| In recent years, with the growing global energy crisis and environmentalproblems, the development and utilization of renewable energy is imminent.Geothermal energy has many advantages, such as, green, continuous, stable, highefficiency, and there are promising prospects for it, which is expected to become animportant part in the future energy structure. In addition to be used directly,geothermal energy can be used to generate electricity and heating. Because of manyadvantages of geothermal energy, it plays more and more important role in alleviatingenergy crisis and improving the ecological environment. Hot dry rock (HDR) is a newtype of geothermal resources, it has caused the attention of people on account of widedistribution and large reserves, and it refers to the heat but lack of fluid due to lowporosity and permeability of rock mass, mainly various metamorphic rocks orcrystalline rocks. Enhanced Geothermal system (EGS) is technical term that used tomine the heat in the HDR, which mainly include artificial fracturing, thermalconversion, and so on. The concept of EGS project is injecting cold water to thereservoir through injection well and extracting hot water through production well forpower generation, so as to achieve the purpose of the development of hot dry rockheat. EGS is a complicated system engineering, and there are a lot of scientific andengineering problems in this process, such as resource target positioning technology,the artificial fracturing, monitoring technology of micro earthquake and tracermaterial, resource evaluation methods, geothermal geological model, undergroundtemperature field coupling process of rock mass, heat transfer characteristics andmechanism of geothermal medium, the energy conversion efficiency evaluation, theusing efficient of power generation system, construction of demonstration site. With the rapid development of China’s economy and the need of saving energyand reducing emissions, the existing energy structure is no longer meeting therequirements of the domestic situation and the development and utilization ofrenewable energy has become the consensus of the whole society. Geothermalresources can play an important role in alleviating energy supply and improving theeconomic environment. As a big energy consumption country, China also needs toexploitation of geothermal resources and the development of geothermal energytechnology, especially HDR. Due to HDR belongs to the new type of geothermalresources, the world has not yet formed a set of mature theory on the development andutilization, and there is a long time for widely promoting it. China is in its initial stagefor HDR development, and many theoretical research, indoor experiment and fieldtest need to be conducted, such as the determination of thermal properties andphysical and mechanical parameters, reservoir reconstruction and evaluation, waterheat exchange, energy conversion efficiency, underground multi-field coupled processof high temperature rock mass.Based on National High-tech R&D Program (863Program)-“Key technologyresearch on HDR resource development and utilization”(NO.2012AA052801), thepaper carried out the analysis of rock thermal physical parameters and research onmulti-field thermal effect coupled model. Firstly, using rock samples of Ordos basin,Songliao basin and Gonghe in China mainland area, the paper selects optical scanningdevice (TCS) and BRR geotechnical heat capacity tester for determining the thermalphysical parameters of rock sample and tests porosity, mineral composition, watercontent, wave velocity and density of these rock samples, further analyzing therelationship between rock thermal physical parameters and these factors andpredicting rock thermal conductivity by using Support Vector Regression (SVR)model. Secondly, the paper researched the variation of rock thermal conductivity in alldirections of the space and determined its anisotropic characteristics. Thirdly, thepaper studied the influence of temperature and pressure on the thermal conductivity ofdifferent type rock by using genetic algorithm inversion model. Fourthly, theEGS-THM coupled analysis program was further modified and improved that hadbeen developed by our research group, the paper established multi-field thermal effect coupled model according to the analyzing results of reservoir fracturing withSTIMPLAN software and validated the program through experimental model andAmerican Desert Peak EGS project, analyzing the accuracy and applicability of theprogram in the process of EGS engineering simulation and coupled process andmechanism of every physical fields. Finally, aiming at China’s first HDRtarget-Xujiaweizi rift zone in northern Songliao basin area, the paper conducted theanalysis and research of THM coupled for the process of HDR exploitation andutilization and proceeded to evaluate the space-time evolution law of various physicalfields in the process of extracting thermal. This study could provide solid theoreticalsupport for the HDR mining engineering design and contribute to further research andthe practical engineering development of the deep geothermal resource in our country.In the evaluation of rock thermal physical parameters and the study ofinfluencing factors, the paper tested thermal conductivity, specific heat, density,porosity, moisture content, mineral composition and wave velocity of rock samplesthat come from Songliao basin, Gonghe basin and Ordos basin, and then analyzed therelationship between rock thermal physical parameters and different factors andforecasted the changing rule of thermal conductivity. First, rock thermal conductivityand specific heat capacity of Gonghe basin change with depth, specific heat capacityof three boreholes rock samples are modest decline with increasing depth, the value is0.7kJ/kg (K) at final stage and the numerical size is between0.7~1.2kJ/kg (K);thermal conductivity is some fluctuations with the increase of layer depth, however,the overall trend is increasing gradually, and the value tends to be3W/(m K) and therange of thermal conductivity is1.0~3.2W/(m K). Second, for thermal conductivity,specific heat capacity and porosity of different type rock under dry state, the paperfound that there is no obvious regularity between specific heat capacity and porosityand present the largening trend of thermal conductivity with the porosity increase;based on the theatrical relation of rock thermal conductivity and porosity, this paperstudied the relationship between different calculation model and the measured thermalconductivity and found no one model is suitable for all of rock samples, so rockthermal conductivity calculation model was proposed according to different rocksamples. Third, the study thought the wave velocity is not the same for different type rock, and the greater rock wave velocity is basically corroding to the greater thermalconductivity, and a theoretical formula of rock samples is put forward by the wavevelocity. Fourth, the relationship between thermal conductivity and density is not thesame for the same type rock in different rock, which reflect the influence ofenvironment on the rock; On the whole, thermal conductivity gradually increased withthe increase of density, both positively related, probing into the relationship betweenthe fitting analysis; there had no obvious relationship between specific heat capacityand density. Fifth, the range of dry thermal conductivity of different type rock of isnot the same, as the same saturated thermal conductivity, the reason is that thermalconductivity is closely related to the rock mineral composition, structure, and so on;the paper analyzed the influence of the degree of saturation on the thermalconductivity through putting forward the saturation effect coefficient, which wascompared with the theoretical saturation effect by calculating with the formula.Finally, since the thermal conductivity is a very important parameter in geothermaldevelopment and influenced by many factors, there is not one of the theoreticalformula can perfectly match the calculation of thermal conductivity; the paperproposed that SVR model was adopted to predict the thermal conductivity; thesepredicted values are closed to measure values, and the fitting accuracy is higher.Thermal conductivity is closely related to the internal structure of the rock andthe external environment as an important thermal physical properties, which must bedifferent in different directions of the space, namely, it can present obvious anisotropy.Through testing58granite, rhyolite, sandstone, mudstone of Songliao basin andgranite of Gonghe basin, the paper preliminarily researched anisotropic characteristicsof thermal conductivity for different rock with polarizing microscope and X-raydiffraction. Due to structural rock sample is not obvious, the range of the anisotropicthermal conductivity factor is0.85~0.85, proposing anisotropy equation conformed torock thermal conductivity.Generally, because of HDR located in the conditions of high temperature andhigh pressure, its thermal conductivity is obviously different from the value under thecondition of room temperature and atmospheric pressure. Though collecting rockthermal conductivity that had been tested in the laboratory by relevant scholars under high temperature and high pressure conditions, such as quartzite, sandstone andgranite. The paper chose thermal conductivity calculation formula that is widely used,and used artificial intelligence theory for unknown parameters inversion, namely,genetic algorithm. Formulas were determined that is suit for the relationship betweentemperature, pressure and thermal conductivity of different rocks, providingtheoretical basis for the numerical simulation in the following sections.By considering the impact of high temperature and high pressure on thermalphysical property parameters, this paper modified and perfected EGS-THM coupledsimulation program that had been developed by our research group. Based on Taron’stwo-dimensional numerical theory example of Pennsylvania University andengineering geological data of Desert Peak geothermal field in America, theapplicability and rationality of modified program was verified in the EGS project. Thepaper compared and analyzed the change of temperature field, seepage field and stressfield, three fields coupled strength and corresponding heat output change since thechange of thermal conductivity according to the simulating results. The researchfound that the change of thermal conductivity had a certain impact on the temperaturefield and displacement field of the reservoir, but had no effect on pore pressure.Furthermore, the paper studied the influence of the change of rock specific heatcapacity on the underground temperature and pore pressure, and found that thedropping trend of the temperature is more obvious with the greater specific heatcapacity, and the corresponding pore pressure is smaller. The paper further analyzedthe influence of injection rate, injection pressure and different ways of well spacingon the heat output capacity.Based on the engineering geological conditions of Songliao basin, the first HDRtarget area is determined in mainland China that is Xujiaweizi rift zone. For HDRtarget areas in northern Songliao basin, this paper studied rock mechanics parametersunder the condition of high temperature and high pressure using the relatedexperiment equipment, determining the elastic modulus, poisson’s ratio and testing therocky Biot coefficient and fracture toughness. According to practical engineeringgeological conditions in the region, the HDR target reservoir and the optimalfracturing scheme was determined; multi-field thermal effect coupled model was established according to fracturing simulation results and geological conditions,analyzing THM coupled effect in the process of reservoir heat transfer using theimproved EGS-THM program, and determining the spatial evolution rule oftemperature field, seepage field and stress field. The paper carried on forecast for theenergy conversion efficiency and power generation capacity of HDR reservoir in theregion. Moreover, the influence of the uncertainty of the reservoir parameters anddrilling parameters were compared on the coupled model, analyzing the change ofextraction temperature and generated energy. |
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