Progress in hot dry rock exploitation

The term hot dry rock (HDR) has been used to describe the exploitation of the thermal energy contained in rocks that have a sufficiently high temperature but contain insufficient fluids to enable the heat to be extracted. The attraction of such a resource is that it is probably available everywhere, but at varying depths. International activity has focused on the problem of circulating fluids through the rock and extracting the heat by conduction from the rock to the fluid. The initial concepts were based on creating individual cracks to interlink two wells approximately 500 m apart. The necessary heat exchange area was to be achieved by using multiple systems of interconnections in parallel. The results from the field work have shown that the interlinking is dominated by stimulated natural joints and the degree of normal dilation that can be achieved is limited by the orientation of the natural discontinuity with the pre-existing stresses and, hence, the shear stress on the joint. The normal dilation is important because it controls the resistance to flow between the wells. The shear mechanism controls the far-field water losses and the direction of growth of the more permeable interwell region and has obvious implications for the proposed geometry of any system. The cost of drilling has been shown to be comparable to deep drilling for other purposes and the development of deviated holes in strong crystalline rocks has been shown to be possible. The progress of the stimulation has been mapped successfully by locating the microseismic events generated by shearing but the relationship of the microseismically active areas and the heat transfer region has yet to be identified. No adverse environmental problems have been identified; the produced fluids are generally benign and the microseismicity is well below any threshold of damage. The goal of a universally available heat source free of stack emissions and waste products that does not consume finite reserves of minerals and hydrocarbon demands substantial investment in the research to determine if it is attainable. The major field programmes should have reached their preliminary conclusions by the middle of 1986.     

Extraction of heat from multiple-fractured dry hot rock

A brief summary of theoretical solutions for the extraction of heat from multiple-fractured dry hot rock is presented. The analysis is based on an infinite series of parallel vertical fractures of uniform aperture and separated by blocks of homogeneous and isotropic impermeable rock with a uniform spacing between fractures. Cold water enters at the bottom of the fractures and the solutions are given in terms of dimensionless parameters from which the exiting water temperature at the top of the fractures can be determined. An example of the application of the theory is given to demonstrate how a multiple, fractured system provides a more efficient mechanism for heat extraction than a single fracture in dry hot rock.     

Stimulation and flow processes in hot dry rock reservoirs

There was less agreement among the stimulation and flow processes group than in the other groups, reflecting the fact that this aspect remains the major uncertainty in the understanding of HDR systems. The differences arose from the different emphasis placed by the various research teams on the role of artificial as opposed to natural fractures. There was, however, general agreement that the flow in HDR reservoirs is predominantly laminar. It was also agreed that impedance can be reduced only by enlarging fractures apertures; the best way forward, however, depends on resolution of the “artificial” versus “natural” fracture controversy. This will require further theoretical work in close association with prolonged circulation in the major deep field experiments.     

Borehole measurements in the field of hot dry rock experiments

The logging group examined the value of different types of borehole logging in the context of HDR developments and considered the limitations of current techniques and the scope for further development. They recommended that the main development effort should concentrate on methods of fracture detection, both at the borehole wall and in the far field. They further recommended that such work would best be done as part of a collaborative international effort.     

Modeling heat extraction from hot dry rock in a multi-well system

A mathematical model is developed for describing the heat energy extracted from a hot dry rock in a multi-well system. The solutions for the water temperature, accounting for a geothermal gradient in a geothermal reservoir, are given in the Laplace domain and computed by numerical inversion, the modified Crump method. The results show that the heat extraction effectiveness is affected significantly by the well spacing, well radius, reservoir thickness, and pumped flow rate in a multi-well system. The water temperature decreases with increasing pumping rate and increases with the well spacing, well radius, and reservoir thickness. The geothermal gradient affects only the early time heat extraction effectiveness significantly and has direct impact on the water temperature all the time if the vertical thickness of geothermal reservoir is large. The present solution is useful for designing and simulating the heat extraction project of geothermal energy exploitation in a multi-well system.     

干热岩热能开发降温压裂技术

针对增强型地热系统压裂工艺,提出了降温压裂技术,利用压裂前置液对裂缝地层进行降温。文章以一口地热井的压裂施工数据,计算分析了射孔位置温度和裂缝内压裂液温度的分布,为压裂前置液注入量的计算提供依据。计算结果表明:压裂液对井筒降温的效果显著;裂缝内压裂液温度梯度分布较大;裂缝前端50 m范围内的压裂液温降明显,可满足耐高温压裂液的温度要求。     

Development of hot dry rock technology at the Hijiori test site

Since 1985, the New Energy and Industrial Technology Development Organization (NEDO) has conducted a Hot Dry Rock project at the Hijiori test site, Yamagata prefecture. The objective of this project is to develop and test technologies such as borehole logging, hydraulic fracturing, fracture mapping and reservoir evaluation, which are essential for the development of a Hot Dry Rock power generation system. In 1991, heat was successfully extracted from a shallow reservoir at a depth of 1800 m for three months using one injection well (SKG-2) and three production wells (HDR-1, HDR-2 and HDR-3). About 80% of the injected water was recovered from these production wells. The thermal output of hot water and steam reached about 8 MW. Since 1992, a deep reservoir at a depth of 2200 m has been developed. In 1995 and 1996, heat extraction tests were conducted using one injection well (HDR-1) and two production wells (HDR-2 and HDR-3). A long-term circulation test, lasting about two years, is planned to evaluate the reservoir, starting in 2000.     

Forecast and evaluation of hot dry rock geothermal resource in China

Utilizing information from plate tectonics characteristics, volcanic activities, and geothermal anomaly, this paper identifies areas where hot dry rock (HDR) may exist as potential geothermal resource in China. Further investigations are also carried out in the paper based on results from regional tectonics, volcanic geology and lithology, as well as data from geothermal displays, geochemistry, geophysics, and shallow borehole temperature measurements. The study reveals several promising areas of HDR geothermal resource in China, including Tengchong of Yunnan province, Qiongbei of Hainan province, Changbaishan of Jilin province, Wudalianchi of Heilongjiang province, and the Southern Tibet area. A 3D static heat conduction model was developed to study the underground temperature gradient characteristics of the Rehai geothermal field in Tengchong and the Yangbajing geothermal field in Tibet. The model adopted is a geological block 10 km deep from the ground surface and 50 km wide in each of the horizontal directions (2500 km² area). The numerical simulation results in evaluations on the quantities of the HDR geothermal resource in Rehai and Yangbajing geothermal fields. The paper shows that there is abundant HDR geothermal resource with large exploitation value in China. If developed with a power capacity of 1×10⁸ kW, the Rehai and Yangbajing fields along would be able to generate electricity for 1560 years.     

Renewable hot dry rock geothermal energy source and its potential in Pakistan

Geothermal energy source, one of the viable renewable energy sources, has encouraging potential to generate full base-load electricity, which has not been explored so far in Pakistan. Though the country can be benefited by harnessing the hydro-geothermal options of energy generation in areas where sources exist, but most of these sources lie in extreme remote and inaccessible rugged mountainous ranges away from the urban-industrial centers. On the other hand, the present study shows that the HDR geothermal option is one of the most viable renewable sources considering the tectonic setup of Pakistan. Results of the study highlight the HDR geothermal energy prospects at relatively deeper depths than hydro-geothermal resources in water-free condition. The basement tectonic analyses reveal that the HDR prospects could be found even just below the urban-industrial centers of Pakistan where there are no hot springs and/or geysers like southern Indus basin in Sindh province or the Kharan trough in the western Balochistan province. Presence of high earth-skin temperature gradient trends derived from satellite temperature data and the high geothermal gradient anomalous zone derived from scanty data of bottom-hole temperatures of some of the oil and gas exploratory wells, indicates encouraging prospects for HDR energy sources in southern Indus and Thar Desert regions inclusive of Karachi synclinorium area. These high geothermal gradients have been inferred to be the result of the deep-seated southern Indus and the Thar fossil-rift structures. Moreover, the prospects of the HDR geothermal energy sources have also been inferred in the Chagai Arc region and the Kharan–Panjgur tectonic depression in the western part of Pakistan based on the analysis of integrated geophysical data. If HDR prospects are developed, they can offer the sustainable, CO₂-free and independent of time, of day, of weather or season, and the base-load energy-generation resource.     

Development program of hot dry rock geothermal resource in the Yangbajing Basin of China

Geothermal energy from hot dry rock (HDR), considered an almost inexhaustible source of “green” energy, was first developed and tested in the 1970s, leading to installations in America, Japan, Britain, France and other countries. In the present work, a liquating rock mass at a depth of 5-15 km in the Tibet Yangbajing region in China was subjected to detailed analysis. The temperature distribution of the geothermal field in the region was determined by the finite element method. The results estimate that the HDR geothermal resource of the Yangbajing region is 5.4 × 10⁹ MW a, representing a huge potential source of HDR geothermal energy for China. Based on detailed research into the continental dynamics of the environment forming the HDR geothermal field of Tibet, along with the tectonic characteristics of the southern slope of Tanggula Mountain and the Dangxiong-Yangbajing Basin, and the magnitude and orientation of the in situ stresses in the region, the design of an arrangement for extracting these HDR geothermal resources is proposed: taking the fault zone nearest the high-temperature liquating rock region as the location of an artificial reservoir, a vertical injection well could be drilled at a low point on the downdip side of the fault, and two dipping production wells drilled higher up. In this way, an artificial reservoir 3 × 10¹¹ m³ in volume would be created: 360 times the volume of the HDR geothermal reservoir in Cornwall, UK, which uses hydrofracturing. An investigation of the reservoir features, including seepage analysis of the heat exchange area, project implementation and investment analysis, indicates that a 10⁴ MW capacity power station with a projected operating life of approximately 100 years could be constructed. An analysis of a geothermal extraction system comprising one injection well and two production wells suggest that a power station of 1000 MW installed capacity could be constructed initially to provide electricity production of 8.64 × 10⁹ kWh per year.     

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。     

高温岩体圈定的思路与方法探讨

高温岩体地热资源开发利用首先要解决的是在目前技术和经济条件下可以开发利用的高温岩体分布地区和赋存深度及规模。提出了高温岩体圈定的基本思路应该是在对高温异常区的岩石类型及分布、断裂构造格架及其活动性、火山活动和岩浆侵入的类型及时代等基础地质研究的基础上,并充分利用地球物理探测技术、遥感技术、地球化学方法,配合少量深钻验证工程来进行。特别强调了在高温岩体地热资源圈定和评价中多种方法和技术的相互配合、相互补充、相互验证。     

Rock-water interactions in the Fenton Hill, new Mexico, hot dry rock geothermal systems I. fluid mixing and chemical geothermometry

The chemistry of fluids circulated through an artificially-stimulated, hot dry rock (HDR) fractured geothermal reservoir system in granitic rock is described in terms of mixing phenomena, geothermometry, and approach to saturation with reservoir rock minerals. Based on the similar dynamic behavior of Na⁺, K⁺, Li⁺, CI⁻, and B species and other isotopic evidence, the presence of a concentrated in-situ pore fluid was identified. Mixing and displacement of this in-situ fluid with meteoric make-up water is responsible for the observed behavior of the major dissolved species in the circulated fluid of this HDR system.     

Hot dry rock: a new geothermal energy source

A project being conducted by the Los Alamos Scientific Laboratory is attempting to demonstrate the technical and economic feasibility of extracting energy from the hot, dry rock geothermal resource. The system being tested is composed of two deep boreholes drilled into hot, impermeable rock and connected by a hydraulically produced fracture. In September 1977, the circulation loop was closed for the first time and water was circulated through the downhole reservoir and through a pair of 10-MW (thermal) heat exchangers. A series of long-term experiments is planned for 1978 in order to evaluate the thermal, chemical and mechanical properties of the energy extraction system.     

The relationship between thermal comfort and user satisfaction in hot dry climates

Reviews on thermal comfort approaches and studies including field studies, laboratory studies and multi-dimensional scaling, have been carried out. It appears that the tendency of current research is towards more sophistication in measuring and relating objective to subjective domains of thermal comfort. The desire for increased accuracy has resulted in a situation where thermal fields have been studied in isolation from behavioural ones and independently of the environmental control system.This paper has encapsulated thermal comfort, the environmental control system and overall user satisfaction in two propositions, which were subjected to an empirical test in a sample from two different house typologies. Statistical analysis of user responses and climatic data revealed significant correlations between thermal comfort and the ability to effect environmental control, and thermal comfort and overall user satisfaction with the house form. Other variables, such as privacy were not significantly correlated with overall user satisfaction.     

On the size of hot spots in the dry sliding of metals

This paper investigates the influence of the interfacial temperature rise, in the dry sliding of metals, on the size of hot spots. An area expansion coefficient that corrects for such thermally induced changes is introduced. It is shown that the discrepancy between theoretical and experimental estimates of the area ratio, reported by some authors, may be avoided by taking into consideration the effects of temperature elevation on the properties of the sliding pair: namely, material softening and thermal expansion.     

The dependence of permeability on effective stress from flow tests at hot dry rock reservoirs at Rosemanowes (Cornwall) and Fenton Hill (New Mexico)

Effective stress is the primary control on permeability and thus on flow and water loss for two-well hot dry rock systems involving injection and production that have been tested to date. Theoretical relations are derived for the flow between an injector and producer, including the dependence of permeability on effective stress. Four relations for permeability as a function of effective stress are used to match field data for the hot dry rock systems at Rosemanowes, Cornwall, and Fenton Hill, New Mexico. The flow and water loss behavior of these systems are well explained by the influence of effective stress on permeability. All four relations for permeability as a function of effective stress are successful in matching the field data, but some have difficulty in determining unique values for elastic and hydrologic parameters.     

Performance analysis and improvement of the use of wind tower in hot dry climate

Wind towers for passive evaporative cooling offer real opportunity for improving the ambient comfort conditions in building whilst reducing the energy consumption of air-conditioning systems.This study aims at assessing the thermal performance of a bioclimatic housing using wind towers realized in a hot dry region of Algeria. Performance monitoring and site measurement of the system provide data which assist model validation. The analysis and site measurement are encouraging, and they confirm the advantage of the application of this passive cooling strategies in hot dry climate.A mathematical model is developed using heat and mass transfer balances. For a more effective evaporative cooling, a number of improvements on wind tower configurations are proposed.