| Since the industrial revolution, the continual development of energy science has greatly promoted social progress, which brought unprecedented convenient and comfortable lifestyle to human being. However, with the population expansion and economic development in the last century, environmental problems have gradually appeared due to the extensive use of fossil fuels. In order to deal with environmental problems and optimize energy structure, many countries have pay attention to hot dry rock resources, which are environmental friendly and with huge potential. HDR resources refer to high temperature rocks that have a very low porosity and permeability. Energy contained in HDR resources can be extracted by artificially stimulating the reservoir to form an artificially altered geothermal system called EGS (enhanced geothermal system).HDR resource development is a complicated engineering; many science and technology issues need to be solved during the process. These issues mainly include rock mechanics, hydrogeology, geophysics, engineering thermophysics, drilling et al. Heat mining is one of the most important issues in the development of HDR. The main physical process of heat mining is the flow and heat transfer of working fluids (H2O or supercritical CO2) through high temperature fractured rock masses.This process can be summarized to two main scientific problems, the first one is the seepage problem in fractured rock mass; the second one is the convection heat transfer problem when fluids flow across rock fracture surface. Another important scientific problem is reservoir modeling and heat production forecast of the site. The optimization design of EGS scheme is of great significance to the development of HDR resources. To reasonably predict the heat production potential, the geological and geophysical condition of reservoir should be studied sufficiently. Meanwhile, the established numerical model should be appropriate and reasonable.Aiming at these problems, laboratory experiments were conducted to study the seepage and convective heat transfer problem of rock single fracture. Combined with geological and geophysical data of Yingshen well regions in northern Songliao Basin, the development of natural fractures and in-situ stress condition were analyzed in detail. To predict the heat production character of naturally fractured reservoir, a numerical multiple interacting continuum model was built, we also conducted an uncertainty analysis of fracture parameters. To predict the heat production potential of tight sandstone reservoir, a detailed experiment on field hydraulic fracturing stimulation was conducted, combining with a pseudo 3 dimension fracturing simulation approaches, the geometric dimension and flow conductivity of the induced fracture are determined by matching the recorded value of well head pressure. A 3D numerical model was established to predict heat production potential of the stimulated reservoir. Additionally, the energy efficiency and environment benefit of the system was evaluated.To study the seepage characteristic of rock fracture, Brazilian split test,3d laser scanning of rough fracture, and scanning electron microscope were conducted. Then the following problems were studied, seepage characteristic of smooth fracture (sandstone and granite); seepage characteristic of granite-proppant system; seepage characteristics of single rock fracture considering stress states and stress history. (1) Seepage experiments results of smooth fracture indicate that flow rates have a linear positive relationship with seepage pressure. When the confining pressure is at a relatively low level, the hydraulic aperture increase with the increase of seepage pressure. When the confining pressure is at a relatively high level, the hydraulic aperture remain unchanged versus the increase of seepage pressure. (2) Seepage experiments results of granite-proppant system indicate that proppants can significantly enhance the flow conductivity of rock fracture. When the confining pressure is high, proppants will crush under the combined effects of seepage and stress. These proppant fragments will block the seepage channel and lead to the decrease of seepage capacity. The asymmetrical distribution of the proppants will create tension stress at local parts of fracture surface, which will generate new crack on the fracture surface. (3) Seepage experiments results of single fracture considering stress states and stress history indicate that cyclic loading process has little influence on seepage capacity of smooth fracture. During a single loading and unloading cycle, the recovery of fracture flow conductivity has a hysteresis effect. For rough fracture, the flow conductivity has significant difference during loading and unloading process. Generally, the seepage capacity of rough fracture can hardly recovery to the level before the confining pressure are loaded.In order to study the seepage and convective heat transfer characteristic of rock fracture, a series of experiments were conducted. In this work, distilled water was used as working fluids. Rock specimen were cut through along the axis to make rough and smooth fracture surfaces. The seepage and heat transfer characteristic of rock fracture were then studied under different flow and temperature condition. Experiments results indicate that the value of convective heat transfer coefficient has a linear positive relationship with flow rate. Compared with smooth fracture surface, the convective heat transfer process of rough fracture surface is more intense. The lithology of rock specimen has no big influence on the convective heat transfer process. It also can concluded that heat transfer intensity is different in different position of the rock fracture, and rock roughness has significant influence on heat transfer process.The relationship between Nu, Re and Pr was fitted using power exponential function. The basic form of the formula is Nu= CRenPrm. The fitted results indicate that the value of C is basically same with the empirical value, while the value of n is distinctly bigger. These results indicate that heat transfer processes in this work are different from smooth flat plate seepage and heat transfer model. This difference may be caused by the fracture surface roughness and the material property of rock samples.By using data from FMI imaging logs, the characterization of the natural fractures in the studied formations are investigated. The Hydraulic Electrical Fracture Apertures of the studied formations are in the range of 0.01-1.4 mm. Apparent fracture densities are in the range of 1.57-5.18/m. Apparent Electrical Fracture Porosities are in the range of 0.002-0.138%. The mean direction of the fast shear waves is mainly in the range of 80° to 110°, indicating that the current day in-situ SHmax strikes nearly in East-West direction. The magnitudes of Shmin and SV are determined to be 82 MPa and 96.5 MPa, respectively. The lower bound on the magnitude of SHmax is determined to be 96.5MPa according to the nearby focal mechanism solutions.In this work, naturally fractured HDR reservoir was numerically simulated and studied. The results indicate that in the very beginning of the system operation, temperature in fracture medium is lower than that of matrix medium. This kind of difference decreases with the distance from the injection well. The decrease of reservoir temperature leads to the increase of fluid density and viscosity, which will cause the increase of injection pressure finally. Research on influence of fracture property on the reservoir heat production indicate that the increase of fracture permeability will speed up the decrease of production temperature. On the other hand, the increase of fracture permeability will cause the decrease of injection pressure, and lead to the decrease of flow impedance. The increase of fracture spacing will decrease the complexity of seepage path and heat exchange area, which will speed up the occurrence of thermal breakthrough. Fracture spacing has little influence on the injection pressure.Research on tight sandstone HDR reservoir in Songliao Basin was conducted numerically and experimentally. Based on the geological data of Dashen well region and a detailed field hydraulic fracturing stimulation experiment, a pseudo 3 dimension fracturing model was established. The numerical model was verified by matching well head pressure. The geometric dimension and flow conductivity of the induced fracture were determined. The forecasted heat production indicates that the maximum production flow rate from the induced fracture is approximately 8 kg/s. During production, flow impedance maintains a level of approximately 9.78-12.32 times that of the commercial standard. Over 30 years of production, the temperature decreases from an initial value of 112.69℃ to a final value of 89.01℃. Moreover, the heat extraction rate decreases from 1.96 MW to 1.17 MW. The thermal breakthrough occurs in the 7th year when the temperature drops by nearly 10%. In 30 years production, the accumulative total produced energy is approximately 1.36×1015 J. About 5.04×107 kg coals can be saved by using the proposed system in 30 years period. Meanwhile, emissions of CO2 and SO2 can be reduced by 1.18×108 kg and 7.75×105kg respectively. |
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