Mechanism And Experimental Research Of Supercritical Carbon Dioxide Fracturing Shale

Greater demands of energy in China were observed with the development of the economy in the recent years.Excessive dependences of energy supply on the traditional fossil fuels results in the low proportion of the green energy in the energy consumption.Challenged by the greater energy demand and the severer environmental problems,the efficient exploitation of shale gas becomes an inevitable method.Shale gas is a kind of unconventional natural gas,and the reserves of shale gas is abundant and widespread.It is necessary to use the fracture method to achieve the commercial capacity because of the low-porosity and low-permeability of shale gas.The hydraulic fracture was attributed to the booming exploitation of shale gas in United States and it is also the dominant technology for shale gas exploitation both at the domestic and abroad.The application of hydraulic fracture is restrained by the deep burial of shale gas resources,high clay content and deficient water resources.SC-CO2 is regarded as a promising fracture fluid because its high permeability,low viscosity,high density and enhanced desorption.SC-CO2 fracture can not only reduce the fracture initiation pressure,but also improve the exploitation efficiency of shale gas and the complexity of fracture network.In addition,the carbon capture and storages were also achieved during the fracturing process.Theoretical analysis,laboratory experiment and the field experiment were conducted in the present work to obtain a better understanding of the fracturing initiation mechanism and the propagation law of fractures.The main work and conclusion are presented below:1.The physical characteristics of carbon dioxide were investigated,and their variations were accurately calculated.The facilitative effects of SC-CO2 physical characteristics(high density,low viscosity and high thermal conductivity)on the fracturing process were analyzed.The influence of the in-situ stress on the initiation and expansion of the cracks were investigated.The stress condition of wall rock was investigated from the perspective of rock mechanics.The equation of breakdown pressure was obtained based on the tensile failure criterion.2.A real triaxial fracturing system of SC-CO2 with acoustic emission device was developed independently.The mechanical and geometric parameters of the laboratory fracturing test were designed on the basis of similarity theory,and the relation between the parameters of the laboratory test and the field tests were obtained.Based on the hundreds of tests,the preparation method of the fracturing samples and natural rock samples were obtained.Both the open hole completion and perforation completion were achieved.The developed triaxial test system and completion method were obtained to ensures the success of the SC-CO2 fracturing test and improve the accuracy and credibility of the physical simulation test.3.The SC-CO2/hydraulic fracture experiments in the cases of open hole/perforation completion were conducted on the real triaxial fracturing system.The results showed that the fracturing pressure of SC-CO2 fracturing was lower than that of hydraulic fracturing.The numbers of main fractures,secondary fracture,fracture steering,fracture bifurcation and fracture surface morphology were analyzed based on the fractured samples.Compared with the hydraulic fracture,the SC-CO2 fracture had more complex crack propagation characteristics,and it's the secondary cracks and bifurcation were easier to occur in the SC-CO2 fracturing process.The fracture surface was also more uneven and complex for the SC-CO2 fracture than that of hydraulic fracture,which was easy to produce secondary cracks and bifurcation phenomena.The blasting sound,gas jet and the difference of surface temperature,occurred in the fracturing process,were explained from the aspects of expansion and phase transition.These unique phenomena of SC-CO2 play dominant roles in facilitating the fracturing process.4.The field experiments were conducted in the optimal well positions.The results showed that the CO2 was in the supercritical state at the early stage of slotting process.The SC-CO2 turned to the liquid state due to the crack propagation before it gets to the well bottom.Then the liquid CO2 return to the supercritical state at the well bottom in a short time.At the same time,the micro-seismic monitoring results showed that the pre-SC-CO2 fracture was able to form a complex fracturing network,and then the slots expansion could be further achieved using hydraulic fracture combined with sand.The low viscosity and the strong permeability of the SC-CO2 impaired the effects of in-situ stress in the fracturing process.As a result,more cracks occurred and the complex fracture networks were formed.