Research On The Hydraulic Fracturing Of The Shale Considering The Influence Of Initial Temperature Of The Reserves And The Stratification Characteristics

The shale gas which belong to unconventional gas is a clean and efficient energy,and its reserve in China ranks the first in the world.Hydraulic fracturing is an effective method for exploiting shale gas.There are abundant stratifications among the shale gas reservoir whose anisotropy and heterogeneity are apparent.The depth of the shale gas reservoir is mostly about 3000 m in China.The propagation path and the crack morphology are affected by the fracturing fluid in the ambient temperature,the thermal stress is induced by the initial geothermal difference and the properties of stratifications.The thesis study the law of the hydraulic fracturing of the gas considering the initial temperature and the properties of the shale gas reservoir with test,theoretical analysis and numerical simulation,respectively.The following results and conclusions are obtained.In the experiment,an experimental system of monitoring the temperature in the process of heating and fracturing the shale gas were designed.Firstly,the parameters and mechanical properties of the original shale gas were obtained according to the experiment.The results of the experiment show that the mechanical properties of the shale gas in the parallel and the vertical direction to the stratification surfaces are greatly different.The tensile and compression strength are high In the parallel direction and low in the vertical direction.In the vertical direction,the elasticity modulus of the shale is small,the compressive strength is relatively high and the tensile strength is relatively low,The coefficient range of linear expansion is relatively large.In the parallel direction,the difference of the mechanical properties are relatively little.The shale is a material approximating transverse isotropy.Based on the measurement experiments of mechanical parameters and the linear expansion coefficient,a large-size triaxial hydraulic fracturing test of the original shale gas considering temperature and a hydraulic fracturing test of similar specimens of shale gas considering the features of stratifications are carried out.The evolution of water injection pressure and the temperature of the monitored node with fractured time is obtained.When the main crack which is perpendicular to the minimum principle stress on stable propagation period expand to stratifications,the smaller the angle between the stratification surfaces and the initial direction of the main crack propagation is,the easier the main crack propagate in the direction paralleling the stratification surfaces.The larger the angle between the stratification surfaces and the initial direction of the main crack propagation is,the easier the main crack propagate in the initial direction of the crack crossing the stratification surfaces.The difference of the direction of the stratification surfaces,the in-situ stress and the tensile strength of the bedrock is little.When the main crack expand to stratifications,the larger the difference of the stratification and the bedrock is,the easier the main crack propagate in the direction paralleling the stratification surfaces is.The smaller the difference of the stratifications and the bedrock is,the easier the main crack propagate in the initial direction of the crack crossing the stratification surfaces.When the difference of the direction of the stratification surfaces and the strength are little,the larger the difference of the in-situ stress and the stress differences are,the easier the main crack propagate in the initial direction of the crack crossing the stratification surfaces.In the theoretical analysis,based on the theory of the heat transfer and the thermal elastic law,a method of calculating the initial temperature of the shale gas reservoir was obtained,and a heat transfer model and a thermal stress field model of the shale gas reservoir was built.Based on these model,the distributions of the initial thermal stress field was obtained,and a criterion of initiation and propagation of cracks was derived.A fracturing model was built based on the results of the temperature field and the thermal stress field during injecting fracturing fluid.In the numerical simulation,using the extended finite element method,based on the experimental data and theoretical model,the effects of different initial temperatures of the shale gas,different stratification dips,different stratification strengths and different maximum principle stresses on the law of the hydraulic fracturing was obtained.The higher the initial temperature of the reservoir is,the lower the initiation pressure and propagation pressure are.The bigger the angle between the stratification dip and the propagation direction of the hydraulic fracture is,the smaller the difference between the stratification strength and the bedrock is,and the higher the maximum principle stress is,the easier the hydraulic fractures propagate crossing the stratifications.