Thermoplastic Fracture Model For Rock In Deep Formations And Its Applications

Energy exploitation in deep formation is a fundamental problem closely related to China's energy strategy.Since thermoplasticity is a major mechanical characteristic for rock in deep formation of high temperatures and pressures,understanding of thermoplastic fracture model and thermoplastic fracture characteristics for high-temperature rock remains challenging and crucial,which determines the safety and efficiency of energy exploitation in deep reservoir.In this paper,the thermoplastic fracture model and its application for high-temperature rock are studied systematically,including rock thermoplastic constitutive model,thermoplastic fracture model,characterizations of thermoplastic fracturing,and the application of thermoplastic fracture model in high-temperature hydraulic fracturing.In this study,a thermoplastic constitutive model is proposed,which considers the effects of hydrostatic pressure,deviatoric stress and temperature on rock thermoplasticity.The proposed constitutive model involves a temperature-dependent Drucker-Prager hardening rule,thermal loading/unloading criteria,and stress-strain-temperature relation.For Tournemire shale,the thermo-plastic properties in the proposed model are determined from the existing thermo-mechanical tests.The model is compared with the thermo-mechanical measurements of Tournemire shale,with the predicted stress-strain curves well consistent with the experiments data(correlation coefficient: 93%),which is sufficient to verify the validity of the proposed thermo-plastic constitutive model.Based on the thermoplastic constitutive modeling method,a thermoplastic fracture model is proposed,which regards the softening fracture process zone(FPZ)as a cohesive crack.The proposed fracture model involves the temperature-dependent yield and softening rules,a thermoplastic loading/unloading criteria and a constitutive relation coupling cohesive stress,crack opening displacement(COD)and temperature.Moreover,thermoplastic fracture characteristics are represented with the thermoplastic fracture model,such as the temperature-dependent cohesive tensile strength(CTS),critical COD,fracture energy and FPZ size.In addition,the characterization model of thermoplastic fracture characteristics under transient temperature differentials(transient-??)is proposed,revealing the effect of transient-?? on thermoplastic fracturing.Based on the thermoplastic fracture model,the experimental methods for characterizing thermoplastic fracturing of high-temperature rock are established,and a thermoplastic fracturing measurement system(including detections of digital image correlation(DIC)and acoustic emission(AE))is designed.With the experimental system,the three-point bending tests are applied to high-temperature Shandong granite specimens at high temperatures and under transient temperature differentials(??),respectively.The combined characterization of loading curves,DIC-based displacement/strain field and AE energy distribution reveals the thermoplastic fracture characteristics,i.e.parameters of the thermoplastic fracture model.The new proposed model is compared with the fracturing measurements of Shandong granite,with the predicted fracture characteristics well consistent with the experiments data(81%?98%),which is sufficient to verify the validity of the proposed thermoplastic fracture model.Four groups of cube granite specimens(dimensions: 300/300/300 mm)are tested to investigate the characteristics of the hydraulic fracture,for two confining pressures(0.1/0.1/0.1 MPa and 10/25/30 MPa)and temperatures(20 ? and 120 ?).AE-based spatial distributions of energy and source mechanisms are employed to characterize the propagation of high-temperature hydraulic fracture and delineate the microcrack band generating surrounding the hydraulic fracture.Based on the measurements,three indicators for hydraulic fracturing effectiveness at different temperatures are proposed: fracture energy of microcrack-band,microcrack-band volume and fracture mechanisms of microcrack-band.The thermoplastic fracture model is used to interpret the change law of hydraulic fracture propagation under the effect of high temperatures and transient-??.For example,the high-temperature hydraulic fracture propagates intermittently with increasing resistances.The effective width of hydraulic microcrack-band is reduced by 40?56.4%,and the fracture energy is reduced by about 75% adjacent to the wellbore in the microcrack-band.The high-temperature(120 ?)reduces the proportion of shear microcracks by 6?12% in hydraulic microcrack-band.Based on the proposed model and experiments,two optimization methods of hydraulic fracturing in high temperature reservoir are proposed.