mainly distributed in the contact zones between magmatic plutons and surrounding rocks, is significantly controlled by faults, followed by neoid volcanic apparatus and magmatic activities. That is, the geothermal system therein is under joint control of structures and magmas. Moreover, fractured zones of neoid transtensional faults conduct the geothermal water in the hydrothermal system and control its shallow discharge. Therefore, the hydrothermal system in the study area is characterized by the control of transpressional tectonic zone and volcanic apparatus, and geothermal water conduction through fractured zones of transtensional faults.

The Possible Source of Abnormally High Sodium Content in Low-Salinity Geothermal Water.

The dissolution of sodium-containing minerals in high-temperature geothermal systems can cause Na+ to exceed 400 mg/L with high salinity. But the Na+ of low-salinity geothermal water is mostly less than 100 mg/L in medium-low temperature geothermal systems. However, geothermal water with Na+ up to 325.4mg/L and total dissolved solids less than 650 mg/L was found in the Huangshadong geothermal field, which is a typical medium-low temperature hydrothermal system in South China. The water chemistry results indicate that thermal groundwater is uniformly HCO3 -Na type with high sodium content (average 240.06 mg/L). All the thermal groundwater and shallow groundwater have the same meteoric origin based on δD and δ18 O. According to water chemical geothermometers and multicomponent mineral equilibrium (MME) method, the reservoir temperature is estimated to be 100 to 130 °C at a maximum depth of 2.43 km. The estimation of the Cl- mixed indicator suggests that geothermal water has mixed with 51% to 72% of shallow groundwater, resulting in the reduction of Na+ content in real geothermal water (Na+ up to 685.2mg/L). The simulated results of water-rock interactions indicate that mineral dissolution and ion exchange have minor contributions to Na+ enrichment in geothermal water. Hydrochemical simulations and Gibbs diagrams suggest an additional source of high sodium: granite fluid inclusions are fractured into geothermal water at high temperatures. Granite fluid inclusions may only account for 3% to 5% of geothermal water, but they provide the main source of Na+ in geothermal water.

An evaluation framework for production performance of high-temperature fractured and karstified geothermal reservoirs: Production mechanism, sensitivity study, and key parameters ranking

High-temperature (>150 °C) geothermal resources have great potential for sustainable power generation. These geothermal resources can be exploited efficiently through highly permeable geological structures, i.e. tectonic fractures and dissolution caves, which thus are categorized as fractured and karstified geothermal reservoirs. However, the study of long-term cold fluids injection into these high-temperature geothermal systems is faced with significant unsolved challenges, i.e. the simulation of coupled porous media flow and free flow in multiscale and heterogeneous fracture-cave networks with distinct thermo-hydro-mechanical (THM) coupling effects. To address these challenges, we develop a novel coupled THM model based on the discrete fracture-cave network approach. We then apply this model to perform the analysis of THM coupling mechanism, sensitivity study, and key parameters ranking to identify the development characteristics of high-temperature geothermal reservoirs. Results show that with respect to coupled THM processes, the fluid flow and heat transfer are dominated by connected fracture clusters and are strongly influenced by the inclusion of caves. However, the behavior of solid deformation is governed by the distribution of caves. As to the reservoir thermal performance under different working fluids, i.e. water and CO2, the water-based geothermal reservoir exhibits longer service life, greater global thermal power, and higher heat extraction rate than the CO2-based one. Regarding sensitivity analysis and key factors ranking, fracture attributes (fracture length and density) are the two most important parameters in regulating the geothermal extraction performance. Caves fall out as the third most significant parameter. However, the operating parameters (i.e. injection temperature, injection pressure, production pressure, well length, and well distance) play a limited role in influencing thermal performance compared with geometrical parameters of fractures and caves. The developed model and insights obtained from our research have crucial implications for understanding and optimizing the thermal production of high-temperature fractured and karstified geothermal reservoirs.

Hydrogeochemical Characteristics and Genesis of the Chazi Geothermal Field Area in Tibet

The Chazi geothermal field area is located in the large region of Shigatse in southern Tibet. The geothermal resources in this area are abundant, but their exploitation and utilization are low. By studying the water chemistry and isotope characteristics of geochemical fluids in the study area, information on water chemistry, heat storage temperature, recharge source, recharge elevation, and circulation depth was obtained. These results provide a scientific theoretical basis for improving the genetic mechanism of high-temperature geothermal systems in the study area. The type of geothermal fluid hydrochemicals in this area is mainly HCO3–Na. The isotopic geochemical method was used to determine that the recharge source of geothermal fluids was atmospheric precipitation, and the recharge elevation was 5200–6000 m. The geochemical thermometer, Na–K–Mg equilibrium diagram, and silica-enthalpy mixed model indicated the shallow and deep thermal storage temperatures of approximately 150 and 200°C, respectively, and the average circulation depth of 1163.38 m in the study area. Combined with the fracture structure and magmatic activity characteristics of the southwest Qinghai-Tibet Plateau, the source, storage, cover, and general situation of the area were preliminarily summarised, and the conceptual model of geothermal origin was established. The results can provide a scientific theoretical basis for the mechanism of high-temperature geothermal systems and subsequent drilling and resource development.

Study on the genetic mechanism of high-temperature geothermal system and its engineering impact in the Woka graben, Tibet

The geothermal resource has become the significant constitution of renewable and clean energies in the world. This study focuses on the genetic mechanism of a high-temperature geothermal system and its engineering impact in the Woka graben, southern Tibet, via hydrochemical and isotopic analyses. The hydrochemical types are mainly SO4-Na type, SO4-Cl-Na type, and HCO3-SO4-Na type. Geothermal water is characterized as medium to alkaline affinity with low total dissolved solids. D-O isotopes indicate that geothermal water is recharged by atmospheric precipitation at the elevation of 5193–5247 m. Na-K-Mg equilibrium diagram shows partial equilibrium or mixed water, and the proportion of cold water mixing is 73–83%. The temperature ranges of shallow and deep geothermal reservoirs are from 96.85°C to 119.57°C and from 120°C to 200°C, respectively. Geothermal water is heated by melting crust and controlled by deep faults. For major construction projects in the Woka graben, detailed investigation and demonstration should be conducted to avoid the geothermal water channel as much as possible, or to divert the geothermal water and reasonably arrange the construction sequence to overcome the problem.

Structural Controls of High-Temperature and Deep Geothermal Systems Associated with Detachment Fault

In this study, the structural controls of a high-temperature system (188 to 287 oC) associated with the detachment fault in Gediz Graben generating approximately half of this electricity generation in Turkey which is in the top five of the world in the generation of electricity from geothermal resources are examined. &#x0D; The high-temperature geothermal reservoirs associated with detachment fault in the study area have formed along the normal faults nearly in E-W orientation and in intensely fractured rocks in a structurally complex manner associated with transfer faults approximately in N-S orientation. Generally, it has been determined that the reservoirs are closely located to strike-slip faults intersecting, overlapping, and/or terminated with Quaternary transfer faults.

Imaging the heat source of the Kangding high-temperature geothermal system on the Xianshuihe fault by magnetotelluric survey

The Kangding geothermal field along the Xianshuihe fault is a typical high-temperature geothermal system with an enormous capacity for geothermal power generation. The Xianshuihe fault is a major active sinistral strike-slip fault in eastern Tibet Plateau, where the fault splits into three parallel branches near Kangding. We collected data from 17 magnetotelluric stations in the Kangding geothermal system. After data processing, the dimensionality of the subsurface structure was qualitatively estimated using the phase tensor. The electrical resistivity model inverted with ModEM shows the Kangding geothermal system from the surface to a depth of 20 km, which correlates closely with the near-surface geology. The synthetic tests confirmed that the main resistivity anomalies identified in the electrical resistivity model were robust. There are several significant low resistivity anomalies of less than 10 Ωm beneath the Kangding geothermal system. The cap rock and liquid-dominated reservoir can be interpreted as shallow conductive zones. The conductors beneath the Kangding geothermal field at a depth of 10 km are interpreted as an underlying zone of partial melt, which provides fluids and heat for the geothermal system. The partial melt fractions are in the range of 4–19%, confirming that there is a magmatic heat source beneath the Kangding geothermal system.

ISOTOPE 18O OF ALTERATION ROCKS AT WELL C-1 KAMOJANG GEOTHERMAL FIELD, WEST JAVA, INDONESIA

The purpose of this study is to determine the reservoir condition system of Well C-1. Kamojang geothermal field located in the Quaternary volcanic complex caldera system (0.452 to 1.2 Ma). The volcanic complex contributes to the formation of vapor-dominated high-temperature geothermal systems (250oC). The potential of the electricity produced from this field was 260 MWe. This study describes a new method, namely the 18O isotope of alteration rock, to determine reservoir conditions. Another approach is to use a petrographic analytical form, X-ray diffraction, and isotope 18O and D on some core and cutting samples. Isotope abundances are measured using an isotope ratio mass spectrometer (IRMS). Zoning mineral alteration in Well C-1 comprises the cristobalite-montmorillonite zone, illite-montmorillonite zone, and chlorite-epidote zone. Hydrothermal fluids that generate steam in geothermal systems were derived from meteoric water. These fluids interact with rocks based on the ratio of reservoir water-rock interaction, Well C-1. The research shows that the value of the fraction of oxygen atoms of rocks in the system (1-x) was 0.16. That indicated that the reservoir containing fluid 16% or reservoir is rock-dominated. This research on 18O isotope is a novelty because it differs from what has been done by previous researchers who studied reservoir characteristics using water 18O isotope. Meanwhile, this study uses the 18O isotope of alteration rock to identify reservoir conditions. It is hoped that this method can be used in other geothermal fields.

Meso-Mechanical Characteristics of Granite with Natural Cracks after Mud Acid Corrosion

Most of the discovered high-temperature geothermal energy systems are often related with granite that is characterized by natural faults, fractures and cracks of different size. However, the porosity and permeability of the granite matrix is very low, greatly limiting the efficiency of heat extraction in granitic rock. Chemical stimulation, which is regarded as one of the most important means of reservoir stimulation, has consequently received more and more attention. In this paper, a Triassic granite obtained from the eastern region of Liaoning Province in China was reacted with three different concentration of mud acid solution (8% HCl + 1% HF, 10% HCl + 2% HF, 12% HCl + 3% HF) and the resulting microstructure changes studied by scanning electron microscopy (SEM) and nuclear magnetic resonance (NMR). The experimental results show that the number of micropores in the granite increases after chemical corrosion by mud acid solution. A higher mud acid solution concentration results in a much higher pore volume. Triaxial compression tests on the granite before and after chemical corrosion were carried out to study the effect of acidification on the mechanical characteristics of granite, showing that the peak stress and elastic modulus of granite decreases 25.7% and 16.5%, respectively, after exposure to mud acid solution (12% HCl + 3% HF) corrosion for three weeks at room temperature. The particle flow program PFC2D based on discrete element method was used to investigate the mechanical response before and after the chemical corrosion. Considering that the granite is rich in microcracks, the study is simplified by considering them all grouped into one main closed fracture. The influences of main crack inclination angle, crack length, friction coefficient and confining pressure on the mechanical response were investigated. Under the triaxial compression loading state, wing cracks appear at the initial crack tip, then secondary cracks begin to appear. The sensitivity analysis shows that three characteristic strengths (crack initiation strength, damage strength and peak strength) are strongly correlated with crack length, crack inclination angle, crack surface friction coefficient and confining pressure. These three characteristic strengths decrease 60%, 59% and 53%, respectively, compared with their initial values with the increase of main crack length from 6 mm to 22 mm, while they present positive correlation with the fracture friction coefficient from 0 to 1.0 and confining pressure from 10 to 50 MPa. There is a critical inclination angle of the main crack (i.e., 45°), meaning that these three characteristic strengths of granite decrease with inclination angles smaller than 45°, while they increase with an inclination angle larger than 45°. After the corrosion effect of mud acid solution on granite, the pore structure was changed and mechanical properties was damaged, which further affect the failure mode and failure process of granite samples affected by mud acid solutions. This paper provides a theoretical reference for evaluating the effect of chemical stimulation technology on the mechanical characteristics of granite, serving for the continuous hydraulic stimulation design after the chemical stimulation.

3-D Joint Inversion of MT and CSEM Data for Imaging a High-Temperature Geothermal System in Yanggao Region, Shanxi Province, China

3-D Joint Inversion of MT and CSEM Data for Imaging a High-Temperature Geothermal System in Yanggao Region, Shanxi Province, China