Temperature–depth relationships based on log data from the Los Azufres geothermal field,Mexico

Equilibrium temperatures based on log data acquired during drilling stops in the Los Azufres geothermal field were used to study the relationship between temperature, depth and conductive heat flow that differentiate production from non-production areas. Temperature and thermal conductivity data from 62 geothermal wells were analyzed, displaying temperature–depth, gradient–depth, and ternary temperature–gradient–depth plots. In the ternary plot, the production wells of Los Azufres are located near the temperature vertex, where normalized temperatures are over 0.50 units, or where the temperature gradient is over 165°C/km. In addition, the temperature data were used to estimate the depth at which 600°C could be reached (5–9 km) and the regional background conductive heat flow (≈ 106 mW/m²). Estimates are also given for the conductive heat flow associated with the conductive cooling of an intrusive body (≈ 295 mW/m²), and the conductive heat flow component in low-permeability blocks inside the reservoir associated with convection in limiting open faults (from 69 to 667 mW/m²). The method applied in this study may be useful to interpret data from new geothermal areas still under exploration by comparing with the results obtained from Los Azufres.     

A spectral model for transient heat flow in a double U-tube geothermal heat pump system

This paper introduces a semi-analytical model based on the spectral analysis method for the simulation of transient conductive-convective heat flow in an axisymmetric shallow geothermal system consisting of a double U-tube borehole heat exchanger embedded in a soil mass. The proposed model combines the exactness of the analytical methods with an important extent of generality in describing the geometry and boundary conditions of the numerical methods. It calculates the temperature distribution in all involved borehole heat exchanger components and the surrounding soil mass using the fast Fourier transform, for the time domain; and the complex Fourier and Fourier-Bessel series, for the spatial domain. Numerical examples illustrating the model capability to reconstruct thermal response test data together with parametric analysis are given. The CPU time for calculating temperature distributions in all involved components, pipe-in, pipe-out, grout, and soil, using 16,384 FFT samples, for the time domain, and 100 Fourier-Bessel series samples, for the spatial domain, was in the order of 3 s in a normal PC. The model can be utilized for forward calculations of heat flow in a double U-tube geothermal heat pump system, and can be included in inverse calculations for parameter identification of shallow geothermal systems.     

Modeling contribution to risk assessment of thermal production power for geothermal reservoirs

We analyze the likelihood of success for heat production strategies in a sandstone reservoir in the north-eastern German basin in a depth of about 2 km by simulating both double and single well configurations. For this test case study we use an exploited oil and gas field. We combine seismic interpretation, numerical modeling, and stochastic estimation of rock properties to predict the transient temperature and pressure variations and their uncertainties in a geothermal reservoir. We demonstrate the essential necessity in geothermal reservoir modeling to account for heterogeneity of rock properties. We use 3D seismic data and stratigraphy data from about 100 wells at 1500 m – 2500 m depth for setting up a 3D stratigraphic model. Rock properties are assigned to this model by a Monte Carlo approach using Sequential Gaussian Simulation. Using 3D inversion of temperature data obtained in the wells we estimate a specific heat flow of 77.7 mW m⁻² ± 1.2 mW m⁻² at 6 km depth, in agreement with a temperature of 87.1 °C ± 1.8 K in the Rhaetian sandstone target layer at a depth of ∼2 km. For different types of potential geothermal well installations inside the Rhaetian sandstone layer the probability of success is just 1.6%.     

Evaluation of heat exchange rate of GHE in geothermal heat pump systems

Total thermal resistance of ground heat exchanger (GHE) is comprised of that of the soil and inside the borehole. The thermal resistance of soil can be calculated using the linear source theory and cylindrical source theory, while that inside the borehole is more complicated due to the integrated resistance of fluid convection, and the conduction through pipe and grout. Present study evaluates heat exchange rate per depth of GHE by calculating the total thermal resistance, and compares different methods to analyze their similarities and differences for engineering applications. The effects of seven separate factors, running time, shank spacing, depth of borehole, velocity in the pipe, thermal conductivity of grout, inlet temperature and soil type, on the thermal resistance and heat exchange rate are analyzed. Experimental data from several real geothermal heat pump (GHP) applications in Shanghai are used to validate the present calculations. The observations from this study are to provide some guidelines for the design of GHE in GHP systems.     

Modeling of hot gas flow through a feed stream within a horizontal pipe

High heat penetration into a feed stream within a horizontal pipe is described mathematically with a gas flow and heat transfer model. Influences of varied factors on the gas flow and heat transfer in porous media are examined for different conditions. The temperature of the packed‐bed particles and the gas velocity distribution curves are obtained for the feeding service at interruption and at regular charge operating conditions. The numerical results show that the thermal effect to the packed‐bed particles by the seepage flow fluid is high only in the position near the gas entrance. The thermal penetration depth tends to increase with the seepage flow velocity and decrease with the feed rate. The operating conditions and the porosity of solid bed have importance effects on the gas velocity and temperature field in the thermal penetration zone. The model results are found to compare favorably with the experimental data available in the literature. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(6): 553–565, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10109     

Spatial data analysis and integration for regional-scale geothermal potential mapping,West Java,Indonesia

Conceptual modeling and predictive mapping of potential for geothermal resources at the regional-scale in West Java are supported by analysis of the spatial distribution of geothermal prospects and thermal springs, and their spatial associations with geologic features derived from publicly available regional-scale spatial data sets. Fry analysis shows that geothermal occurrences have regional-scale spatial distributions that are related to Quaternary volcanic centers and shallow earthquake epicenters. Spatial frequency distribution analysis shows that geothermal occurrences have strong positive spatial associations with Quaternary volcanic centers, Quaternary volcanic rocks, quasi-gravity lows, and NE-, NNW-, WNW-trending faults. These geological features, with their strong positive spatial associations with geothermal occurrences, constitute spatial recognition criteria of regional-scale geothermal potential in a study area. Application of data-driven evidential belief functions in GIS-based predictive mapping of regional-scale geothermal potential resulted in delineation of high potential zones occupying 25% of West Java, which is a substantial reduction of the search area for further exploration of geothermal resources. The predicted high potential zones delineate about 53–58% of the training geothermal areas and 94% of the validated geothermal occurrences. The results of this study demonstrate the value of regional-scale geothermal potential mapping in: (a) data-poor situations, such as West Java, and (b) regions with geotectonic environments similar to the study area.     

An improved thermal response test for U-tube ground heat exchanger based on optical fiber thermometers

As part of a new thermal response test (TRT) and to determine ground thermal conductivities, vertical temperature profiles were obtained using retrievable optical fiber sensors inserted into the U-tubes of two ground heat exchangers (GHEs) installed at Maebaru City (Fukuoka, Kyushu) and Kushiro City (Hokkaido), Japan. Measured profiles and outlet temperatures from TRTs were history-matched with the cylindrical source function. Nonlinear regression was used to estimate the vertical distribution of ground thermal conductivities. The computed distribution is consistent with measured data indicating both the reliability of the optical fiber thermometer and TRT interpretation. It is expected that TRTs and optical fiber thermometers will prove to be increasingly useful for optimizing the depth of the GHEs installed in heterogeneous formations, and consequently will minimize installation costs of geothermal heat pump systems.     

Heat flow in Albania

As part of a joint geothermal project between Albania and the Czech Republic, a field expedition was organized to Albania in summer 1993 to measure temperature profiles in selected boreholes and to collect rock samples to determine their thermal conductivity. Fourteen localities were visited and nine detailed temperature-versus-depth profiles were obtained. These results were completed with numerous industrial temperature records from other deep holes. The regional patterns of temperatures at 100 m depth and of characteristic near-surface temperature gradients were constructed, and the effect of the topography on the subsurface temperature field was assessed to calculate a total of 49 heat flow density data. A generally low geothermal gradient exists in all of the country, ranging from 7–11 mK/m in the synclinal belt, 11–13 mK/m in southernmost Albania, to a maximum of 18–20 mK/m in the central part of the Pre-Adriatic Depression. These values correspond to a low heat flow zone of 30 to 45–50 mW/m² extending from the north and bordering the Adriatic coast. Heat flow generafly increases from west to east, but its distribution in the Inner Albanides is not clear.     

Temperature field distribution from cooling of a magma chamber in La Primavera Caldera, Jalisco, Mexico

The temperature field distribution in La Primavera geothermal area, Jalisco, located in the western part of the Mexican Volcanic Belt (MVB), has been simulated from cooling of a shallow magma chamber (assumed as the primary heat source) during the entire volcanic history of the caldera. Similar to the other two geothermal fields of the MVB (Los Humeros and Los Azufres), it is considered that the evolution of the magma chamber is controlled by the processes of fractional crystallization as well as magma recharge. Besides these processes, heat contribution is also taken into account from decay of natural radioactive elements, U, Th, and K, present in all geological materials. In some models presented in this work, convection in the geothermal reservoir is simulated by assigning higher values of thermal conductivities (up to 20 times the rock conductivities) to respective geologic units. The heat transfer equation has been solved by a finite element implicit method. The results of temperature simulations from the magma chamber are compared with undisturbed formation temperatures in three drill wells. The subsurface depth of the top of the magma chamber is varied from 5 to 7 km. Similarly, the horizontal dimensions of the chamber are varied from 12 km (which is approximately the diameter of the La Primavera caldera) to 10 km. The thermal effects of this change in depth and horizontal dimensions of the magma chamber are readily seen in the predicted temperature distribution for this rather young caldera.     

A Compact Thermal Model for Data Center Analysis using the Zonal Method

Modeling the thermal environment for data centers, including prediction of airflow and temperature distributions, is generally an extremely time-consuming process using full-scale CFD analysis. Reduced order models are necessary in order to provide real-time assessment of cooling requirements. The use of a coarse-grained zonal model is being investigated as a predictive tool, and this article details the development and implementation of a three-dimensional, pressurized zonal model. To construct and validate the zonal model, a basic data center configuration was analyzed using a finite volume software package. The calculated flow fields provide the spatial flow coefficients required in the zonal model, which is based on the power law method (PLM). A physically-based mapping between the controllable spatial mass flow rate and temperature distribution was obtained. Good agreement (within 10% average relative error) was obtained between the zonal model predictions and the CFD results. These preliminary results show promise that zonal models may yield an effective real-time thermal management design tool for data centers.     

Effective thermal conductivity of Mexican geothermal cementing systems in the temperature range from 28°C to 200°C

The effective thermal conductivity of six Mexican cementing systems used in geothermal well completion were experimentally determined in the temperature range from 28°C to 200°C. Measurements were carried using the classical line-source method. The experimental system was calibrated by measuring the thermal conductivity of standard fused quartz samples. An experimental procedure for preparation of the cement specimen samples was also developed. Results show that thermal conductivity depends on the particular cement system and tends to increase with temperature for most cement systems. Experimental uncertainties of thermal conductivity were less than 4%. From this experimental work, new empirical equations for correlating thermal conductivity with temperature for geothermal cementing samples were obtained.     

多孔介质中高温气体非稳态渗流传热数值计算

针对水平导管中填充颗粒物料层内的高温气体参流传热现象，考虑渗流与传热的相互作用并采用局部非平衡假设建立多孔介质中的瞬态渗流传热物理数学模型。研究不同情况下填充物料中的渗流速度和气固温度分布。计算结果表明，高温热气体对水平导管中移动颗粒料层的热渗透主要发生在渗流入口端区域，随着渗流时间延长，热渗透深度沿导管推进。增大入口渗流速度以及减小出料速度，将导致物料温度沿导管慢速下降，热渗透深度扩大，热渗透作用区域内的物料温度水平提高。在热渗透作用区域，孔隙率对流场和温度场有很大的影响。研究对于高温反应器的颗粒输运和给料器的设计与运行有一定的参考作用。     

Transient and steady natural convection from a heat source embedded in thermally stratified porous layer

Numerical results are presented for transient and steady natural convection from a heat source buried in a saturated, porous layer. The porous layer is thermally stratified with a negative temperature gradient but its corresponding Rayleigh number is not greater than that of the critical value (Ra_c = 40). The effects of the size of the heat source and its buried depth are also investigated. The results, which include the buoyancy-induced flow patterns and temperature profiles, as well as the heat transfer coefficient in terms of the Nusselt number, are presented for a wide range of Rayleigh number and stratification parameter. The results thus obtained have important implications for applications in geothermal energy and underground disposal of nuclear wastes.     

利用测温资料判别热储流体的运动方向

热储温度数据主要通过钻探过程中温度测井来获取。由于在钻井过程中，钻孔周围的原始热动力条件常受到扰动。因此，地层温度需要通过对测温曲线进行细致地解释来获得。我们以Kaldarholt地热田为例，通过对34口勘探孔测温曲线的解释，分析了地热田的温度场分布特征。据此，建立了Kaldarholt地热田地下热水流动的概念模型，并为地热田开采井KH-36的成功定位起到关键的作用。     

A novel flow-resistor network model for characterizing enhanced geothermal system heat reservoir

The fracture characteristics of a heat reservoir are of critical importance to enhanced geothermal systems, which can be investigated by theoretical modeling. This paper presents the development of a novel flow-resistor network model to describe the hydraulic processes in heat reservoirs. The fractures in the reservoir are simplified by using flow resistors and the typically complicated fracture network of the heat reservoir is converted into a flow-resistor network with a reasonably simple pattern. For heat reservoirs with various fracture configurations, the corresponding flow-resistor networks are identical in terms of framework though the networks may have different section numbers and the flow resistors may have different values. In this paper, numerous cases of different section numbers and resistor values are calculated and the results indicate that the total number of flow resistances between the injection and production wells is primarily determined by the number of fractures in the reservoir. It is also observed that a linear dependence of the total flow resistance on the number of fractures and the relation is obtained by the best fit of the calculation results. Besides, it performs a case study dealing with the Soultz enhanced geothermal system (EGS). In addition, the fracture numbers underneath specific well systems are derived. The results provide insight on the tortuosity of the flow path between different wells.     

Evidence for thermal mining in low temperature geothermal areas in Iceland

This study deals with thermal mining in several geothermal systems in Iceland. A number of 2500- to 3000-m deep drillholes have been drilled into low temperature geothermal areas in the country. The conductive gradient outside active geothermal areas has also been mapped, and shows a systematic variation from lower than 50°C/km in the outer parts of the Tertiary basalts to over 100°C/km on the borders of the volcanic zones (rift zones). The difference between formation temperatures inside geothermal systems and the surrounding conductive gradient can be computed as a function of depth. This difference is termed ΔT in this paper. The ΔT-curves show that the upper parts of the geothermal systems are heated and the lower parts are cooled compared to the undisturbed conductive gradient. In many cases the cooling of the lower part is greater than the heating in the upper part, so that a net thermal mining has occurred. This thermal mining is calculated for several geothermal systems, and the systems are compared. The net thermal mining in the top 3000 m appears to be much greater in formations of Pleistocene and Pliocene age. It gradually decreases to zero for formations older than 6 million years. However, the net thermal mining is critically dependent on the maximum depth of water convection in these systems, which is unknown.     

Hydrogeochemistry and geothermometry of Changal thermal springs,Zagros region,Iran

This study addresses the hydrogeochemistry of thermal springs that emerge from the Asmari limestone in a gorge at Changal Anticline in the vicinity of the Salman-Farsi dam. The Changal thermal springs vary in temperature between 28 and 40 °C. Chemical and isotopic compositions of the thermal waters suggest two distinct hydrogeological systems: a deep, moderate-temperature (∼40 °C) geothermal system recharged by deeply circulating meteoric waters, and a shallow cold aquifer system related to local groundwater. The source geothermal fluid temperature was calculated using different geothermometers and mineral saturation indexes. Based on chemical and isotopic data, it is hypothesized that: (1) mixing occurs between the ascending geothermal water and shallow cold water; (2) the resulting thermal waters reaching surface are a mixture of 80% local, shallow meteoric water and 20% geothermal water; and (3) the circulation depth of the meteoric water is about 1500 m. The thermal reservoir temperature is estimated to be between 70 and 80 °C according to calculations using different geothermometers and computation of saturation indices for different solid phases.     

The geothermal exploration of Campanian volcanoes: Historical review and future development

Since Roman time, the heat produced by Neapolitan volcanoes was an appeal for people living in and outside the area, for the fruition of the famous thermal baths. This very large area, which spans from Campi Flegrei and Ischia calderas to Somma-Vesuvius volcano, is characterized by high temperature at shallow depth and intense heat flow, and is yet utilized for the bathing and spa treatment industry, while only in the middle of the 20th century a tentative of geothermal exploitation for energy production was performed. Pioneering researches of geothermal resource were carried out in Campanian region since 1930, until 1985, during which a large amount of geological data were collected. In this paper, we make for the first time a review of the history of geothermal explorations in the active Campanian volcanic area. By the analysis of a great amount of literature data and technical reports we reconstruct the chronology and the main information of the drillings performed since 1930 by the SAFEN Company and successively in the framework of the ENEL-AGIP Joint Venture for geothermal exploration. The available data are utilized to correlate the temperatures measured within the deeper wells with the possible sources of geothermal heat in the shallow crust, down to about 8-10 km of depth. Finally, we assess the geothermal potential of the hottest areas, Ischia Island and Campi Flegrei, which have shown the best data and favorable physical conditions for a reliable, and cost-effective, exploitation for thermal and electric purposes.     

U型井开发干热岩井筒温度场研究

干热岩开发主要采用水力压裂的方式,而以U型井方法取代固有开发模式,具有减少水损、提高能量衰减周期,避免诱发地震等显著优势。因此,建立了U型井井筒和干热岩储层非稳态流动传热耦合模型,采用有限体积法结合Crank-Nicolson全隐式格式进行离散求解,并通过地热井数据验证了模型的可靠性。研究了干热岩开采过程中井筒及地层的温度变化特征,定量分析不同的敏感性因素对出口温度的影响,利用正交试验法明确影响取热量大小的因素排列。结果表明：各个因素的大小对出口温度影响明显,而影响取热量的因素由主到次为注入流量、水损率、岩石导热系数、水平段长度、入口温度、井筒直径。研究成果进一步完善了干热岩高效热提取理论,可为我国干热岩地热能的开发利用提供借鉴作用。     

Negative buoyant plume model for solar domestic hot water tank systems incorporating a vertical inlet

Thermal stratification in solar energy storage tanks plays an important role in enhancing the performance of solar domestic hot water systems. The mixing that occurs when hot fluid from the solar collector enters the top of the tank is detrimental to the stratification. Mathematical models that are used for system analysis must therefore be able to capture the effects of this inlet jet mixing in order to accurately predict system performance. This paper presents a computational study of the heat transfer and fluid flow in a thermal storage tank of a solar domestic hot water system with a vertical inlet under negative buoyant plume conditions. The effects of parameters such as the fluid inlet velocity and temperature as well as inlet pipe diameter on the thermal mixing were considered. The work culminated in the development of a one-dimensional empirical model capable of predicting the transient axial temperature distribution inside the thermal storage tank. Predictions from the new model were in good agreement with both experimental data and detailed computational fluid dynamics predictions.