| The energy structure based on fossil fuel, has a significant effect on livingenvironment of mankind. Hot dry rock (HDR), as an extension of conventionalhydrothermal resources, due to its environment-friendly, clean, renewable and widelyspatial distribution characteristics, is regarded as the most potential new energy in the21stcentury.Currently, the geothermal resources for exploitation and utilization are mainlyhydrothermal. HDR is a kind of high temperature rock in depth without water orsteam, mainly consisting of metamorphic rock or crystalline rock. A conservativeestimation of energy of the HDR in the earth’s crust (depth of3~10km, temperatureof150~650℃) is equivalent to the30times of the total energy contained in the oil,gas and coal all over the world. The latest evaluation result of the China GeologicalSurvey shows that the resource amount of the HDR in the mainland of China is about860trillion tons of standard coal and if2%of the resource can be utilized, it isequivalent to the5300times of the total disposable energy consumption of China.Enhanced Geothermal System concepts would exploit economically deepgeothermal energy contained in rocks with low permeability by creating an artificialreservoir, which is also called engineering geothermal system. Cold water is injectedinto the artificial thermal reservoir through injection well, heated through contactingwith the rocks, and returns to the surface via production well. The processes extractheat from a closed loop and don’t need supplement of amounts of water after thesystem reaches a steady condition.EGS development includes two important stages: hydrofracturing stage andextracting heat stage. At the hydrofracturing stage, fluids with low temperature and high pressure are injected into fractured reservoir and induce a decrease in effectivestress. Consequently, shear slip occurs on the fracture plane and permeability isenhanced. At the extracting heat stage, cold water flows through the injected well andgoes into hot reservoir which is fractured, and exchanges heat with hot rock. At theend, hot fluids are pumped out through production well. Unreasonable injectionstrategy would lead to thermal breakthrough early and have to end the project. Fromthe view point of numerical simulation, stages mentioned above are actually coupledprocesses of mechanical (M) and thermal-hydrodynamic (TM). This paper will firstlyestablish a coupled THM model and then develop a coupled simulator. The developedsimulator is used to analyze the spatial and temporal evolution of pressure,temperature and stress during shear stimulation and evaluate the enhancement ofpermeability. Based on coupled wellbore/reservoir simulator, the optimal injectionstrategy, evolution of pressure and temperature, and production performance areestimated in wellbore/reservoir system.On the aspect of program development, Terzaghi consolidation theory andextended Biot equation considering pressure and temperature effects are combinedwith coupled TM model in TOUGH2to form two coupled THM models:(1)1-Dmechanics coupled with3-D TH processes,(THMC-Terzaghi)(2) general3-Dmechnics coupled with3-D TH processes (TOUGH2Biot). Based on finite elementmethod for mechanics and integral finite difference method for coupledthermal-hydrodymanic processes and using a sequential calculation approach betweenM and TH, a mechanical module is developed in the framework of TOUGH to forman integral THM simulator.1-D consolidation problem with analytical solution isemployed to verify coupled mechanical-hydrodynamic processes in simulators ofTHMC-Terzaghi and TOUGH2Biot. While the problem of1-D settlement induced bythermal conduction is used to verify coupled mechanical-thermal processes. Moreover,a geothermal field problem associated with coupled THM processes is employed tocompare the results of TOUGH2Biot and TOUGH2-FLAC3Das well as field observations. According to analysis of calculation time on coupled THM problems oftypical CO2geological sequestration and geothermal development, we can concludethat iterative calculation for coupled TH processes and mechanics is the mosttime-consuming, which provides an important refereence for parallelization of THMsimulator. In addition, some modification for TOUGH are conducted, such asnon-Darcy flow at a low or high velocity, local thermal non-equilibrium andcomputational efficiency.Based on the data of Deserk Peak EGS field, a model with random distributedfractures for shear stimulation is constructed and two modules of an equivalentpermeability tensor and permeability enhancement model are added to TOUGH2Biotfor specifying Desert Peak shear stimulation. During the well27-15multiple stages ofshear stimulation (wellhead pressure limited to the maximum of4.7MPa),permeability is not increased at the first and second stages. The equivalentpermeability in the vicinity of the well27-15is about1mD under the naturalcondition and the initial injectivity is0.13kg/s/MPa. In the third and fourth stage,permeabilities are enhanced significantly and injectivities reach about1.0kg/s/MPaand1.2kg/s/MPa, respectively. The first and second permeability enhancements are50and10times. The distances of permeability enhancement after the third stageextend to about60m in the vertical and the horizontal maximum principal stressdirections and about40m in the horizontal minimum principal stress direction, whichpresents apparent directivity in the evolution of permeability. Permeabilityenhancement is mainly derived from temperature decrease inducing shear failure atthe fracture planes. Relative high pressure only promotes the migration of the coldwater to far away. During the phase of well shut-in, permeability is reduced by55percent comparing to the third stage’s permeability gains. Pressure recovers to thelevel of initial state. But temperature doesn’t bounce back to that of initial state, whichinhibits the performance of fracturing in the fourth stage. Sensitivity analysis ofinjection temperature indicates that using moderate injection temperature fluid can obtain the maximum permeability enhancement. There is about15%difference inwell injectivity between considering and without considering new fractures generationduring shear stimulations.Based on the geological condition of Yingshen well region of Songliao Basin, aconceptual model with an assumed artifical reservoir of500m×500m×140m isestablished. Production performances (wellhead temperature, production mass rateand heat extracting rate) for the well pattern with one injection and one productiononly depend on pressure difference of injection and production wells. Regarding thepressure difference and injection temperature as controlled optimal parameters and alimited temperature drop of10oC at the downhole of production well after30years ofproduction, the optimal injection pressure and temperature are obtained at1.7MPaand20oC, respectively. Mass rate and net heat extracting rate are about12.0kg/s and4.5MW, respectively. The cycle time period is0.2year for the shortest flow path, andis2.5year for the longest flow path. Considering the optimal injection strategy, waterpressure and temperature increase from wellhead to downhole of injection well,howerver, they decrease from downhole to wellhead for the production well. Changein pressure is almost contributed by water gravity. Change in temperature iscontributed by heat conductive exchange with the surrounding rock and gravitationalpotential energy. The rate of contribution is7:3. Sensitivity analyses of well pattern,permeability of fracture, fracture space, wellbore diameter and reservoir burial depthindicate that (1) horizontal wells are better than vertical wells;(2) fracturepermeability with the magnitude of10mD is suitable for30years of production in thenormal pressure range;(3) a small fracture space takes a large heat exchange area,which contributes to the great net heat extracting;(4) production performance is betterusing a smaller diameter well than using a big one;(5) a small depth of reservoirdoesn’t obviously lead to decrease in net heat extraction, but a large depth of reservoirresults in a significant increase. Cold water injection will induce land subsidence,which extends from injection well to production well. The maximal land subsidence is about0.25m, which occurs tens of meters away from injection well. Changes inpressure and temperature contribute to the deformation of formation and the latterdominates the magnitude and extension of subsidence. |
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