| In oil and gas reservoirs,the development of natural fractures occurs in multiple structural stages,and subsequently,induced fractures contribute to the formation of a complex fracture system characterized by multiple origins and varying scales.Understanding the elastic wave anisotropy and scattering characteristics induced by fractures of different scales constitutes a fundamental scientific challenge in the field of oil and gas exploration and production.However,traditional studies in oil and gas geophysics face certain limitations:(1)Many studies rely on simplified elastic wave equations,such as TTI,VTI,and HTI effective fractured media models,which tend to oversimplify the complexity of actual fracture distribution;(2)The focus is primarily on analyzing the elastic wave equations of fractured media,neglecting the comprehensive analysis of scattering characteristics resulting from complex fractures at different scales;(3)The correlation mechanism and superposition effect of elastic wave responses between multi-scale fractures should be considered.This thesis establishes a finely detailed theoretical model of fractured media,serving as a prerequisite for wave field forward modeling and as a theoretical basis for predicting fracture parameters using seismic data.This study uses advanced image processing technology to extract natural complex fractures from geological exploration data,enabling the construction of an actual complex fractured medium model.The finite element method is employed for wave field forward modeling,initially verifying the accuracy and effectiveness of the numerical approach using a single fracture medium model.The numerical results demonstrate that variations in fracture length,source frequency,and fracture angle decrease the velocity of elastic waves and result in more pronounced scattering characteristics.Furthermore,the study expands to encompass a multi-fracture medium model,revealing how changes in the distance between fractures and the scale of fractures affect the propagation of elastic waves.Interestingly,even with the same crack density,different scattering characteristics manifest due to the significant influence of the number of cracks on the propagation of elastic waves.By finely describing the theoretical model of fractured media and employing a sound numerical calculation solution,this research conducts wave field forward modeling investigations on complex fractured media at different scales.Based on the insights gained from single-fracture and multi-fracture media models,the thesis analyzes the propagation tendencies of multi-scale fracture systems,investigates the impact of fracture characteristics on propagation velocity,and explores the scattering characteristics of elastic waves within natural oil and gas reservoirs.Additionally,this study delves into the correlation mechanism and superposition effect of elastic wave responses in multi-scale fracture systems,thereby providing novel insights and a necessary theoretical foundation for unconventional oil and gas exploration. |
