| Hydraulic fracturing is the main technical means for unconventional reservoir reconstruction of oil and gas resources.Low permeability glutenite reservoirs are widely distributed in continental tight oil and gas reservoirs in China.However,due to the characteristics of deep glutenite reservoirs,such as large burial depth,complex pore structure and strong heterogeneity,its hydraulic fracturing mechanism and reconstruction technology have always been an important frontier topic of concern to engineering and academic circles.Because of the difficulties faced by field observation technology and laboratory testing methods,numerical simulation has become an effective means to understand and quantitatively describe the characteristics and influencing factors of hydraulic fracturing of low permeability glutenite.Yet as the result of the challenges in obtaining deep conglomerate core,heterogeneous particle model construction,fracture propagation simulation,mesh generation,algorithm efficiency of large-scale model and experimental verification,the numerical simulation method of hydraulic fracturing of low permeability conglomerate has become a common concern.The main research work carried out in this paper is as follows:(1)Based on the typical deep sandy conglomerate cores from oil field,the composition,physical properties,geometric characteristics and spatial distribution of natural sandy conglomerate,as well as the macro-physical and mechanical properties of sandy conglomerate,were obtained by X-ray diffraction,micro-focus CT scanning and image analysis.The implementation scheme of the reconstructed model can reconstruct the model consistent with the physical and mechanical properties of the real glutenite core.(2)A numerical simulation method for continuous-discontinuous process of hydraulic fracturing of low permeability glutenite has been developed,and the numerical simulation of fracturing fracture propagation of glutenite has been realized by using the numerical reconstruction model.The three-dimensional parallel finite element-discrete element simulation technology of complex particle structure is improved,and the efficient and reliable simulation of three-dimensional fracturing network of glutenite is realized.The three-dimensional spatial shape and distribution characteristics of fracturing fracture propagation of glutenite under different in-situ stress differences are obtained,and the influence of the spatial distribution of glutenite particles on the law of fracturing propagation of hydraulic fracturing is analyzed.The finite element-discrete element-finite volume method considering fluid-solid coupling effect is developed to simulate the evolution process of solid and fluid in fracturing of glutenite.The fractal characteristics of tensile and shear fracture types and fracture complexity in fracturing considering fluid-solid coupling effect are discussed.Aiming at the phenomena of filtration and induced microseismicity during hydraulic fracturing of sandy conglomerate reservoirs,an adaptive finite element-discrete element simulation method considering fluid-solid coupling and filtration failure was developed to quantitatively analyze and reveal the influence of geometrical and distribution characteristics of heterogeneous sandy gravel on fracture propagation,convergence behavior,filtration and microseismic effect.(3)A physical reconstruction model of sandy conglomerate is developed,which is consistent with the grain distribution characteristics and macroscopic physical and mechanical properties of natural sandy conglomerate.Physical experiments of true triaxial hydraulic fracturing and fracture net CT scanning are carried out.The composition of natural sandy conglomerate,physical properties,geometric characteristics and spatial distribution characteristics of heterogeneous particles were tested and analyzed.The physical model of glutenite was prepared by gradation selection and preparation method of manually prepared concrete specimens.Through the true triaxial hydraulic fracturing experiment,the hydraulic fracturing crack propagation behavior and distribution law of glutenite model with different in-situ stress levels and particle distribution characteristics were analyzed,and compared with the numerical simulation results.The research in this paper shows that:(1)Fluid-solid coupling and filtration failure during hydraulic fracturing should be the important factors affecting fracture propagation in glutenite.In this paper,a three-dimensional digital reconstruction model of sandy conglomerate based on the composition and structural characteristics of natural sandy conglomerate is proposed,and a finite element-discrete element-finite volume simulation method for fracturing fracture propagation considering the continuous-discontinuous characteristics of hydraulic fracturing of low permeability sandy conglomerate,fluid-solid coupling and filtration failure is presented.It can effectively simulate and quantitatively analyze the discontinuous propagation behavior of fractured fractures and the influence of non-uniformly distributed particles on the discontinuous propagation behavior.The obtained three-dimensional fracture propagation and spatial distribution laws are in agreement with the experimental results of true triaxial hydraulic fracturing of the physical model of glutenite.Compared with the previous numerical simulation methods,it is found that whether the composition and structural characteristics of glutenite can be accurately reflected,and whether the fluid-solid coupling and filtration failure should be taken into account,will significantly affect the simulation results of fracture propagation and spatial morphology of hydraulic fracturing.(2)Geometric and distribution characteristics of heterogeneous gravel are important factors affecting fracture propagation,fracture network morphology and micro-earthquake induced by hydraulic fracturing of gravel.When the fracturing fracture extends to the edge of gravel grains,the gravel hinders the fracture propagation,leading to the fracture stop or expand around the edge of gravel;the more the number of gravel,the greater the effect of preventing the fracture propagation.Distribution characteristics of gravel have a significant impact on fracture propagation and spatial distribution.With the increase of grain size distribution of gravel,the total length,volume and filtration capacity of crack propagation decrease at first and then increase.Microseismic events are easily induced around gravel.(3)The in-situ stress level of low permeability reservoir is the main factor controlling the fracture propagation behavior of hydraulic fracturing.With the increase of in-situ stress difference,the fractal dimension of fractured fractured network of low permeability glutenite tends to decrease gradually.When the horizontal in-situ stress ratio approaches 2.0,the fractured fracture is easy to form single fracture,and the fractal dimension of fractured network reaches the minimum.Under different in-situ stress difference conditions,the hydraulic fracturing cracks of glutenite are mainly tension cracks,and a few shear cracks appear near the wellbore. |
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