Three Dimensional Fracture Propagation Simulation Coupled With Thermal-Hydro-Mechanical Processes For Carbon Dioxide Fracturing

In the more than 200 years after the industrial revolution,a large amount of carbon dioxide produced by the burning of fossil fuels was directly discharged into the atmosphere without treatment,resulting in an increase in the concentration of carbon dioxide in the atmosphere,which in turn led to an increase in global temperatures.Some scholars have proposed the idea of supercritical carbon dioxide to promote the efficient development of shale gas and the integration of underground storage to solve the contradiction between human energy demand caused by carbon dioxide emissions and climate warming.In the supercritical state,carbon dioxide has the advantages of good solubility high density,and strong diffusion and low interfacial tension.In recent years,supercritical carbon dioxide has been used as a fracturing fluid.Most of the previous studies on the fracture propagation of carbon dioxide fracturing focused on physical experiment simulation.Due to the size effect,the laws revealed by physical experiments are often difficult to meet the actual engineering needs.Fortunately,numerical simulation techniques can solve the scale effect problem,However,in the previous numerical simulation of supercritical carbon dioxide,the difference in physical properties between liquid and supercritical carbon dioxide was neglected,and the fracture propagation process in the height direction and the horizontal direction was separated,resulting in limitations of the fracture propagation in height direction and the calculation of carbon dioxide physical parameters.In this paper,the fracture propagation of supercritical carbon dioxide fracturing is studied basing previous researches.The theory of fracture mechanics,fluid mechanics,thermodynamics,heat transfer and mass transfer,and the mathematical calculation methods of boundary element method,finite volume method and finite difference method are used to establish three-dimensional fracture propagation model coupled thermo-hydro-mechanical process of supercritical carbon dioxide fracturing.Based on this model,a simulation program of fracture propagation during supercritical carbon dioxide fracturing was developed.And the influence of different factors,such as the reservoir temperature,reservoir in-situ stress,Young's modulus,Poisson's ratio,fluid type,injection rate and bottom-hole temperature,on the fracture propagation have been analyzed.The research contents and results are summarized as follows.(1)Analysis of supercritical carbon dioxide fracturing and numerical simulation technology of fracture propagation.The characteristics of supercritical carbon dioxide have been analyzed,and the common difficulties of supercritical carbon dioxide fracturing fracture propagation simulation were sorted out.Aiming at the particularity of the dynamic change of fluid physical parameters during supercritical carbon dioxide fracturing and the high requirements for phase state control,the existing fracture propagation model is analyzed,and the modeling idea of carbon dioxide fracturing fracture propagation simulation is proposed.(2)Established a three-dimensional fracture propagation model coupled hydro-mechanical process.Based on the three-dimensional boundary element method and the finite volume method,considering the leak-off of fracturing fluid,the physical and mathematical model of three-dimensional hydraulic fracturing fracture propagation coupled hydro-mechanical process is established.To calculate the position of the fracture front,the level set function in the extended finite element is introduced.The three-dimensional fracture propagation model is solved based on the Newton-Raphson iteration.(3)Established a three-dimensional fracture propagation model coupled thermo-hydro-mechanical process during supercritical carbon dioxide fracturing.Considering the influence of temperature and pressure on the physical properties of carbon dioxide,based on the energy conservation criterion,the temperature field calculation model of the fluid in the fracture is established.The Span-Wagner model and the Vesovic model are used to calculate the carbon dioxide phase and physical parameters in the fracture during the fracturing process.The temperature field model and the carbon dioxide parameter calculation model are coupled with the three-dimensional fracture propagation model,established in the previous step,to construct a three-dimensional fracture propagation model coupled thermo-hydro-mechanical process during supercritical carbon dioxide fracturing.(4)The factors affecting the fracture propagation of carbon dioxide fracturing are revealed.Based on the above model,a three-dimensional fracture propagation simulation program coupled thermo-hydro-mechanical process during supercritical carbon dioxide fracturing has been developed.And the influence of different factors,such as the reservoir temperature,reservoir in-situ stress,Young's modulus,Poisson's ratio,fluid type,injection rate and bottom-hole temperature,on the geometry of the fracture and the bottom-hole pressure have been analyzed.It is found that in addition to the rock mechanics parameters of the reservoir,the geometry of the supercritical carbon dioxide fracture crack is also affected by the temperature of the reservoir and the in-situ stress.And the influence of the injection rate on the fracture morphology cannot be ignored.