| The development of every trade consumes lots of energy in new century. On the one hand, fossil energy such as coal, oil and natural gas are non-renewable in a short term; on the other hand, a large amount of burning fossil fuels will bring about more serious air pollution and greenhouse effect. So development and research of renewable energy becomes more and more significant. Geothermal energy is the world’s third largest renewable energy, only after the water and biomass. Geothermal energy is clean and renewable energy source of widely distributed, and takes more and more attention of the energy researchers.Enhanced Geothermal System is a kind of project which develops and uses deep geothermal energy. In 2012, National 863 Program launched the project of research on the key develop and comprehensive utilization technology of hot dry rock, this project regarded Xujiaweizi in songliao as exploration target. This paper bases on research of water-rock interaction in the project, and takes Yingcheng Formation of songliao basin as the main research object, then carries on research through the method of experiment study and numerical simulation. Rhyolite of Yingcheng Formation is main material in the experiment, besides this experiment there has anther experiment of granite for comparison. Observation the change of main ionic concentration in solution and electron microscopic morphology of rock slice in experiment, analysis and study phenomenon and mechanism of minerals dissolution/precipitation in the process of water rock interaction. The main conclusions of experiment are as follows:1) In the initial stage of rhyolite water rock interaction all kinds of minerals have been dissolved, with the reduction of the experimental temperature quartz precipitation, calcite dissolution and feldspar selective dissolution, albite dissolution but k-feldspar precipitation. Concentration of K+ changed slightly and concentration of Na+, Ca2+, Si changed obviously during experiment.2) In the initial stage of granite water rock interaction quartz and feldspar dissolution but calcite precipitation. After the reduction of the experimental temperature, dissolution/precipitation phenomenon of quartz, feldspar and calcite is likes the rhyolite experiment. The volume of mineral dissolution/precipitation is little in this experiment compare with rhyolite experiment. Besides, there have been observed little kaolinite precipitation by electron microscope.Because of the flow of heat transfer fluid in EGS, the trend of dissolution/precipitation is different between injection well and production well, this cause physical characteristics(such as porosity and permeability) of geothermal reservoir changes differently. Therefore this paper simulates water rock interaction in EGS to explore the trend of mineral dissolution/precipitation and change of physical characteristics in geothermal reservoir. There are three simulations in this paper: rhyolite water-rock simulation of inject water temperature at 20℃, rhyolite water-rock simulation of inject water temperature at 65℃ and granite water-rock simulation of inject water temperature at 20℃. Through the analysis of the results of numerical simulation, the conclusions can be concluded as following:1) In the numerical simulation of rhyolite reservoir, when inject water temperature is 20℃, quartz precipitation in most simulation areas; Calcite and albite dissolution near injection well but precipitation near production well; K-feldspar precipitation near injection well but become dissolution near injection well. Dissolution and precipitation of this mineral lead to porosity and permeability increase near inject well and decrease toward product well. When inject water temperature is 65℃, porosity and permeability are bigger than the result of 20℃ inject water.2) In the numerical simulation of granite reservoir, dissolution/precipitation trend of quartz, feldspar and calcite is likes the rhyolite reservoir, but the change of volume fraction of quartz, k-feldspar and calcite is smaller. Besides, the change of volume fraction chlorite is litter and has litter effect on porosity. The change of porosity and permeability is small than the result in rhyolite water-rock simulation.3) According to the result of numerical simulation, the order from big to small of mineral volume fraction change is: calcite, k-feldspar, quartz, albite and chlorite. With the increase of simulation time, mineral volume fraction changes bigger, and porosity changes obviously after enough simulation time. According to the simulation results it can be considered that porosity and permeability are mainly affected by dissolution and precipitation of calcite in water-rock interaction. |
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