Thermal deterioration of high-temperature granite after cooling shock: multiple-identification and damage mechanism

Cooling shock has a significant thermal deterioration (TD) on the physical and mechanical properties of rocks in high-temperature conditions, which is a critical concern for the engineering application of the cyclic hydraulic fracturing technique in an enhanced geothermal system (EGS). In this work, cooling shock tests were carried out on granite samples to evaluate the actual TD of high-temperature rocks by cooling shocks, and multiple test methods were used to explore the effect of TD on the corresponding physical and mechanical properties of high-temperature granite. Some core conclusions from the study are as follows: (1) The wave velocity and apparent resistivity (AR) can reflect the thermal damage effect of cooling shocks on high-temperature granite. Notably, the higher the temperature of granite, the more significant change in wave velocity and AR. (2) The stress-strain curve tends to be smooth with the granite temperature increases and the cooling shocks intensify, the quiet period of acoustic emission (AE) events is lengthened, and the number is gradually reduced. (3) The TD effect of the cooling shock tends to be more significant for the samples at temperatures above 550 °C, and the peak stress continues to decrease with cooling shock strengthen. Furthermore, thermal stress is the main cause of TD to high-temperature granite. This study has the potential to guide the use of the cooling shock effect in extraction applications of geothermal engineering.

     

Experiments on rockburst proneness of pre-heated granite at different temperatures: Insights from energy storage,dissipation and surplus

Many underground engineering projects show that rockburst can occur in rocks at great depth and high temperature, and temperature is a critical factor affecting the intensity of rockburst. In general, temperature can affect the energy storage, dissipation, and surplus in rock. To explore the influence of temperature on the energy storage and dissipation characteristics and rockburst proneness, the present study has carried out a range of the uniaxial compression(UC) and single-cyclic loading-unl...     

Waiweras Warmwasserreservoir – Welche Aussagekraft haben Modelle?

The warm water geothermal reservoir below the village of Waiwera in New Zealand has been known by the native Maori for centuries. Development by the European immigrants began in 1863. Until the year 1969, the warm water flowing from all drilled wells was artesian. Due to overproduction, water up to 50 °C now needs to be pumped to surface. Further, between 1975 and 1976, all warm water seeps on the beach of Waiwera ran dry. Within the context of sustainable water management, hydrogeological models must be developed as part of a management plan. Approaches of varying complexity have been set-up and applied since the 1980s. However, none of the models directly provide all results required for optimal water management. Answers are given simply to parts of the questions, nonetheless improving resource management of the geothermal reservoir.     

A comparative performance analysis of solar heat exchangers for solar hot water application under controlled laboratory conditions

The paper presents the experimental performance evaluation of a novel retrofit heat exchanger (‘SolaPlug’) developed for solar hot water storage applications. The performance of this system was compared with a traditional dual-coil (‘Coil’) solar cylinder under controlled operating conditions. The tests were conducted under different solar-simulated conditions with a 30 and 20 evacuated tube collector. The results showed that after a 6-h test period, the average water temperatures within the store for the ‘SolaPlug’ system were 58.8°C and 40.5°C at 860 and 459?W?m^?2, respectively, and for the ‘Coil’ system were 60.5°C and 40.6°C when a 30 tube collector was used. The performance of the ‘SolaPlug’ system was marginally better than the ‘Coil’ system under the low solar input condition. Under high insolation condition, the overall ‘SolaPlug’ system efficiency was found to be 4.3% lower than that of the ‘Coil’ system. The ‘SolaPlug’ heat exchanger rating was 222?W?K^?1.     

Carbon sequestration potential of the Habanero reservoir when carbon dioxide is used as the heat exchange fluid

The use of sequestered carbon dioxide(CO_2) as the heat exchange fluid in enhanced geothermal system(EGS) has significant potential to increase their productivity, contribute further to reducing carbon emissions and increase the economic viability of geothermal power generation. Coupled CO_2 sequestration and geothermal energy production from hot dry rock(HDR) EGS were first proposed 15 years ago but have yet to be practically implemented. This paper reviews some of the issues in assessing these systems with particular focus on the power generation and CO_2 sequestration capacity. The Habanero geothermal field in the Cooper Basin of South Australia is assessed for its potential CO_2 storage capacity if supercritical CO_2 is used as the working fluid for heat extraction. The analysis suggests that the major CO_2 sequestration mechanisms are the storage in the fracture-stimulation damaged zone followed by diffusion into the pores within the rock matrix. The assessment indicates that 5% of working fluid loss commonly suggested as the storage capacity might be an over-estimate of the long-term CO_2 sequestration capacity of EGS in which supercritical CO_2 is used as the circulation fluid.     

Effect of high temperature and cooling conditions on aerated concrete properties

In this study, effect of elevated temperatures and various cooling regimes on the properties of aerated concrete is investigated. Air cooled materials are tested at room temperature and in hot condition right after the fire. Water quenching effect is determined by testing the material in wet condition right after the quenching and in dry condition at room temperature. Unstressed strength of the material tested hot is relatively higher than air cooled unstressed residual strength up to 600 °C. On the other hand, water quenching decreases the percentage of the strength particularly when the material is wet right after the quenching; strength is lost gradually as the temperature rises. As a result, if the quenching effect is disregarded, temperature rise does not have a considerable effect on the strength of the aerated concrete approximately up to 700–800 °C. It is able to maintain its volumetric stability as well. However, more care needs to be taken in terms of its use above 800 °C for fire safety.     

Nitrification in porous media during rapid,unsaturated water flow

A substantial nitrification in rapid infiltration (RI) systems for wastewater treatment is a prerequisite for obtaining good N removal by denitrification. The purpose of this study is to investigate nitrification in porous media at conditions corresponding to RI treatment systems. Nitrification in six 50-cm porous media columns (98% weathered granite or sand and 2% field soil) during unsaturated leaching at constant flow rates of synthetic wastewater was investigated. Concentrations of NH₄-N between 20 and 60 mg l⁻¹ were applied and vertical concentration profiles of NO₃-N, NO₂-N and NH₄-N were measured for 54 d at 30°C (three columns) and for 140 d at 10°C (three columns). A time lag in nitrification of 20 d was found at 10°C. Complete nitrification was obtained after 3–5 at 30°C and after approximately 50 d at 10°C. Assuming first-order nitrification at steady-state, the corresponding first order reaction rate coefficients (k₁) for NO₃ production in the columns were estimated to be between 0.4 and 1 h⁻¹ at 10°C and between 6 and 9 h⁻¹ at 30°C. Steady-state NO₃ profiles were obtained between 1.5 and up to 9 weeks after the experiments were started. At the actual soil-air contents (0.10 cm³ air phase cm⁻³ soil), oxygen limitations were not observed during the experiments. Nitrogen loadings (water flow times N concentration) above 100 mg N 1⁻¹ cm h⁻¹ (1 g N m⁻² h⁻¹) caused NH₄ accumulation in the columns at 10°C and should probably be avoided during operation of RI system.     

Numerical modeling of thermal breakthrough induced by geothermal production in fractured granite

It is well known that the complicated channeling of fluid flow and heat transfer is strongly related with the intricate natural fracture system. However, it is still challenging to set up the fracture network model which is strong heterogeneous. Compared with other methods (e.g. equivalent continuum model (ECM), discrete fracture model (DFM), and ECM-DFM), the fracture flow module in the COMSOL Multiphysics simulator is powerful in definition of fractures as the inner flow boundary existing in the porous media. Thus it is selected to simulate the fluid flow and heat transfer in the geothermal-developed fractured granite of Sanguliu area located at Liaodong Peninsula, Eastern China. The natural faults/fractures based on field investigation combined with the discrete fracture network (DFN) generated by the MATLAB are used to represent the two-dimensional geological model. Numerical results show that early thermal breakthrough occurs at the production well caused by quick flow of cold water along the highly connected fractures. Suitable hydraulic fracturing treatments with proper injection rates, locations, etc. can efficiently hinder the thermal breakthrough time in the natural fracture system. Large well spacing helps the long-term operation of geothermal production, but it is highly dependent on the geometrical morphology of the fracture network. The enhancement of reservoir properties at the near-well regions can also increase the geothermal production efficiency. The results in this study can provide references to achieve a sustainable geothermal exploitation in fractured granitic geothermal reservoirs or hot dry rocks at depth.     

Prospect of HDR geothermal energy exploitation in Yangbajing,Tibet,China,and experimental investigation of granite under high temperature and high pressure

Hot dry rock(HDR) geothermal energy,almost inexhaustible green energy,was first put forward in the 1970s.The development and testing of HDR geothermal energy are well reported in USA,Japan,UK,France and other countries or regions.In this paper,the geological characters of Yangbajing basin were first analyzed,including the continental dynamic environments to form HDR geothermal fields in Tibet,the tectonic characteristics of south slope of Nyainqêntanglha and Dangxiong-Yangbajing basin,and the in-situ stress...     

Analysis of a combined photovoltaic–geothermal gas-fired absorption heat pump system in a Canadian climate

This study examines the technical feasibility of using a geothermal gas-fired absorption heat pump (A-GSHP) for space conditioning and domestic hot water heating in a Canadian climate. The A-GSHP is coupled to a photovoltaic (PV) system with battery storage intended to ensure the full autonomy of the heating, ventilating and air conditioning (HVAC) system from the electric grid. The system is modelled using TRNSYS with standard models and a new performance-based A-GSHP model, which accounts for part-load operation. Results indicate that the coefficient of performance (COP) is equal to 1.12, 0.55 and 1.79 for heating only, cooling only and simultaneous cooling and domestic hot water (DHW) heating, respectively. A 13.5 kWp PV array and a 400 kWh battery storage are necessary to provide the electrical power required to operate the A-GSHP and the associated HVAC system at all times without importing electrical energy from the grid.     

Coupled mechanical and chemical processes in engineered geothermal reservoirs with dynamic permeability

A model is developed to represent mechanical strain, stress-enhanced dissolution, and shear dilation as innately hysteretic and interlinked processes in rough contacting fractures. The model is incorporated into a numerical simulator designed to examine permeability change and thermal exchange in chemically active and deformable fractured reservoirs. A candidate engineered geothermal reservoir system (EGS) is targeted. The mechanistic model is able to distinguish differences between the evolution of fluid transmission characteristics of (1) small scale, closely spaced fractures, and (2) large-scale, more widely spaced fractures. Alternate realizations of fracture frequency and scale, exhibiting identical initial bulk permeability, lead to significantly different conclusions regarding permeability evolution and thermal drawdown within the reservoir. Reactivation, primarily through mechanical shear, of pervasive, large-scale fractures is shown capable of causing both hydraulic and thermal short circuiting. Small variations in fracture scale impact the balance between the efficiency of thermal transfer and the rate of fluid circulation. Stress-enhanced chemical dissolution, initially at equilibrium within the reservoir, may be reactivated as fractures are forced out of equilibrium during hydraulic fracturing. At the conditions examined (250 °C reservoir with 70 °C injection), however, shear dilation exerts dominant control over changes to permeability. Heterogeneity in permeability, generated from a normal distribution of fracture spacing, impacts thermal breakthrough times at the withdrawal well, as well as withdrawal rates. For the given conditions, spatial variability over ～1 order of magnitude leads to a reduction of ～10% in withdrawal rates compared to a spatially uniform system. Permeability is a strongly dynamic property and at geothermal conditions is influenced by the full suite of THMC interactions.     

河南新商断裂对地热资源形成的控制作用及资源潜力分析

新商断裂是一条几乎横贯河南的区域性深大断裂,构成了济源-开封凹陷和通许凸起两大地热单元的边界。在其及影响带附近分布着新乡-延津、兰考、民权、宁陵-商丘等大型地热田,蕴藏着丰富的中深层地热资源。该文通过分析新商断裂的构造特征和地热流体特征,系统分析了深大断裂对地热资源的控制作用,以商丘项目为例分析了其资源潜力。区内地下热水的分布特征严格受控于新商断裂的展布方向,地下热水的温度取决于盖层的厚度及围岩的放射性含量,地热资源潜力巨大,地热可采资源量约140.37×10⁴ m³/a,地热流体含热能量71.94×10¹² J/a,折合标准煤2454.6 t/a,可减少CO₂排放5856.8 t/a,该研究为今后在该地区开展地热资源勘查工作具有一定的指示意义。     

Simulation of Legionella concentration in domestic hot water: comparison of pipe and boiler models

The energy needed for the production of domestic hot water (DHW) represents an important share in the total energy demand of well-insulated and airtight buildings. DHW is produced, stored and distributed above 60°C to kill Legionella pneumophila. This elevated temperature is not necessary for DHW applications and has a negative effect on the efficiency of hot water production units.

In this paper, system component models are developed/updated with L. pneumophila growth equations. For that purpose, different existing Modelica pipe and boiler models are investigated to select useful models that could be extended with equations for simulation of bacterial growth. In future research, HVAC designers will be able to investigate the contamination risk for L. pneumophila in the design phase of a hot water system, by implementing the customized pipe and boiler model in a hot water system model. Additionally it will be possible, with simulations, to optimize temperature regimes and estimate the energy saving potential without increasing contamination risk.     

Mechanisms of arsenic enrichment in geothermal and petroleum reservoirs fluids in Mexico

The lack of chemical similarity between thermal fluids in geothermal and petroleum reservoirs in Mexico indicates a distinct origin for arsenic in both types of reservoirs. Deep fluids from geothermal reservoirs along the Transmexican Volcanic Belt (TMVB) are characterized by elevated arsenic concentrations, within a range between 1 and 100 mg L^?1 at a depth from 600 to 3000 m b.s.l. Based on hierarchical cluster analysis (HCA), arsenic is linked to typical geothermal species like lithium, silica, and boron. The lack of correlation between arsenic and salinity reflects the importance of secondary water-rock interaction processes. The predominance of arsenic compared to Fe- and Cu-concentrations, and the occurrence of secondary minerals (sulfides and clay minerals) in temperature-dependent hydrothermal zones, supports this hypothesis. Neither magmatic fluids input, nor As mineralization is a prerequisite for As enrichment in Mexican geothermal fluids. In contrast, petroleum reservoir waters from sedimentary basins in SE-Mexico show maximum As concentrations of 2 mg L^?1, at depths from 2900 to 6100 m b.s.l. The linear chloride–arsenic correlation indicates that evaporated seawater represents the major source for aqueous arsenic in oil reservoirs, and only minor arsenic proportions are derived from interaction with carbonate host rock. Speciation modeling suggests the lack of arsenic solubility control in both geothermal and petroleum reservoirs, but precipitation/co-precipitation of As with secondary sulfides could occur in petroleum reservoirs with high iron concentrations. Geothermal fluids from magmatic-type reservoirs (Los Azufres and Los Humeros at the TMVB and Las Tres Vírgenes with a granodioritic basement) show relative constant arsenic concentrations through varying temperature conditions, which indicates that temperatures above 230–250 °C provide optimal and stable conditions for arsenic mobility. In contrast, temperature conditions for sedimentary-type reservoirs are irrelevant for water-rock interaction processes, as suggested by low arsenic aqueous concentration for both Cerro Prieto geothermal fluids (high temperature – sedimentary type) and oil field formation water (low temperature – sedimentary type).     

Analyses of non-aqueous reactive polymer insulation layer in high geothermal tunnel

The scenario of geothermal tunnel is commonly observed around the world, and increases with the new constructions in the long and deep tunnels, for example in China. Tunnel insulation is generally divided into active and passive insulation. In passive insulation, it is an effective way to set low thermal conductivity materials as the thermal insulation layer as the choice of insulation material mainly depends on the thermal conductivity. Polymer is a kind of material with good geothermal performance, but there are relatively few studies. In this context, the transient plane source (TPS) method was used to measure the thermal conductivity of the developed polymer. Then, the temperature field of the high geothermal tunnel insulated by the non-aqueous reactive polymer layer was simulated. With the parametric analysis results, the suggestions for the tunnel layers were proposed accordingly. It revealed that the thermal conductivity of polymer first increases and then decreases with temperature. There are two rising sections (?40–10 °C and 20–90 °C), one flat section (10–20 °C) and one descending section (>90 °C). It is observed the thermal conductivity of polymer increases with increase of the density of insulation layer and the density, and the thermal conductivity decreases when exposed to high temperatures. The temperature of the surrounding rocks increases with increase of the thermal conductivity and the thickness of polymer. Finally, a more economical thickness (5 cm) was proposed. Based on the parametric study, a thermal insulation layer with thermal conductivity less than 0.045 W/(m K), thickness of 5 cm and a density less than 0.12 g/cm³ is suggested for practice.     

Innovative closed-loop underground pumped hydroelectricity geothermal storage system

A mixed methods approach with an embedded case study was used to develop a decision-making model for generating up to 2,489,116 GW of clean renewable energy. The case study site was an abandoned underground iron ore mine located within one of the largest natural park-preserves in the USA. The research included a literature review, comparative field research, data collection from the site, and parametric statistical techniques. A novel concept was to harness readily available geothermal hot dry rock heat in the lower reservoir over 1.2 km below the surface to lower the water density and reduce return-flow gravitational resistance in order to increase overall system efficiency. An operational risk factor was estimated by reference to 37 comparable pumped hydro plants. Normality tests were used to determine the underlying distribution, and a simulation was executed with 1000 iterations to estimate operational certainty. The inferential model was statistically proven to be likely to occur 95 out of every 100 years, based on the case study.     

水源热泵在低温地热生活热水系统的应用

提出在常用低温地热生活热水系统中增设回水加热器,以回收生活热水系统的排水。对可选的5种回水加热热源———燃煤锅炉、燃油锅炉、燃气锅炉、电锅炉及水源热泵的节能性、经济性进行了比较,提出水源热泵辅助低温地热生活热水系统。结合工程实例,对常用低温地热生活热水系统与水源热泵辅助低温地热生活热水系统进行了经济性比较。水源热泵辅助低温地热生活热水系统可有效降低生活热水系统排水温度及排水量,节能、节水效果显著。     

Forschungsbohrung Altdorf bei Landshut/Niederbayern – Erkundung einer geothermischen Anomalie im Bereich des Landshut-Neuöttinger-Hochs

In 1999 and 2000 on behalf of the Bavarian Geological Survey and the Wasserwirtschaftsamt Landshut a research bore hole was drilled in Altdorf near Landshut. In contrast to what had been expected this 796 m deep drilling on the Landshut-Neuöttinger-Hoch (LNH), a crystalline barrier in the deeper underground of the Molasse basin, met the Malm (Upper Jurassic) in an almost unreduced thickness of about 400 m. The temperature at the top of the Malm in a depth of about 400 m amounted to 58 °C, yielding an extraordinary temperature gradient of about 12.5 °C per 100m. At final depth the borehole temperature was 71 ° C. Even in the overlying Tertiary several thermal water aquifers were found. Hydrochemical investigations and age determinations lead to the assumption that the heat flow is controlled by both, convection and thermal conduction. Results from a pumping test confirm the high productivity of the Malm aquifer. The Altdorf drilling not only produced a remarkable geothermal anomaly suggesting possible energetic and balneological utilization, but also supplied results that make it necessary to revise the doctrine of the LNH and how it relates to the hydrology in the East Bavarian Molasse basin.     

Diversity of sulfate-reducing bacteria in a plant using deep geothermal energy

Abstract Enhanced process understanding of engineered geothermal systems is a prerequisite to optimize plant reliability and economy. We investigated microbial, geochemical and mineralogical aspects of a geothermal groundwater system located in the Molasse Basin by fluid analysis. Fluids are characterized by temperatures ranging from 61°C to 103°C, salinities from 600 to 900?mg/l and a dissolved organic carbon content (DOC) between 6.4 to 19.3?mg C/l. The microbial population of fluid samples was analyzed by genetic fingerprinting techniques based on PCR-amplified 16S rRNA- and dissimilatory sulfite reductase genes. Despite of the high temperatures, microbes were detected in all investigated fluids. Fingerprinting and DNA sequencing enabled a correlation to metabolic classes and biogeochemical processes. The analysis revealed a broad diversity of sulfate-reducing bacteria. Overall, the detection of microbes known to be involved in biocorrosion and mineral precipitation indicates that microorganisms could play an important role for the understanding of processes in engineered geothermal systems.     

高地温隧道地温特征及预测研究

尼格隧道属长大深埋隧道,兼有高水温与高岩温,最高水温达63.4 ℃,最高岩温达88.8 ℃,最高气温达56.4 ℃。为了研究隧道地温特征并进行地温预测,针对性地设计了一系列地温测量方案,研究成果表明:灰岩段表现为高水温,水温＞气温＞岩温,水温与气温随着隧道进深及埋深呈现上升趋势,出水量及水温在接触带附近达到高值,洞内气温受水温、隧道出水量、积水量影响大;花岗岩段表现为高岩温,无水,岩温与气温随隧道进深及埋深呈现上升趋势,两者差值约为25~30 ℃;超前钻孔在孔深＞2 m时岩温达到稳定;一个完整施工循环的施工环境气温呈现4个阶段:气温下降阶段(打钻施工环节),气温骤升阶段(爆破施工环节),气温快速上升阶段(新爆围岩散热),气温缓降阶段(出渣施工环节),出渣环节由于车辆及挖机等机械作业影响,气温出现多处异常高值;施作二衬后,二衬内外壁温差约3.4 ℃;利用热量传递理论、地热成因理论预测的最高地温值与实测值较为吻合。该工程案例颇为典型,本文研究对西南高地热区隧道工程建设具有指导意义。