GIS integration model for geothermal exploration and well siting

The work involved in identifying geothermal fields can be simplified by means of a Geographical Information System (GIS), a decision-making tool used to determine the spatial association between exploration and environmental thematic maps. This methodology has been applied to part of the Sabalan geothermal field in northwestern Iran, and to the siting of exploration wells. The datasets used in the analyses consist of geological, geochemical and geophysical information. Boolean and Index Overlay knowledge-driven models were developed for site selection; the findings from these two models correlated well with the exploration data modeling. The results of the Index Overlay model were combined with those of an environmental suitability analysis for final selection of well sites. The results of exploration and environmental data modeling were combined to select and prioritize the sites of three exploratory wells in the study area.     

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.     

Development of a numerical hydraulic model of the Los Azufres steam pipeline network

The development and documentation of a hydraulic model of the steam pipeline network at the Los Azufres geothermal field is presented, as well as the results obtained using one- and two-phase numerical simulators. Flow simulations were performed to determine pressure and heat losses, flow directions and velocities in that network. Computed well pressures agree within ±10% with measured values, except in three of the 41 wells in the system where the differences are between 10% and 13%. Computed and measured steam flow rates entering the Los Azufres geothermal power plants agree within 10%, with the exception of one that showed a 26.7% difference. This is most likely due to a mismatch between the reported and actual flow rates delivered by the pipeline network. The computed results are considered highly satisfactory given the complexity of the Los Azufres network.     

Main aspects of geothermal energy in Mexico

With an installed geothermal electric capacity of 853 MW_e, Mexico is currently the third largest producer of geothermal power worldwide, after the USA and the Philippines. There are four geothermal fields now under exploitation: Cerro Prieto, Los Azufres, Los Humeros and Las Tres Vírgenes. Cerro Prieto is the second largest field in the world, with 720 MW_e and 138 production wells in operation; sedimentary (sandstone) rocks host its geothermal fluids. Los Azufres (88 MW_e), Los Humeros (35 MW_e) and Las Tres Vírgenes (10 MW_e) are volcanic fields, with fluids hosted by volcanic (andesites) and intrusive (granodiorite) rocks. Four additional units, 25 MW_e each, are under construction in Los Azufres and due to go into operation in April 2003. One small (300 kW) binary-cycle unit is operating in Maguarichi, a small village in an isolated area with no link to the national grid. The geothermal power installed in Mexico represents 2% of the total installed electric capacity, but the electricity generated from geothermal accounts for almost 3% of the national total.     

SolGeo: A new computer program for solute geothermometers and its application to Mexican geothermal fields

The freely available computer program Solute Geothermometers (SolGeo) was written and tested using geochemical data and reported geothermometric temperatures from several geothermal wells from around the world. Subsurface temperatures for the Mexican geothermal fields of Cerro Prieto, Las Tres Vírgenes, Los Azufres, and Los Humeros were estimated based on different solute geothermometers and found to be generally in close agreement with measured well temperatures when considering errors in the calculations and measurements. For Los Humeros wells it was concluded that a better agreement of chemical geothermometric temperatures is observed with static formation than with bottom-hole temperatures (BHTs). It was also found that the widely used Na–K geothermometric equations generally give more consistent and more reliable temperature estimates than the other geothermometers, which should therefore be applied with caution.     

Residual steam to energy: a project for Los Azufres geothermal field, Mexico

More than 500 t/h of residual steam are discharged into the atmosphere at Los Azufres geothermal field. Steam comes from nine back-pressure turbines that are at present generating 45 MW. A significant increase in output can be obtained if low pressure turbines are installed to expand residual steam from atmospheric pressure up to a vacuum pressure in condensing cycles. A net output optimization process for each unit in the condensing cycle is presented here. The exergy concept is also applied to compare efficiencies of back pressure cycles with new condensing schemes that include low pressure turbines. Results show that 27.8 MW of additional net output can be generated with the new schemes at competitive production costs and higher conversion efficiencies.     

Element mobility during the hydrothermal alteration of rhyolitic rocks of the Los Azufres geothermal field, Mexico

The effects of hydrothermal alteration on major, rare-earth, and other trace-element concentrations in rhyolitic rocks of the Los Azufres geothermal field, Mexico, were investigated by statistically comparing the chemical compositions of altered drill cuttings (taken above 450 m depth) with those of fresh rock outcrop samples. Altered rhyolitic rocks show predominantly vitreous and fluidal textures, with alteration products (mainly clay minerals, chlorites and, less commonly, sericite) comprising up to 40% of the rock mass; cryptocrystalline quartz, chlorite, and zeolites fill fractures and vesicles. In altered rocks the greater statistical variances of several elements (Y, Ce, Pr, Nd, Sm, Lu, and Pb) are probably due to alteration effects, whereas smaller variances for CaO, Sr, Rb/Sr, and Rb/Ba suggest that alteration processes have resulted in more uniform chemical compositions. Only MnO, P₂O₅, Ta, Zr, and Nb have significantly different concentrations in hydrothermal altered rocks as compared to fresh rocks. MnO, P₂O₅, Ta, Rb/Zr, and Rb/Nb decrease, whereas Zr, Nb, and Nb/Y increase in the altered rocks. The present study stresses that caution should be taken when using these chemical parameters for petrogenetic studies of old hydrothermally altered areas, particularly with rhyolitic rocks. Rare-earth element (REE) concentrations were not significantly different between fresh and altered rhyolitic rocks. This may indicate that these elements were relatively immobile during the hydrothermal alteration processes affecting the rhyolites at Los Azufres, or more likely that they were reincorporated into hydrothermal minerals after being mobilized from the primary phases.     

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.     

Thermodynamic evolution of the Los Azufres,Mexico, geothermal reservoir from 1982 to 2002

An investigation has been made of the response of the Los Azufres geothermal reservoir to 20 years of development, beginning in 1982. The simulator WELFLO was used to characterize the thermodynamic conditions of the reservoir fluids. The first response to exploitation consisted of a decrease in pressure and an increase in enthalpy. Small decreases in reservoir pressure associated with large increases in fluid enthalpy characterize the long-term response in the northern production area. In the southern production area, long-term changes include decreases in pressure and mass flow rate, increases in steam production and, in wells affected by injection, increases in both pressure and total mass flow rate. These changes reflect the effects of boiling, cooling and fluid mixing, processes resulting from large-scale fluid production.     

Isotopic (δO, δD) patterns in Los Azufres (Mexico) geothermal fluids related to reservoir exploitation

Isotopic patterns for the year 2000 in the Los Azufres geothermal reservoir were related to injection of a condensed steam–water–air mixture as well as to the occurrence of reservoir physical processes resulting from exploitation. Reservoir boiling and mixing of reservoir fluids with cooler fluids were the most important processes identified. Boiling takes place in two zones of the field. In the north, the boiling area includes wells AZ-13, AZ-28, AZ-48, AZ-43 and AZ-32, while, in the south, boiling affects wells AZ-16AD, AZ-22, AZ-18, AZ-26 and AZ-36. Mixing of reservoir fluids with cooler waters was identified in wells AZ-2, AZ-33, AZ-16 and AZ-46 located in the southern zone and in well AZ-4 in the north. The isotopic (oxygen-18 and deuterium) patterns of fluids collected in September 2000 show that the original convective process found in the unperturbed reservoir is still taking place, although mixing of reservoir and reinjected fluids is also indicated. According to N₂ data, the effects of reinjection on the physical and chemical characteristics of the reservoir fluids can be observed in the northern part of the field. Until now, however, only a steam phase, resulting from boiling and steam separation of the re-injected mixture, is evident in the fluids discharged by the northern wells.     

Mineralogy and distribution of hydrothermal mineral zones in Los Azufres (Mexico) geothermal field

The general features of the geometry of Los Azufres reservoir have been defined through the mapping of hydrothermal mineral alteration zones. Hydrothermal alteration has been studied in cuttings and drill cores from most of the active wells. X-ray diffraction microprobe analysis and classical optical methods have been employed for the identification of primary and authigenic minerals in fresh and altered samples. Observed patterns of alteration have been correlated with temperature and patterns of fluid circulation. The resulting model depicts a body of geothermal fluid at depth, which ascends and discharges through two main fracture systems. These two circulation zones are characterized by concentric aureoles of increasing hydrothermal alteration towards quasivertical axes. The overall pattern could be described as a dome structure produced by the abnormal thermal gradient, distorted by the effects of active upward circulation of the fluids.     

A generalized analytical model for hydrocarbon production using multi-fractured horizontal well with non-uniform permeability distributions

The multi-fractured horizontal well (MFHW) has significantly facilitated the exploitation of unconventional hydrogen-carbon compound reservoirs, which is an abundant source of fossil hydrogen energy. The composite linear flow models were frequently adopted to analytically evaluate the hydrocarbon production using MFHW. Most of the existing composite linear flow models assume uniform permeability in the primary fractures and the Stimulated Reservoir Volume (SRV) region. However, due to the non-uniform proppant distribution in the primary fractures and the heterogeneity in the density of the secondary fractures in SRV region, the permeability field in both regions are non-uniform. For the first time, a generalized analytical model incorporating continuous permeability heterogeneity in both the primary fractures and SRV region is proposed for the hydrocarbon production of MFHW. Any arbitrary permeability distribution functions can be treated as inputs to the proposed model. The mathematical model is solved using the perturbation technique and the Laplace transform method.The good match with the numerical solution verifies the accuracy of the proposed analytical model. Subsequently, the proposed solution is assigned with different modes of permeability heterogeneity to investigate their impacts on well production performances, i.e., cumulative hydrocarbon production and transient bottom-hole pressure (BHP). The results reveal that the incorporation of permeability heterogeneity will enhance the production rate and lower the BHP drawdown compared to the homogeneous case with the same average permeability. Besides, a field case is presented to demonstrate the applicability of the proposed model.     

A novel approach for prediction of mass yield and higher calorific value of hydrothermal carbonization by a robust multilinear model and regression trees

This study shows a mathematical and statistical analysis to generate models based on multiple linear regression (MLR) and regression trees (RT) that allow a reliable prediction of the Mass Yield (MY) and the Higher Heating Value (HHV) of the final solid product obtained by Hydrothermal Carbonization, called hydrochar. MLR models were obtained for lignocellulosic and non-lignocellulosic biomass using a set of experimental data with more than 500 points collected from the literature. A new approach based on dimensionless groups of variables that describe the composition of biomass and operational conditions was used. The analysis for each equation indicated that the MY depends on the process conditions and the biomass composition, which is proportional to the Polarity Index (IP) and Reactive Index (IR) values. On the other hand, the severity factor (log Ro) and the initial calorific value (HHVo) were the main factors for the HHV, but also the raw biomass composition (IP and H/C ratio) had an opposite and equal significant effect. For these equations, the results indicated an adjusted R² (R²_a) of about 0.90 and an average RMSE of 6% and 1.7 MJ/kg for MY and HHV, respectively.Besides, explanatory variables were analyzed by their Relative Importance for the RT models. The severity factor (65%) and the IR (18%) were the most decisive variable in the MY prediction. The R² and RMSE were 0.73 and 2%, respectively. For HHV, the variables with the most significant impact were the HHVo (33%), the log Ro (24%), and the IP (22%). In this case, the R² and RMSE were 0.87 and 0.68 MJ/kg, respectively. Therefore, the model equations obtained are a powerful tool to predict the mass yield and the energetic value of the hydrochar before developing an experimental study.     

Application of multi-model control with fuzzy switching to a micro hydro-electrical power plant

Modelling hydraulic turbine generating systems is not an easy task because they are non-linear and uncertain where the operating points are time varying. One way to overcome this problem is to use Takagi–Sugeno (TS) models, which offer the possibility to apply some tools from linear control theory, whereas those models are composed of linear models connected by a fuzzy activation function. This paper presents an approach to model and control a micro hydro power plant considered as a non-linear system using TS fuzzy systems. A TS fuzzy system with local models is used to obtain a global model of the studied plant. Then, to combine efficiency and simplicity of design, PI controllers are synthesised for each considered operating point to be used as conclusion of an electrical load TS Fuzzy controller. The latter ensures the global stability and desired performance despite the change of operating point. The proposed approach (model and controller) is tested on a laboratory prototype, where the obtained results show their efficiency and their capability to ensure good performance despite the non-linear nature of the plant.     

Gas permeation through H2-selective mixed matrix membranes: Experimental and neural network modeling

Gas permeability through synthesized polydimethylsiloxane (PDMS)/zeolite 4A mixed matrix membranes (MMMs) were investigated with the aid of artificial neural network (ANN) approach. Kinetic diameter and critical temperature of permeating components (e.g. H₂, CH₄, CO₂ and C₃H₈), zeolite content and upstream pressure as input variables and gas permeability as output were inspected. Collected data of the experimental operation was used to ANN training and optimum numbers of hidden layers and neurons were obtained by trial-error method. The selected ANN architecture (4:10:1) was used to predict gas permeability for different inputs in the domain of training data. Based on the results, the predicted values demonstrate an excellent agreement with the experimental data, with high correlation (R² = 0.9944) and less error (RMSE = 1.33E−4). Furthermore, using sensitivity analysis, kinetic diameter and critical temperature were found as the most significant effective variables on gas permeability. As a result, ANN can be recommended for the modeling of gas transport through MMMs.     

On the gas permeability of the microporous layer used in polymer electrolyte fuel cells

In an attempt to obtain accurate values of the gas permeability of the microporous layer (MPL), substrates with negligible MPL penetration and of known gas permeability, i.e. membrane filters, have been employed. The values of the MPL permeability obtained using the membrane filters were compared with those obtained conventionally using the carbon substrates. Due to MPL penetration, the MPL permeability obtained using the carbon substrate were found to decrease with carbon loading. On the other, due to negligible penetration, the MPL permeability obtained using the membrane filters were found to be almost invariant with the carbon loadings. Furthermore, the MPL permeability was found to be sensitive to the substrate used: more cracks (and subsequently substantially higher permeability) were shown by the MPLs coating the carbon substrates. This implies that the MPLs coating the carbon substrates and the MPLs coating the membrane filters are structure-wise different. It subsequently means that the MPL permeability obtained using the membrane filters cannot be used to estimate the MPL penetration into the carbon substrates.     

Estimation of pruning biomass of olive trees using airborne discrete-return LiDAR data

The use of residual biomass from fruit tree pruning requires the creation of logistic schedules based on the knowledge of the amount of residues available in each plot. Remote sensing is shown as an important tool to quantify these resources. The objective of this research was to estimate the amount of pruned materials from olive trees (Olea europea L.) using dendrometric parameters obtained from field measurements and airborne LiDAR (Light Detection and Ranging) data. Pruning, mean crown diameter, stem diameter, total height and crown height were measured in the field for a set of 25 trees in Viver, Central East Spain. Airborne discrete LiDAR data were acquired and used to extract the crown area of the trees and metrics based on the height distribution of the LiDAR points. Two types of regression models to predict the amount of pruned material were computed, firstly using dendrometic measurements obtained in the field, and then based on tree metrics derived from LiDAR data. The dendrometric variables obtained from field measurements were also compared with the metrics obtained from LiDAR data. High values of R² were obtained for the models, ranging from 0.86 to 0.89. The regression models derived from LiDAR data used the area as an explanatory variable. The results of this study show the potential of airborne LiDAR data to predict pruning residues in olive trees.     

低渗透油藏开发效果综合评判体系新探讨

针对辽河油田低渗透油藏的特点,建立了低渗透注水油藏评价标准,将反映主观认知度的层次分析法和反映指标重要性的相似权二者结合起来,通过偏好系数确定出方案指标权重,然后用模糊综合评价模型和未确知测度模型对油藏开发效果进行评价。结果表明,开发效果等级处于较好以上储量占22％,虽然近4／5的储量开发效果处于中等以下的水平,但它们采出程度和含水相对较低,挖掘潜力较大。     

Error of Darcy's law for serpentine flow fields: An analytical approach

Serpentine flow fields and other flow fields with partial under-land cross-flow are commonly used in various energy devices, such as proton exchange membrane (PEM) fuel cells and redox flow batteries, due to their higher mass transfer rate to reaction sites and better product removal capability. Accurately predicting the under-land cross-flow rate and pressure drop in such flow fields is crucial in flow field design optimizations. Darcy's law is the most commonly used model in predicting the under-land cross-flow and pressure drop in such flow fields. However, since the Darcy's law neglects inertial effect, its validity in different designs and operating conditions needs to be carefully studied. In this work, mathematical models for a serpentine flow field are developed based on both the Darcy's law and a modified Darcy's law that includes the inertial effect. Both models are solved and analytical solutions are obtained. The predicted pressure drops and under-land cross-flow rates from the two models are compared with experimental data and the results show that under some conditions, both the Darcy's law and the modified Darcy's law can predict pressure drop and under-land cross-flow rate reasonably well. However, under other conditions the Darcy's law can result in significantly large errors in predicting both pressure drop and under-land cross-flow rates. Further studies provide the variations of errors from the Darcy's law with different parameters, including channel length, gas diffusion layer (GDL) thickness, land width, inlet flow rate, GDL permeability and GDL inertial coefficient.     

Large eddy simulation of heat and mass transport in turbulent flows. Part 1: Velocity field

Localized residual or subgrid-scale (SGS) models are presented for use in large eddy simulation of heat and mass transport in turbulent flows. In part (1) (this paper), we discuss the SGS stress models for the velocity field. The models for the scalar field are presented in part (2). The new SGS stress closures are compared with the dynamic-Smagorinsky model (DSM) and the dynamic two-parameter mixed model (DTMM). All models are applied “locally” and their performances are assessed via both a priori and a posteriori analyses with detailed comparisons against data obtained from direct numerical simulation of homogeneous isotropic, homogeneous shear and temporal mixing layer flows. The results of a priori assessments indicate that the new closures predict the SGS stresses better than both DSM and DTMM in all simulated flows. The results of a posteriori assessments also show that the SGS stresses and the statistics of the filtered velocity are more accurately predicted with the new models.