| Enhance Geothermal System is mainly utilized for generating power through extract the thermal energy of hot dry rock in the depths of the earth’s crust, with feature that high temperature,usually rang 150℃ to 300℃ or even higher. On the one hand, during the process of EGS drilling will encounter cracks and lead to lost circulation, it must be controlled. If the lost-circulating zone locates in the thermal reservoir, the plugging operation cannot reduce the permeability of the reservoir and impede the flow of the fluid. On the other hand, fracturing is one of the key technologies of the EGS development, for it can reduce the cost of development. Also, temporary plugging technology is very important for the multilayer hydraulic fracturing. We can use the temporary plugging material to seal the first crack and fracturing the second crack, even the third crack. After completion and undergo high temperature, the temporary plugging material could self-degradation and recover the permeability of the rock when encountering water.Slag is a kind of particles that are rich in vitreous body, having fine activity when it is milled into powder. High-calcium fly ash(class C) has volcanic ash and hydraulic. These materials will form very high compressive strength when are activated by alkali. With the curing temperature rising, the strength will gradually increase. The hydration products of the alkali-activated slag and class C fly ash at the normal temperature mainly consist of calcium silicate hydrate, calcium hydrosilicate and so on. However, at higher temperature, it will generate zeolites which harder than those hydration products above. When high temperature decomposition, the sodium carboxymethyl cellulose(CMC) could product a great deal of carbon dioxide and acetic acid steam what can increase porosity and destroy the pore structure of cement. Those decomposition products will react with sodium silicate which unreacted or partial reaction. A large amount of heat will be generated in the reaction and promoting the alkali-activated cement product cracks.In this paper, we mainly dependent on the characteristics of the EGS drilling and fracturing, utilizing features of high temperature and corrosion resistance of the alkali-activated slag / class C fly ash cement and polymer thermal degradation to start the experimental study of self-degradation cement.We discuss and analysis the basic rules that amount of sodium silicate, ratio of slag with fly ash effect on setting time and compressive strength of the alkali-activated cement. The experimental results reveal that setting time can be delayed with the increasing of the amount of sodium silicate and fly ash, compressive strength would increase if the curing temperature rises range from 200℃ to 300℃. Also the compressive strength could reach up to 30 MPa about two times of class G oil well cement after curing five hours at 200℃.The setting time reaches up to 90 minutes at 90℃.According to the experiments, we obtained the best basic formula: the ratio of slag with fly ash is 80/20, the added amount of sodium silicate is 6%. Also the compressive strength could reach up to 30 MPa about two times of class G oil well cement after curing five hours at 200℃.The setting time reaches up to 90 minutes at 90℃. The experimental results reveal another phenomenon that LV-CMC could promote the alkali-activated cement’s self-degradation and effective delay the setting time, but the added volume should not over 1.5%. Entirely performance evaluating experiments show that: alkali-activated self-degradable cement has good performances of sedimentary stability, water loss, free fluid and corrosion resistance. |
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