Simulation And Comparison Of Seismic Wave Fields In Natural Gas Hydrate Formations

Natural gas hydrate is a new type of clean energy with abundant reserves and broad prospects.Due to the different formation conditions,the occurrence state of natural gas hydrate in the reservoir is diverse.Forward modeling is the basis of seismic detection of hydrate deposits,and it is a more effective method to study the seismic wave field of hydrate deposits.Based on different forward modeling models,the wave field characteristics of hydrate deposits with different occurrence states vary widely.This paper mainly focuses on the differences in seismic response of hydrates in different occurrence states.According to the different contact relationships between hydrates and sediment particles in the formation,this article summarizes the six occurrence states of natural gas hydrates below the seafloor,which are inter-particle cementation type,particle-encapsulation type,particle support type,pore filling type,Matrix and filler coexistence type and nodule / crack filler type.The calculation methods of the physical property parameters of the hydrate-free hydrates and hydrates with different occurrence states in the sediment layer are listed.The basic principle of the staggered-grid finite difference method and the calculation of difference coefficients are introduced.Based on this,the first-order stress-velocity wave equations after the dispersion of the acoustic wave model,the elastic wave model,and the two-phase medium model are listed.The treatment of initial conditions,PML(Perfectly Matched Layer)absorption boundary conditions,as well as the stability conditions of the differential format and the influencing factors of the dispersion phenomenon are elaborated in detail.This article designed a variety of geological models.Based on the previous understanding of the physical properties of each mineral phase,the porosity of the sedimentary layer and the saturation of each mineral phase are reasonably selected,and the corresponding physical property parameters are obtained.The programming method is used to realize the numerical simulation process,and three forward models are used to discuss the seismic response characteristics of hydrates with different occurrence states.Changing the thickness of the hydrate layer,the presence or absence of the free gas layer,and the level of the main frequency of the wavelet,etc.,mainly discussed the wave field snapshot characteristics of the hydrate layer and the polar characteristics of the top and bottom boundaries in the seismic records,The relationship between the strength of the reflection amplitude and the change with the offset.Finally,starting from the three aspects of amplitude response,polar response and response of special faults,the characteristics of various wave fields are visually compared,and the differences in seismic response of hydrates with different occurrence states are analyzed.