Conventional Geothermal Systems and Unconventional Geothermal Developments: An Overview

This paper provides an overview of conventional geothermal systems and unconventional geothermal developments as a common reference is needed for discussions between energy professionals. Conventional geothermal systems have the heat, permeability and fluid, requiring only drilling down to °C, normal heat flow or decaying radiogenic granite as heat sources, and used in district heating. Medium-temperature (MT) 100°C - 190°C, and high-temperature (HT) 190°C - 374°C resources are mostly at plate boundaries, with volcanic intrusive heat source, used mostly for electricity generation. Single well capacities are °C - 500°C) and a range of depths (1 m to 20 Km), but lack permeability or fluid, thus requiring stimulations for heat extraction by conduction. HVAC is 1 - 2 m deep and shallow geothermal down to 500 m in wells, both capturing °C, with °C are either advanced by geothermal developers at <7 Km depth (Enhanced Geothermal Systems (EGS), drilling below brittle-ductile transition zones and under geothermal fields), or by the Oil & Gas industry (Advanced Geothermal Systems, heat recovery from hydrocarbon wells or reservoirs, Superhot Rock Geothermal, and millimeter-wave drilling down to 20 Km). Their primary aim is electricity generation, relying on closed-loops, but EGS uses fractures for heat exchange with earthquake risks during fracking. Unconventional approaches could be everywhere, with shallow geothermal already functional. The deeper and hotter unconventional alternatives are still experimental, overcoming costs and technological challenges to become fully commercial. Meanwhile, the conventional geothermal resources remain the most proven opportunities for investments and development.

Effects of fracture evolution and non-Darcy flow on the thermal performance of enhanced geothermal system in 3D complex fractured rock

In fractured geothermal reservoirs,the fracture networks and internal fluid flow behaviors can significantly impact the thermal performance.In this study,we proposed a non-Darcy rough discrete fracture network(NR-DFN)model that can simultaneously consider the fracture evolution and non-Darcy flow dynamics in studying the thermo-hydro-mechanical(THM)coupling processes for heat extraction in geothermal reservoir.We further employed the model on the Habanero enhanced geothermal systems(EGS)project located in Australia.First,our findings illustrate a clear spatial-temporal variation in the thermal stress and pressure perturbations,as well as uneven spatial distribution of shear failure in 3D fracture networks.Activated shear failure is mainly concentrated in the first fracture cluster.Secondly,channeling flow have also been observed in DFNs during heat extraction and are further intensified by the expansion of fractures driven by thermal stresses.Moreover,the combined effect of non-Darcy flow and fracture evolution triggers a rapid decline in the resulting heat rate and temperature.The NR-DFN model framework and the Habanero EGS's results illustrate the importance of both fracture evolution and non-Darcy flow on the efficiency of EGS production and have the potential to promote the development of more sustainable and efficient EGS operations for stakeholders.

A combined method using Lattice Boltzmann Method(LBM)and Finite Volume Method(FVM)to simulate geothermal reservoirs in Enhanced Geothermal System(EGS)

With the development of industrial activities,global warming has accelerated due to excessive emission of CO_(2).Enhanced Geothermal System(EGS)utilizes deep geothermal heat for power generation.Although porous medium theory is commonly employed to model geothermal reservoirs in EGS,Hot Dry Rock(HDR)presents a challenge as it consists of impermeable granite with zero porosity,potentially distorting the physical interpretation.To address this,the Lattice Boltzmann Method(LBM)is employed to simulate CO_(2)flow within geothermal reservoirs and the Finite Volume Method(FVM)to solve the energy conservation equation for temperature distribution.This combined method of LBM and FVM is imple-mented using MATLAB.The results showed that the Reynolds numbers(Re)of 3,000 and 8,000 lead to higher heat extraction rates from geothermal reservoirs.However,higher Re values may accelerate thermal breakthrough,posing challenges to EGS operation.Meanwhile,non-equilibrium of density in fractures becomes more pronounced during the system's life cycle,with non-Darcy's law becoming significant at Re values of 3,000 and 8,000.Density stratification due to buoyancy effects significantly impacts temperature distribution within geothermal reservoirs,with buoyancy effects at Re=100 under gravitational influence being noteworthy.Larger Re values(3,000 and 8,000)induce stronger forced convection,leading to more uniform density distribution.The addition of proppant negatively affects heat transfer performance in geothermal reservoirs,especially in single fractures.Practical engineering considerations should determine the quantity of proppant through detailed numerical simulations.

The role of temperature‐enhanced fault closure in promoting postinjection pressure diffusion and seismicity in enhanced geothermal systems

Post shut‐in seismic events in enhanced geothermal systems(EGSs)occur predominantly at the outer rim of the co‐injection seismic cloud.The concept of postinjection fracture and fault closure near the injection well has been proposed and validated as a mechanism for enhancing post shut‐in pressure diffusion that promotes seismic hazard.This phenomenon is primarily attributed to the poro‐elastic closure of fractures resulting from the reduction of wellbore pressure after injection termination.However,the thermal effects in EGSs,mainly including heat transfer and thermal stress,may not be trivial and their role in postinjection fault closure and pressure evolution needs to be explored.In this study,we performed numerical simulations to analyze the relative importance of poro‐elasticity,heat transfer,and thermo‐elasticity in promoting postinjection fault closure and pressure diffusion.The numerical model wasfirst validated against analytical solutions in terms offluid pressure diffusion and against heatedflow‐through experiments in terms of thermal processes.We then quantified and distinguished the contribution of each individual mechanism by comparing four different shut‐in scenarios simulated under different coupled conditions.Our results highlight the importance of poro‐elastic fault closure in promoting postinjection pressure buildup and seismicity,and suggest that heat transfer can further augment the fault closure‐induced pressure increase and thus potentially intensify the postinjection seismic hazard,with minimal contribution from thermo‐elasticity.

Surface geochemical anomaly pattern of medium-to-high temperature geothermal systems in South China and its application

The existence of thermal storage will correspondingly increase the temperature of surrounding strata and promote the continuous expansion,volatilization,upward migration,and loss of gas in the strata.As a result,a low-concentration gas field will be formed in the strata above geothermal reservoirs.Geothermal reservoirs could in turn heat formation water and increase the solubility of soluble inorganic salts in the surrounding rocks and the total dissolved solids(TDS)content in the formation water.Since water can strongly wet and permeate strata,the dissolved inorganic salts migrate into upper strata along with water,giving rise to the formation of a high-concentration inorganic salt field in the strata above geothermal reservoirs.A higher geothermal reservoir temperature corresponds to more significant characteristics mentioned above.Therefore,a medium-to-high temperature geothermal system has a surface geochemical anomaly pattern of high inorganic salt concentrations and low gas concentrations(also referred to as the high-salt and low-gas pattern).This pattern is applied to the surface geochemical exploration of the two geothermal fields in Guangdong Province,i.e.,the Huangshadong geothermal field in Huizhou City and the Xinzhou geothermal field in Yangjiang City,revealing low-concentration gas fields above both.The application results also show that the exposed thermal spring water in both geological fields has higher concentration of dissolved inorganic salt than the surface water and nearby seawater,forming high-amplitude anomalies on the surface above geothermal reservoirs.These characteristics,as well as the measured temperature at known geothermal wells,verify the validity of the high-salt and low-gas pattern of medium-to-high temperature geothermal systems proposed in this study.Moreover,the high-salt and low-gas pattern proposed predicts three favorable medium-to-high temperature geothermal zones in the surface geochemical exploration of the Shiba Basin near the Huangshadong geothermal field.

Catalog of Enhanced Geothermal Systems based on Heat Sources

It is common sense that a deeper well implies higher temperature in the exploration of deep geothermal resources, especially with hot dry rock(HDR) geothermal resources, which are generally exploited in terms of enhanced geothermal systems(EGS). However, temperature is always different even at the same depth in the upper crust due to different heat sources. This paper summarizes the heat sources and classifies them into two types and five sub-types: crustorigin(partial melting, non-magma-generated tectonic events and radiogenic heat production), and mantle-origin(magma and heat conducted from the mantle). A review of global EGS sites is presented related to the five sub-types of heat sources. According to our new catalog, 71% of EGS sites host mantle-origin heat sources. The temperature logging curves indicate that EGS sites which host mantle-origin magma heat sources have the highest temperature. Therefore, high heat flow(>100 m W/m^(2)) regions with mantle-origin magma heat sources should be highlighted for the future exploration of EGS. The principle to identify the heat source is elucidated by applying geophysical and geochemical methods including noble gas isotope geochemistry and lithospheric thermal structure analysis. This analytical work will be helpful for the future exploration and assessment of HDR geothermal resources.

Comparative Study of Acid and Alkaline Stimulants with Granite in an Enhanced Geothermal System

The Enhanced Geothermal System(EGS) is an artificial geothermal system that aims to economically extract heat from hot dry rock(HDR) through the creation of an artificial geothermal reservoir. Chemical stimulation is thought to be an effective method to create fracture networks and open existing fractures in hot dry rocks by injecting chemical agents into the reservoir to dissolve the minerals. Granite is a common type of hot dry rock. In this paper, a series of chemical stimulation experiments were implemented using acid and alkaline agents under high temperature and pressure conditions that mimic the environment of formation. Granite rock samples used in the experiments are collected from the potential EGS reservoir in the Matouying area, Hebei, China. Laboratory experimental results show that the corrosion ratio per unit area of rock is 3.2% in static acid chemical experiments and 0.51% in static alkaline chemical experiments. The permeability of the core is increased by 1.62 times in dynamic acid chemical experiments and 2.45 times in dynamic alkaline chemical experiments. A scanning electron microscope analysis of the core illustrates that secondary minerals, such as chlorite, spherical silica, and montmorillonite, were formed, due to acid-rock interaction with plagioclase being precipitated by alkaline-rock interactions. Masking agents in alkaline chemical agents can slightly reduce the degree of plagioclase formation. A chemical simulation model was built using TOUGHREACT, the mineral dissolution and associated ion concentration variation being reproduced by this reactive transport model.

Discussion on heat source mechanism and geothermal system of Qinghai Gonghe-Guide Basin

The Qinghai Gonghe-Guide Basin together with the alternatively distributed mountainous region shows characteristics that the conductive geothermal resource of the basin has high geothermal gradient, the granite occurs in the bottom of borehole for geothermal exploration, and the convective hot springs in the basin-edge uplift fracture are in zonal distribution and with high-temperature geothermal water. There are still some divergences about the heat source mechanism of the basin. In this paper, queries to the view of mantle-derived heat source have been put forward, coming up with geochemical evidences to prove that the radiogenic heat of granite is the heat source within the mantle. Additionally, temperature curve is drawn based on the geothermal boring and geochemical geothermometer has been adopted for an estimation of the temperature and depth of the geothermal reservoir, it has been found that the surrounding mountains belong to the medium-temperature geothermal system while the area within the basin belongs to the high-temperature geothermal system with the temperature of borehole bottom reaching up to 175-180 ℃. In this paper, discussions on the problems existing in the calculation of geothermal gradient and the differences generated by the geothermal system have been carried out.

Thermo-economic Investigation of an Enhanced Geothermal System Organic Rankine Cycle and Combined Heating and Power System

As a potentially viable renewable energy, Enhanced Geothermal Systems(EGSs) extract heat from hot dry rock(HDR) reservoirs to produce electricity and heat, which promotes the progress towards carbon peaking and carbon neutralization. The main challenge for EGSs is to reduce the investment cost. In the present study, thermo-economic investigations of EGS projects are conducted. The effects of geofluid mass flow rate, wellhead temperature and loss rate on the thermo-economic performance of the EGS organic Rankine cycle(ORC) are studied. A performance comparison between EGS-ORC and the EGS combined heating and power system(CHP) is presented. Considering the CO_(2)emission reduction benefits, the influence of carbon emission trading price on the levelized cost of energy(LCOE) is also presented. It is indicated that the geofluid mass flow rate is a critical parameter in dictating the success of a project. Under the assumed typical working conditions, the LCOE of EGS-ORC and EGS-CHP systems are 24.72 and 16.1 cents/k Wh, respectively. Compared with the EGS-ORC system, the LCOE of the EGS-CHP system is reduced by 35%. EGS-CHP systems have the potential to be economically viable in the future. With carbon emission trading prices of 12.76 USD/ton, the LCOE can be reduced by approximately 8.5%.

Geothermal distribution characteristics and sedimentary basin geothermal system in the severe cold region of Northeast China

In this study,Shengli fault depression,Tangyuan fault basin,and northern Songliao Basin in Yitong‒Yilan fault zone of Heilongjiang province are considered the research areas for geothermal anomaly.Based on the temperature of the deep thermal reservoir,the hydrothermal fl uid channel,caprock thickness,and the mode of heat transfer,which are the main factors controlling the geothermal reservoir formation,we examined geothermal resource system of the underground HDR in this area.First,we inversed the aeromagnetic data,calculated the Curie isotherm depth,analyzed the geothermal distribution characteristics,and estimated the temperature of the deep heat source.Second,we applied the controlled source audio frequency magnetotelluric(CSAMT)and magnetotelluric(MT)methods to obtain the deep electrical structure of the study area.We determined the thickness of the caprock and the hydrothermal fluid channel.Finally,we obtained the borehole geothermal steady-state temperature measurement data and water sample chemical analysis data from the logging temperature curves of 24 wells to infer the mode of heat transfer.Based on the results,we built a model of the geothermal system of the sedimentary basin in this area.The results show that the depth of Curie isotherm in the study area is 17–39 km.The resistivity of sedimentary caprock in the north of Songliao basin is low,and there exists a deep heat source,which is mainly thermal convection.In contrast,in Shengli and Tangyuan fault basins,heat conduction is dominant.Based on the geothermal system model,we conclude that the area from Daqing to Lindian in Songliao basin has a thermal-convection-dominated sedimentary basin geothermal system.Heat exchange is realized by the upwelling of mantle-derived thermal materials through fracture channels.The thick sedimentary caprock reduces the heat loss.It can be a target for sustainable development and utilization of HDR.

O,H,and Sr isotope evidence for origin and mixing processes of the Gudui geothermal system,Himalayas,China

Tho Gudui geothermal field records the highest temperature at equivalent borehole depths among the lainland hydrothermal systems in China's Mainland.Located about 150 km southeast of Lhasa City,the capital of Tibet,the Gudui geothermal field belongs to the Sangri-Cuona rift belt,also known as the Sangri-Cuona geothermal belt,and is representative of the non-volcanic geothermal systems in the Himalayas.In this study,oxygen-18 and deuterium isotope compositions as well as 87Sr/86Sr ratios of water samples collected from the Gudui geothermal field were characterized to understand the origin and mixing processes of the geothermal fluids at Gudui.Hydrogen and oxygen isotope plots show both,deep and shallow reservoirs in the Gudui geothermal field.Deep geothermal fluids are the mixing product of magmatic and infiltrating snow-melt water.Calculations show that the magma fluid component of the deep geothermal fluids account for about 21.10%-24.04%;magma fluids lay also be a contributing source of lithium.The linear relationship of the 87Sr/86Sr isotopic ratio versus the 1/Sr plot indicates that shallow geothermal fluids form from the mixing of deep geothermal fluids with cold groundwater.Using a binary mixing model with deep geothermal fluid and cold groundwater as two end-members,the nixing ratios of the latter in most surface hot springs samples were calculated to be between 5% and 10%.Combined with basic geological characteristics,hydrogen and oxygen isotope characteristics,strontium concentration,87Sr/(86)Sr ratios,and the binary mixing model,we infer the 6 th-Class Reservoirs Evolution Conceptual Model(6-CRECM) for the Gudui geothermal system.This model represents an idealized summary of the characteristics of the Gudui geothermal field based on our comprehensive understanding of the origin and mixing processes of the geothermal fluid in Gudui.This study may aid in identifying the geothermal and geochemical origin of the Gudui high-temperature hydrothermal systems in remote Tibet of China,whose potential for geothermal development and utilization is enormous and untapped.

Multi-objective Optimization of Geothermal Extraction from the Enhanced Geothermal System in Qiabuqia Geothermal Field, Gonghe Basin

A geothermal demonstration exploitation area will be established in the Enhanced Geothermal System of the Qiabuqia field, Gonghe Basin, Qinghai–Xizang Plateau in China. Selection of operational parameters for geothermal field extraction is thus of great significance to realize the best production performance. A novel integrated method of finite element and multi-objective optimization has been employed to obtain the optimal scheme for thermal extraction from the Gonghe Basin. A thermal-hydraulic-mechanical coupling model(THM) is established to analyze the thermal performance. From this it has been found that there exists a contraction among different heat extraction indexes. Parametric study indicates that injection mass rate(Q_(in)) is the most sensitive parameter to the heat extraction, followed by well spacing(WS) and injection temperature(T_(in)). The least sensitive parameter is production pressure(p_(out)). The optimal combination of operational parameters acquired is such that(T_(in), p_(out), Q_(in), WS) equals(72.72°C, 30.56 MPa, 18.32 kg/s, 327.82 m). Results indicate that the maximum electrical power is 1.41 MW for the optimal case over 20 years. The thermal break has been relieved and the pressure difference reduced by 8 MPa compared with the base case. The optimal case would extract 50% more energy than that of a previous case and the outcome will provide a remarkable reference for the construction of Gonghe project.

Numerical simulation of operation performance on production and injection of a double well geothermal system in Kailu Basin, Inner Mongolia

Inner Mongolia is abundant in geothermal resources,but the development and utilization of medium-depth geothermal resources for clean heating in winter is still in the preliminary stage compared with the neighboring provinces.In this paper,a recently developed geothermal heating system using the Mesozoic sandstone reservoirs in Baokang of Kailu Basin,Eastern Inner Mongolia was investigated,a threedimensional geological model of a pair of production and injection well was established,and numerical simulations on the long term operation performance were conducted and verified by pumping test and water level recovery test data.The effects of flow rates,the direction of wells,injection temperature and ratios on the flow field and water level in the thermal reservoir were analyzed.The results show that considering a 30-year operation period and a production rate from 90 m^(3)/h to 110 m^(3)/h,the optimum well spacing can be increased from 225 m to 245 m,with an average value of 235 m.With the decrease of the injection temperature,the cold front of the injection water has an increasing influence on the temperature in the production well.A complete injection or the principle of production according to injection is recommended in order to maintain the long-term operation stability.In addition,the location of the injection well should be arranged in the downstream of the natural flow field.The present results can provide a useful guide for the optimum design and performance prediction of geothermal wells,thus maintaining the production and injection balance and promoting the sustainable development and utilization of medium-depth and deep geothermal resources.

Evaluation of the Factors Controlling Concentration of Non-Condensable Gases and Their Possible Impact on the Performance of Geothermal Systems: Case Study of Olkaria Wells in the Kenyan Rift Valley

The Olkaria geothermal field is located in the Kenyan Rift valley, about 120 km from Nairobi. Development of geothermal resources in the Olkaria area, a high temperature field, started in the early 1950s. In the subsequent years numerous expansions have been carried out with additional power plants being installed in Olkaria. These include a binary plant at Olkaria South West (Olkaria III) in 2000, a condensing plant at Olkaria North East (Olkaria II) in 2003, another binary plant at Olkaria North West (Oserian) in 2004 and finally condensing plants in the year 2014 within East production field (EPF) and Olkaria Domes (OD) areas. The total generation from this field is about 730 Mw. The study considered samples from 4 producing wells from 3 fields of the Olkaria geothermal area (OW-44 from the Olkaria East, OW-724A from the Olkaria North East, and OW-914 and OW-915 from the Olkaria Domes field). The chemical data were first analyzed using SOLVEQ. This helped in the determination of the equilibrium state of the system, the reservoir temperatures and the total moles to be run through CHILLER. The run CHILLER considered the processes that have been proven to be occurring in the Olkaria field i.e., boiling and condensing processes, fluid-fluid mixing rocks and titration resulting from water-rock interaction. The effects on gas evolution were evaluated based on the resulting recalculated gas pressures. The results indicate that the gas species are not in equilibrium with the mineral assemblages. The CHILLER evaluation shows boiling as the major process leading to the evolution of gases. OW-44 had the least gas concentrations, arising from the considered reservoir processes due to degassing, and near surface boiling, besides the removal of NH3, H2 and H2S are through the reaction with steam condensate. The gas breakout is most likely in OW-914 and least in OW-44. The study proposes different reservoir management strategies for the different parts of the Olkaria geothermal field. That is by increasing hot reinjection in the eastern sector around well OW-44. The reservoir around OW-914 is to be managed by operating the wells at a minimum flow rate (or even to close them) or the use of chemical inhibitors to prevent calcite scaling.

The Coupled Effects of Dryness and Non-condensable Gas Content of Geothermal Fluid on the Power Generation Potential of an Enhanced Geothermal System

The Enhanced Geothermal System(EGS) is a recognized geothermal exploitation system for hot dry rock(HDR), which is a rich resource in China. In this study, a numerical simulation method is used to study the effects of geothermal fluid dryness and non-condensable gas content on the specific enthalpy of geothermal fluid. Combined with the organic Rankine cycle(ORC), a numerical model is established to ascertain the difference in power generation caused by geothermal fluid dryness and non-condensable gas content. The results show that the specific enthalpy of geothermal fluid increases with the increase of geothermal fluid temperature and geothermal fluid dryness. If the dryness of geothermal fluid is ignored, the estimation error will be large for geothermal fluid enthalpy. Ignoring non condensable gas will increase the estimation of geothermal fluid enthalpy, so the existence of the non-condensable gas tends to reduce the installed capacity of a geothermal power plant. Additionally, both mass flow of the working medium and net power output of the ORC power generation system are increased with increasing dryness of geothermal fluid, however there is some impact of geothermal fluid dryness on thermal efficiency.

A New Combinational Terminology for Geothermal Systems

A geothermal resource can be defined as a reservoir inside the Earth from which heat can be extracted economically Geothermal resources are classified on the basis of different aspects, such as heat source, heat transfer, reservoir tem perature, physical state, commercial utilization and geological settings. Unfortunately most of the current classifications that are used for geothermal systems are not complete. So, a combinational terminology of geological and tempera ture-based classifications would be more complete. This terminology can explain all geological situations, temperature and physical state of geothermal reservoir altogether. According to geological settings, in combinational terminology (from left to right), the class of geothermal resource’s name would be placed at first, then the physical state of reservoir (Liquid-dominated or Two-phase or Vapor-dominated) would be written and finally the class of the geothermal reser voir which is related to its temperature, is written.

Investigations of Water Flow Behaviors Induced by Local Temperature Variations through a Single Rough Fracture for the Enhanced Geothermal Systems

In recent years,Enhanced Geothermal System(EGS)technologies have been applied to the geothermal resources production in the Hot Dry Rock(HDR).The core of EGS technologies is to adopt hydraulic fracturing in the reservoir to create a connected network of discrete fractures with the consideration of water as a working fluid for hydraulic fracturing and heat production.This paper investigates the characteristics of water flow behaviors through a single rough fracture under different temperature and pressure conditions.A single fracture model with rough fracture surfaces is constructed and then characterized,and influences of the anisotropic factor on the average tortuosity and frictional resistance coefficient of water flow through a single fracture with rough surfaces have been compared and analyzed.With consideration of other impacting factors(temperature,pressure,fracture roughness),the impact of mass flow rate has also been presented.Numerical simulation results present that changes of average tortuosity for water flow through a single rough facture are mainly affected by temperature.It can be observed that higher temperature leads to larger average tortuosity but the frictional resistance coefficient shows an opposite trend.As for pressure conditions,it is found that effects of pressure on average tortuosity and frictional resistance coefficient is very small,which can be neglected under high pressure conditions.Furthermore,the average tortuosity shows a progressively linear relationship with the mass flow rate.On the contrary,the frictional resistance coefficient has a negative relationship with the mass flow rate.It is revealed that when the mass flow rate reaches a critical point,the influences of temperature on the frictional resistance coefficient will be negligible.Comparisons of single rough fractures with different anisotropic factors show that values of average tortuosity and frictional resistance coefficient have positive relationships with the increase of anisotropic factors.

Effect of temperature-dependent rock thermal conductivity and specific heat capacity on heat recovery in an enhanced geothermal system

The modeling of heat recovery from an enhanced geothermal system(EGS)requires rock thermal parameters as inputs such as thermal conductivity and specific heat capacity.These parameters may encounter significant variations due to the reduction of rock temperature during heat recovery.In the present study,we investigate the effect of temperature-dependent thermal conductivity and specific heat capacity on the thermal performance of EGS reservoirs.Equations describing the relationships between thermal conductivity/specific heat capacity and temperature from previous experimental studies were incorporated in a field-scale single-fracture EGS model.The modeling results indicate that the increase of thermal conductivity caused by temperature reduction accelerates thermal conduction from rock formations to fracture fluid,and thus improves thermal performance.The decrease of specific heat capacity due to temperature reduction,on the contrary,impairs the thermal performance but the impact is smaller than that of the increase of thermal conductivity.Due to the opposite effects of thermal conductivity increase and specific heat capacity decrease,the overall effect of temperature-dependent thermal parameters is relatively small.Assuming constant thermal parameters measured at room temperature appears to be able to provide acceptable predictions of EGS thermal performance.

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 heatflow 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.

Characteristics,Formation Periods,and Controlling Factors of Tectonic Fractures in Carbonate Geothermal Reservoirs:A Case Study of the Jixianian System in the Xiong'an New Area,China

As typical carbonate geothermal reservoirs with low porosity in northern China,the Jixianian System in the Xiong’an New Area is the main target for geothermal fluid exploration.The Jixianian System comprises the Gaoyuzhuang,Yangzhuang,Wumishan,Hongshuizhuang,and Tieling formations.The characteristics,formation periods,and controlling factors of reservoir tectonic fractures have been determined based on analyses of outcrops,cores,thin sections,and image logs.The results show that unfilled fractures account for over 87% and most tectonic fractures are high-angle shear fractures with angles concentrated at 40°to 70°and the fracture porosity increases linearly with an increased fracture aperture.Within the same tectonic setting and stress field,the lithology and layer thickness are the dominant factors governing the development of tectonic fractures,which are the most developed in dolomites and thin layers.Tectonic fractures were most likely formed in regions near faults or areas with larger stress gradients.The tectonic fractures in the carbonate geothermal reservoirs are roughly divided into four sets:NNW-SSE and NNE-SSW oriented‘X’-conjugated shear fractures formed from the Paleozoic to the pre-Yanshanian Movement;NE-SW-oriented shear fractures,formed in episode B of the Yanshanian Movement,occurred at the Early Cretaceous;nearly E-W-oriented tensional fractures formed in the late Yanshanian Movement at the Late Cretaceous to Paleogene,and NEE-SW-oriented shear fractures formed during the Himalayan movement.