Sustainability analysis of the Ahuachapán geothermal field: management and modeling

The Ahuachapán geothermal field (AGF) is located in north western El Salvador. To date, 53 wells (20 producers and 8 injectors) have been drilled in the Ahuachapán geothermal field and the adjacent Chipilapa area. Over the past 33 years, 550 Mtonnes have been extracted from the reservoir, and the reservoir pressure has declined by more than 15 bars. By 1985, the large pressure drawdown due to over-exploitation of the resource reduced the power generation capacity to only 45 MW_e. Several activities were carried out in the period 1997–2005 as part of “stabilization” and “optimization” projects to increase the electric energy generation to 85 MW_e, with a total mass extraction of 850 kg/s.     

西藏羊易EGS开发储层温度场与开采寿命影响因素数值模拟研究

西藏羊易地区具有丰富的地热能,单井开发潜力接近10 MW,对其深部热储进行EGS开采,可缓解西部能源紧缺问题。本文建立二维理想EGS开发模型,探讨深层地热开采过程中开采流量、注采方式、注入温度等参数对热储温度场分布及开采寿命的影响。基于羊易温度信息设计了12个数值模型,对比研究发现,开采流量对EGS开采的影响较大,为保证开采50年内的商业利用价值,最大开采流量应控制在0.028 kg/s以下;考虑到钻井成本,注采方式的选择以高注高采和中注高采为最佳;注入温度对热储开采影响较小,可选择40℃ ~ 80℃之间任意温度的地热尾水进行回灌,实现地热资源梯级利用。     

地热资源的开发利用及可持续发展

地热资源作为一种新型能源矿产,具有分布广泛、易于开发等特点,其利用方式主要有地热发电和地热直接利用两种.我国具有良好的地热资源条件,主要为中低温地热资源.据计算,我国12个主要沉积盆地的地热可开采资源量为7500×1018J,相当于2560×108t标煤.当前,我国地热资源利用方式主要以供暖、洗浴、种植等直接利用为主;地热发电发展缓慢,主要分布在西藏;利用热泵技术开发地热资源得到了快速发展;油区地热资源的开发利用也取得了良好的经济和社会效益.但同时我国地热资源产业也面临着一些问题,包括大部分地区尚未开展地热资源勘查评价,影响了地热资源规划的制订及地热产业的发展;防腐、防垢技术还需要进一步加强研究;地热回灌率普遍过低;增强型地热系统研究有待加强等.为了促进地热资源的可持续发展,建议在加大地热资源勘查力度的同时,应以浅层地温能和热水型地热资源为主,发挥热泵技术的优势,开展地热资源的综合利用及梯级利用;重视和加快油气区地热资源的利用;在西藏等适宜地区加大高温地热能发电利用;集中全国优势技术力量,在一两个有利区域开展增强型地热系统技术探索;此外,走回灌开发道路是地热资源开发利用的必然选择.     

Optimization of the exploitation system of a low enthalpy geothermal aquifer with zones of different transmissivities and temperatures

Market penetration of renewable energy sources, such as geothermal energy, could be promoted even by small cost reductions, achieved through improved development design. This paper deals with optimization of the exploitation system of a low enthalpy geothermal aquifer, by means of the method of genetic algorithms, which has been successfully used in similar problems of groundwater resources management. With respect to water flow, the aquifer consists of two zones of different transmissivities, while from the thermal point of view it may bear any number of zones with different temperatures. The optimization process comprises the annual pumping cost of the required flow and the amortization cost of the pipe network, which carries the hot water from the wells to a central water tank, situated at the border of the geothermal field. Results show that application of the proposed methodology allows better planning of low enthalpy geothermal heating systems, which may be crucial in cases of marginal financial viability.     

Detailed study of sulfide scaling at la courneuue nord, a geothermal exploitation of the Paris Basin, France

The fluid of the Dogger aquifer is always used through a closed loop formed by the production well, the heating plant and the injection well. After two or three years of exploitation of the geothermal doublets in the northern part of the Paris basin, scaling and plugging problems have appeared in some cases. The results of the detailed study carried out at La Courneuve Nord, a typical site of this area, are presented.The drawdrown of production rate, scaling in the heat exchanger and the increase of injection pressure required a rapid decision for workover operations on the wells. These cleaning operations and joint research studies allowed us to identify the cause of the plugging as well as to locate these deposits and to estimate their importance. Iron sulfide was found from the base of the production well casing all the way to the injection well casing. The thickness of this scale induces hydraulic pressure loss along the casings. Moreover, part of this sulfide is carried away by the fluid and fills up the bottom of the open hole injection well. After the cleaning operations, the hydraulic properties of the reservoir seem to be recovered.Chemical and mineralogical analyses of theses deposits identified the presence of a large variety of iron sulfide (mackinawite, pyrite, pyrrhotite) and a typical corrosion product (Fe₂(OH)₃Cl). Biochemical and bacteriological studies show a very high content of micro-organisms. At the end of the workover operations, several geophysical logs gave information on the corrosion of the casing as well as the residual scale. A balance of the iron content is calculated, between the volume of deposits and the iron removed by the corrosion of the casing.A chemical model, TPDEGAZ, is used to calculate the evolution of the saturation indexes of the fluid with respect to iron sulfide phases. The effects of parameters such as pressure, temperature, degassing and addition of iron by corrosion are simulated. The results of the observation and modeling approaches are compared.     

An updated numerical model of the Larderello–Travale geothermal system, Italy

Larderello–Travale is one of the few geothermal systems in the world that is characterized by a reservoir pressure much lower than hydrostatic. This is a consequence of its natural evolution from an initial liquid-dominated to the current steam-dominated system. Beneath a nearly impermeable cover, the geothermal reservoir consists of carbonate-anhydrite formations and, at greater depth, by metamorphic rocks. The shallow reservoir has temperatures in the range of 220–250 °C, and pressures of about 20 bar at a depth of 1000 m, while the deep metamorphic reservoir has temperatures of 300–350 °C, and pressures of about 70 bar at a depth of 3000 m. The 3D numerical code “TOUGH2” has been used to conduct a regional modeling study to investigate the production mechanism of superheated steam, the interactions between the geothermal field and the surrounding deep aquifers, and the field sustainability. All the available geoscientific data collected in about one century of exploration and exploitation have been used to provide the necessary input parameters for the model, which covers an area (4900 km²) about 10 times wider than the Larderello–Travale geothermal field (400 km²). The numerical model explains the origin of the steam extracted in about one century of exploitation and shows that, at the current level, the production is sustainable at least for the next 100 years.     

A review of energy in Rwanda

During the last two decades, Rwanda has experienced an energy crisis mostly due to lack of investment in the energy sector. With the growing of the population and increasing industrialization in urban areas, energy provided by existing hydro and thermal power plants has been increasingly scarce with high energy costs, and energy instability. Furthermore, as wood fuel is the most important source of energy in Rwanda, the enduring dependence on it and fossil fuel consumption as well, will continue to impact on the process of environmental degradation. Rwanda is rich with abundant renewable energy resources such as methane gas in Lake Kivu, solar, biomass, geothermal; and wind energy resource is currently being explored. Recently, the Government has given priority to the extension of its national electrical grid through development of hydro power generation projects, and to rural energy through development of alternative energy projects for rural areas where access to national grid is still difficult. This paper presents a review of existing energy resources and energy applications in Rwanda. Recent developments on renewable energy are also presented.     

Geothermal development in Thailand

San Kampaeng and Fang geothermal areas are considered areas of interest for the exploitation of geothermal energy. The technologies of exploration and development have been studied by Thai scientists and engineers during the past four years. The first geothermal deep exploration well was drilled, in cooperation with Japan International Cooperation Agency (JICA), in the San Kampaeng geothermal area. In 1985, supplementary work is planned to define the deep structural setting in greater detail before starting to drill the next deep exploration well. In Fang geothermal area some shallow exploitation wells have been drilled to obtain fluid to feed a demonstration binary system of 120 kWe, with the technical cooperation of BRGM and GEOWATT, France. The plant will be installed next fiscal year.     

黄河三角洲可再生能源利用现状分析及对策

黄河三角洲具有丰富的可再生能源,科学合理地开发利用可再生能源,对于黄河三角洲的可持续发展具有十分重要的意义。文章阐述了黄河三角洲风能、太阳能、地热能、生物质能等可再生能源资源及利用现状;深入分析了开发利用中存在的问题;有针对性地提出了加快制定黄河三角洲可再生能源专项规划、组建黄河三角洲可再生能源专门管理机构、制定黄河三角洲可再生能源利用运行机制、深化拓展黄河三角洲可再生能源开发利用和营造可再生能源开发利用的社会氛围等相关对策。     

The geothermal system of Ischia Island (southern Italy): Critical review and sustainability analysis of geothermal resource for electricity generation

In this paper we analyze the main available data related to the geothermal system of Ischia Island, starting from the first geothermal exploration in 1939. Our aim is to define a conceptual model of the geothermal reservoir, according to geological, geochemical, geophysical and stratigraphic data. In recent times, the interest on geothermal exploitation for electricity generation in Italy is rapidly increasing and the Ischia Island is one of the main targets for future geothermal exploitation. Nowadays, one of the main economic resources of the island is the tourism, mainly driven by the famous thermal springs; so, it is crucial to study the possible interaction between geothermal exploitation and thermal spring activities. To this aim, we also analyze the possible disturbance on temperature and pressure in the shallow geothermal reservoir, due to the heat withdrawal for electric production related to small power plant size (1–5 MWe). Such analysis has been performed by using numerical simulations based on a well known thermofluid-dynamical code (TOUGH2^®). Obtained results show that such geothermal exploitation generates a perturbation of temperature and pressure field which, however, is confined in a small volume around the well. At shallow level (0–100 m) the exploitation does not produce any appreciable disturbance, and can be made compatible with thermal spring exploitation. Moreover, such results are crucial both for the evaluation of volcanological processes in the island and for the general assessment of geothermal resource sustainability.     

地热发电的投资经济分析

地热能是一种清洁的可再生能源,越来越多的国家宣布支持地热开发。地热发电必须考虑到影响成本的各种因素,地热发电的成本主要由初始投资和电力生产运行及维护成本两部分组成。地热项目具体的投资成本与资源特征和现场条件有着非常密切的关系,资源的温度、深度、化学特性和渗透性是影响发电成本的主要因素。与传统化石燃料发电相比,地热发电已具有相当的竞争力,在生命周期内地热发电厂的平均成本大大低于传统燃料发电厂。另外,地热发电还有抵消化石燃料价格波动对电力市场影响的作用,有利于促进农村和偏远地区经济发展,有利于能源供应多元化。当然,地热能发展也面临着一些障碍,包括钻井的成功率、地热技术尚不够完善以及项目启动成本高等。建议今后地热资源的利用不再仅局限于极少数高温地热项目中,而是尽可能发掘地热资源的所有潜力。     

Geothermal assessment of the deep aquifers of the northwestern part of the Bohemian Cretaceous basin, Czech Republic

Groundwater in the Benešov-Ústí aquifer system in the northwestern Bohemian Cretaceous basin has been intensely exploited since the twentieth century. Apart from providing drinking water, it contains the most extensive accumulation of thermal water in the country. However, excessive exploitation can result in temperature declines and changes in the quality of the groundwater in the future. More than a hundred in situ temperature measurements were used to assess the geothermal gradient and heat flux. However, intense groundwater vertical flow across the well significantly controls the heat flux distribution, resulting in a huge range of values—from less than 50 mW/m² within infiltration areas to more than 125 mW/m² in drainage areas. Certain simplifications and corrections considering the vertical flow between different permeable zones were developed, and the correction for topography as well as lithological variability have been applied to improve accuracy of the geothermal gradient assessment. Despite the fact that the Bohemian Cretaceous basin is tectonically very complex, it is concluded that tectonics [with the exception of the Eger (Oh?e) rift] has only a secondary effect on the thermal field. Two longitudinal W-E areas in the Benešov-Ústí aquifer system have elevated heat flux values. The calculated heat flux values are useful for heat transfer modelling and the assessment of the sustainable limits of thermal water exploitation.     

Geothermal electric power in Iceland: Development in perspective

Geothermal energy is extensively used in thermal (direct) applications in Iceland. More than 70% of the total population enjoy geothermal district heating. Hydro-power provides most of the electricity generated in Iceland, with less than 10% of the potential harnessed. Iceland is well endowed with both geothermal (high- and low-temperature) and hydro-power resources. At the end of 1980, the installed geothermal power in Iceland was 818 MW₁ in direct applications and 41 MW_e in electric power generation. This exploitation represents a few percent of the estimated geothermal resources of Iceland. Plans to develop geothermal electric power in Iceland date back to the early 1960s. The first geothermal electric power plant (3 MW_e) was installed in 1969. In recent years, several small-scale (two 1 MW_e and one 6 MW_e) geothermal power units have been installed in a cogeneration plant for district heating purposes. There is one major (30 MW_e) geothermal electric power plant in Iceland, which became operational in 1978. Hydro-power, geothermal energy and oil provide consumers in Iceland with about 18, 38, and 44% of their energy needs, respectively.     

Low enthalpy geothermal fluids from the Paris sedimentary basin—1. Characteristics and origin of gases

Fifty-seven wells tapping low enthalpy geothermal waters from the Dogger limestone reservoir of the Paris Basin have been sampled and analysed for their gas composition. Methane/ethane ratios indicate that hydrocarbons originate from both biogenesis and thermogenesis. Relatively high and variable H₂ concentrations are likely to result from fluid interaction with geothermal installations. Regional trends among main species (N₂, CO₂, CH₄) reflect large-scale heterogeneities which were already revealed by the geochemistry of the fluids (Criaud et al., 1986). Northern sites are generally nitrogen-rich and southern sites methane-rich, a characteristic which may be linked to the occurrence of oil-bearing zones in the south. N₂, Ar and He absolute contents show clear positive correlations. In particular, N₂—Ar trends are best explained by the occurrence of a paleocomponent, likely to be an evolved seawater. Helium model ages are consistent with the geological age of the host formation but are in contradiction with hydrologic ages. Assuming an exotic flux of helium into the aquifer this discrepancy may be overcome, but its computed rate apparently exceeds the current estimates for the continental degassing flux.     

Thermal and geochemical structure of the Uenotai geothermal system, Japan

The Uenotai geothermal area is located in southern Akita prefecture of northern Honshu Island. The Uenotai geothermal system is a liquid-dominated system with a central zone of aquifer boiling. The two-phase reservoir has evolved from liquid in the natural state due to exploitation. Gas composition of the vapor phase in the reservoir is nearly in equilibrium and correlates with the vapor fraction in the reservoir and with discharging steam quality. The marginal part of the Uenotai system has cooled with the drop in ground-water level. The chemical characteristics of the geothermal water indicate mixing of the immature high Cl source water with conductively heated or steam-heated shallow water or surface water, as well as boiling and steam gain.     

The United Nations' approach to geothermal resource assessment

Although the emphasis of United Nations' assisted geothermal projects has been on demonstrating the feasibility of producing geothermal fluids, the potential capacity of individual fields has been estimated by both the energy in place and decline curve methods. The energy in place method has been applied to three geothermal fields resulting in total resource estimates ranging from 380 to 16,800 MW-yr. The results of these studies must be considered highly tentative, however, due to inadequate reservoir data and a poor knowledge of producing mechanisms. The decline curve method has not given quantitative results concerning ultimate field potential because of the relatively short duration of well tests (several weeks to a maximum of 11 months). In all cases, however, the decline of flowing wellhead pressure, field pressure, and flow rate has continued to decrease with time.A new method for making regional assessment of geothermal potential is described, which is based, in part, on an assessment of the probable range of the power potential of geothermal fields as inferred from a frequency distribution analysis of fields already under development throughout the world. Depending on the reservoir containing dry steam or water, and its location in a region of groundwater recharge or discharge, average power potentials can be expected to range from 36 to 3360 MW.     

Mixing of thermal and non-thermal waters in the Steamboat Hills area, Nevada, USA

Groundwater monitoring began in 1985 at two geothermal facilities in the Steamboat Hills area, Nevada. Wells representing non-thermal, thermal, and mixed waters are evaluated by assessing temporal variations in B and Cl concentrations, water levels, and temperature. The objective is to assess the hydrologic and geochemical connection between the fractured bedrock geothermal reservoir and the alluvial aquifer. Results suggest that fault-controlled groundwater flow between the geothermal system and the alluvial aquifer is the dominant hydrologic process. Temporal trends suggest that the thermal water component in the alluvial aquifer has increased in most areas but decreased in at least one area.     

高温岩体地热开发的技术经济评价

介绍了对美国Fenton Hill(芬登山)地区高温岩体地热开发所作的投资分析，以及应用MIT经济模型进行的高温岩体地热发电开发成本的预测与评价。研究认为，高温岩体地热资源从技术上来说是重要的潜在能源，但钻井技术和人工储留层的建造技术是阻碍其商业发展的关键因素；目前高地温梯度的高温岩体地热发电已具有商业竞争力，采用先进的线性钻井技术后，将使所有等级的地热梯度的高温岩体地热发电电价具有商业竞争能力。     

Direct uses of earth heat

Potential resources and applications of earth heat in the form of geothermal energy are large. World-wide direct uses amount to 7072 MW thermal above a reference temperature of 35°C. District heating is the major direct use of geothermal energy. Equipment employed in direct use projects is of standard manufacture and includes downhole and circulation pumps, transmission and distribution pipelines, heat exchangers and convectors, heat pumps and chillers. Direct uses of earth heat discussed are district heating and cooling, greenhouse heating and fish farming, process and industrial applications, combined and cascading uses. The economic feasibility of direct use projects is governed by site specific factors such as location of user and resource, resource quality, system load factor and load density, as well as financing. Examples are presented of district heating in Reykjavík, Klamath Falls, Melun l'Amont and Svartsengi. Further developments of direct uses of geothermal energy will depend on matching user needs to the resource, and improving load factors and load density.     

Cold groundwater temperatures and conductive heat flow in the Mt. Amiata geothermal area, Tuscany, Italy

The hydrogeochemical study of the cold springs present in the Mt. Amiata geothermal area, where the Bagnore and Piancastagnaio geothermal fields are situated, has defined the different shallow groundwater systems.The cold groundwater temperature of the volcanic phreatic aquifer is largely correlated with the geothermal heat flow. Through the analysis of the temperature of cold groundwaters, a possible method for geothermal prospection has been developed, supported by the results obtained in the studied area. Through the enthalpic balance of the aquifer and in agreement with available data, a geothermal flow of about 200 mW m^-2 has been calculated.