Experiments On The Subcritical Fracture Propagation Of Granite Under The Action Of SC-CO₂

Hot dry rock Geothermal resource is clean and renewable resources,which is distributed widely and has great potential for development.Enhanced Geothermal System(EGS)is usually used to develop hot dry rock.Hydraulic fracturing is a key technique in the EGS with disadvantages of high injection pressure and single fracture.This paper combined with hydraulic fracturing and existing subcritical extension theory to propose a step loading method for hydraulic fracturing.The comparison experiments of conventional and step loading hydraulic fracturing were carried out with granite specimen.Based on multiple sets of granite specimen for different step loading time,differential stress and fracturing fluid(water and supercritical carbon dioxide)for fracturing experiments.Microcrack initiation and propagation are analyzed using injection pressure and bottom hole temperature.Fracture morphology of the specimens is used to analyzed experimental variables of subcritical crack propagation.The most favorable conditions for subcritical fracture propagation were obtained through breakdown pressure and fracture morphology,which is useful for subcritical fracture propagation in hot dry rock development.The experimental results show that :(1)the breakdown pressure of granite decreases under the long-term step loading period,and the fracture surface becomes more complex;(2)under the high stress difference,the pore pressure around the existing crack is larger,resulting in lower breakdown pressure;(3)fracturing fluid type has various impacts on subcritical fracture growth.The lower the fracturing fluid viscosity,the stronger the permeability,reducing the breakdown pressure and enhancing fracture complexity.