Scientists and engineers funded by the U. Department of Energy DOE are working on that. Once the fractures are open, water is pumped into the wells. It circulates through the hot rock absorbing heat and generates steam in a process similar to what happens naturally at The Geysers.
The first enhanced geothermal system test site in France is already online. When creating an enhanced geothermal system an EGS , the goal is to apply small forces to stimulate rock layers to slip. Creating an enhanced, or engineered, geothermal system requires improving the natural permeability of rock. Rocks are permeable due to minute fractures and pore spaces between mineral grains. Injected water is heated by contact with the rock and returns to the surface through production wells, as in naturally occurring hydrothermal systems.
For an interactive look at the process of how Enhanced Geothermal Systems are created, read through the text version. The concept is described in Potter et al.
HDR was also known as hot fractured rock because of either the need to fracture the virtually impermeable formations or the presence of natural fractures in the hot reservoir Wyborn et al. All of the above usually imply the use of petrothermal systems Ilyasov et al. Schulte et al. Iceland , metamorphic e. Lardarello, Italy , magmatic e.
Soultz, France and sedimentary e. According to Potter et al. Over the years, different definitions of EGS have been proposed, covering a broad variety of rock types, depth, temperature, reservoir permeability and porosity, type of stimulation technique involved, etc. Below are four examples of recent EGS definitions in the public domain.
For this assessment, this definition has been adapted to include all geothermal resources that are currently not in commercial production and require stimulation or enhancement. EGS would exclude high-grade hydrothermal but include conduction dominated, low permeability resources in sedimentary and basement formations, as well as geopressured, magma and low grade, unproductive hydrothermal resources.
Williams et al. The BMU defines enhanced geothermal systems as creating or enhancing a heat exchanger in deep and low permeable hot rocks using stimulation methods. Following BMU's definition, EGS embraces not only HDR but also deep heat mining, hot wet rock, hot fractured rock, stimulated geothermal systems, and stimulated hydrothermal systems.
This lack of clarity may constitute a potential obstacle to the implementation of tailored subsidy programmes. In this study, the MIT definition is adopted with the only difference that geopressured and magmatic systems and also co-produced hot water from hydrocarbon wells are excluded. The reasons for this particular choice are that the MIT report was and still is regarded as a milestone report towards the development of EGS; also, from an engineering point of view, it is perhaps one of the most comprehensive definitions.
On the other hand, it does not enter into the details of the different stimulation approaches and associated consequences for different EGS systems. Recently, for example, Jung has reconstructed the background to contemporary EGS: from the original HDR concept based on multi-zone hydraulic fracturing in competent crystalline formations, through that of open-hole massive injection in naturally fractured crystalline formations and finally to the proposed multi-zone massive injection with the objective of generating multiple wing cracks in naturally fractured crystalline formations.
As this review does not aim at a project-by-project evaluation of the geomechanics that occur during EGS stimulation, the modified MIT definition is considered to be suitable for generating the database proposed in this study.
During the last four decades, there have been some key milestones towards the development of EGS for heat production and electricity generation.
The information that follows is based on the report by Tenzer , supplemented by additional information:. Gallen put on hold due to induced seismicity events with a maximum magnitude of 3. The following review should not be considered exhaustive as it is based exclusively on the information available in the public domain. Yet, to the authors' knowledge, this is the first public attempt to formally collate a large database of information on EGS worldwide, from the first HDR project at Fenton Hill in to date.
The objective of this review is to present key information on past and present EGS experience worldwide, from which key lessons can be learnt for the future.
The 31 EGS projects identified during this review are classified by country, reservoir type, depth, reservoir and wellhead temperature, stimulation methods, induced seismicity and radioactivity, plant capacity, flow rate and current status. Table 1 comprises basic information about EGS projects that are still under development.
It does not include pending commercial projects that are either at the status of raising funds e. Munster in Germany and Eden in the UK or still need governmental approval. Tables 2 and 3 present projects that are already in the power generation phase.
Table 4 gives information about experimental projects that were developed to test single phase of an EGS project rather than the whole process to generate electricity. Table 5 presents information on projects that are aimed for electricity generation but were abandoned due to various problems. Input information was drawn from different sources available in the public domain; all of which are cited in the titles of the tables.
This grouping criteria allow the reader to have an immediate overview of past vs. See the following paragraphs for more discussions on induced seismicity in EGS projects. The authors of this manuscript have not performed an independent review or assessment of the specific stimulation methods implemented in or planned for each individual project, as this falls outwith the scope of this broader EGS review.
Overall, the tables above capture a detailed database of 31 EGS projects worldwide. Based on the tables, the following plots provide a way to extract trends and common characteristics of EGS. Note that only 25 projects are displayed in Figure 1 ; the remaining 6 projects St. Gallen are excluded because flow rate data could not be found in the public domain. Figure 3 displays EGS projects classified on the basis of rock types. Although it appears that EGS activities can be implemented in any of the three major groups of rocks on earth, most projects are developed in igneous rocks, following the original HDR concept.
The recorded maximum magnitudes of induced seismic events associated with the development of EGS projects worldwide are shown in Figure 4. Originally, the Richter scale was developed as a mathematical device to compare local earthquake sizes. The magnitude is defined as the logarithm of the wave amplitude recorded by seismographs. At that time, the smallest measurable earthquakes were assigned with values close to zero.
However, due to the higher accuracy of modern seismographs, the Richter scale now measures earthquakes having negative magnitudes. Majer et al. Later, Majer et al. According to EGEC , microseismic activity is less than 3. Stimulation methods that are applied in EGS developments are summarized in Figure 5 , which reveals that hydraulic stimulation is the most commonly used method, independently of the rock type concerned.
In addition, there are relatively few cases where chemical or thermal stimulation technologies are applied. This often leads to the assumption that the EGS definition only applies to hydraulically fractured systems. The installed electrical and thermal capacity of EGS projects are summarized in Figure 6. Since EGS is still a developing concept, the database contains only 14 projects carried out with electricity generation.
Note that the thermal capacities of Bouillante, Soultz, Lardarello, Desert Peak, Cooper Basin and Coso are missing as data could not be found in the public domain. The variation of production scale causes great capacity differences among the projects. Installed electrical and thermal capacity of worldwide EGS projects.
Figure 7 shows the rock type and well depth of all the studied EGS projects worldwide. From the information provided in the tables and the plots shown earlier, it appears that EGS projects currently under development are still on the learning curve, overcoming problems, gaining experience and trying to introduce advanced technology; the projects already concluded provide relevant history and analogy for upcoming developments and the projects that have been temporarily halted or abandoned give an insight into issues that must be avoided in the future.
Below are field cases where breakthrough methodologies were first implemented to validate the EGS concept. Unplanned events and issues that needed addressing in order to ensure feasibility and commerciality of EGS are discussed, and the corresponding lessons learnt are highlighted.
The project has currently solved the problem of salt deposition, which has led to a suspension of the production test Genesys This single well concept has the advantage of lower drilling costs as only one wellbore is needed to be drilled.
However, since the circulating fluid moves through fractures, it is in direct contact with the rock formation, which leads to salt deposition risk. This experience has taught the geothermal community that flow assurance needs to be addressed ahead of time to prevent issues triggered by the chemical interaction between the injected fluid and the receiving rock, which can impair the overall success of an EGS project.
The reservoir can be investigated by logging tools during production using a special Y-tool which is attached to the production string Henninges et al. This system allows measurements with electrical tools and fibre-optic distributed temperature sensing. However, the data transfer to surface is problematic and can only be done discontinuously Huenges This suggests that further technology advances are needed in the area of well logging for this type of applications.
Using a newly developed fluid monitoring system, fluid physicochemical properties were measured online and in situ Feldbusch et al. Worldwide, it is the only facility for the investigation of sedimentary large-scale structures under natural conditions. A 7-day long-term production-injection experiment between both boreholes to investigate the sustainability of the reservoir was completed in April Feldbusch et al.
A corrosion test will permit the verification of the long-term reliability of the system's components Bine 4 A thermal fluid loop as the initial phase of fluid production was established and continuously operated for 7 days Feldbusch et al.
The Altheim project in Austria uses a special working fluid, which was never used before - a non-flammable, non-corrosive fluid with no ozone depletion activity Bloomquist One of the main lessons learnt from the Fenton Hill project is that an engineered hot reservoir should first be created from the preliminary borehole and then by connecting the enhanced reservoir and the injection borehole with the production boreholes Brown One of the most significant lessons learnt from this project is that natural fractures and engineered fractures are almost unrelated.
The natural fracture network plays a more important role compared with hydraulically enhanced fractures MIT et al. Also, as reported by Jung , until then, the basement had been regarded as a competent rock mass, realizing that in reality, the basement contains open natural fractures even at great depth led to the abandonment of the HDR multi-fracture concept and the adoption of the hydraulic stimulation EGS concept.
A power generation phase was never intended. However, several basic technologies were successfully developed for general EGS activities through the project, which were later applied in another EGS programme in the Cooper Basin, South Australia in Kaieda et al. The Basel area has a history of natural seismic activity; the city was severely damaged by a 6. However, following a 3-year study after the seismic events recorded in connection with the geothermal project activities, the Basel project was cancelled.
Induced seismicity associated with water injection and particularly hydraulic fracturing activities due to changing stress patterns in reservoir rocks has caused wide concern among the public Majer et al. A so-called side-leg concept for the injection well was implemented to solve the problem BINE b.
This concept enables pressure distribution during fluid injection over two separated ends of the injection well, thus minimizing the risk of induced seismicity.
However, in , another induced seismic event with a magnitude of 2. Seismic events of 2. As a consequence of these events, water has to be reinjected at a reduced pressure to avoid induced seismicity, resulting in reduced power generation. The problem is planned to be tackled by implementing in the same side-leg concept that was used in Insheim BINE b. Many experiments were conducted during the first 21 years of the project's life before the power plant was built. Different stimulation techniques, such as hydraulic fracturing with and without proppants and chemical stimulation were applied.
Chemical stimulation has resulted in less seismic activity than other methods. Change of hydraulic parameters due to fracturing has resulted in an instantaneous variation of seismic activity. Seismic events with magnitudes greater than 2 have occurred during the shut-in phase.
Although minor damages were caused by this EGS project, it did generate concern among the local population. Microseismic monitoring has become an indispensable technology for the acceptance of EGS developments as it is the case for other applications of hydraulic fracturing and high-pressure water circulation e. The experience gained from preliminary projects has led to a common view that induced seismicity associated with EGS activities can halt further development of this concept particularly in densely populated areas.
More recently, though, despite the 3.
0コメント