ABSTRACT:

Recently, water injectivity index was found to be huge different between process of hydraulic fracturing and long-term water injection during enhanced geothermal energy development. This discrepancy is due to improving the conductivity of natural fractures in geothermal reservoirs is essentially different from traditional hydraulic fracturing in conventional oil and gas reservoirs. The effective conductivity aperture of natural fractures becomes an important parameter to characterize fracture conductivity in geothermal reservoirs, which is affected by initial fracture roughness, shear-induced dilation, thermoelasticity as well as chemistry of hot dry rocks. A coupled thermal-hydro-mechanical-chemistry model was developed to analyze the composition of the total hydraulic aperture and primary role in different fracture section. The result shows that thermal effect plays a major role in increasing the conductive aperture in the near borehole area, which is also an effective way to reduce the surface pumping pressure at the hydraulic fracturing site. However, relative to thermal induced aperture, shear dilation remains stable along the fracture and increases with the injection pressure. The effect of chemical dissolution/precipitation of short-term hydraulic fracturing on fracture aperture is too small to be negligible. Therefore, Long term high pressure injection is the best way to enhance the conductivity of several hundred meters of underground water circulation and heat exchange fracture network.

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