Analysis On Seismic Response Characteristics In Shale Reservoirs

As a new kind of natural gas resources,shale gas will greatly expand worldwide energy supply.However,the seismic prospecting technology applicable for geological and geophysical characteristics in China is in the early research stage,especially in the basic research on wave propagation mechanism and identification methods for shale gas reservoir.Therefore,this paper carries out the research of seismic response characteristics in shale reservoirs.Based on the introduction to widely used rock physics models,this paper discusses and analyzes the geological characteristics from the aspects of shale gas' s occurrence,inorganic minerals and organic matter in shale gas reservoir.Combined with the geological features of shale,this paper establishes a series of effective medium models on the multi-scales of nanometer-submicron,submicron-micron and millimeter.For the coupling problem of different effective medium models,a statistical model has been constructed by multiple linear regression using the least square method to constraint equivalent medium model on millimeter scale.Focused on organic matter,its maturity,pore fluid submission and anisotropic characteristics of rock,equivalent medium model has been analyzed.Then,this paper presents a multi-scales modeling method to construct the effective medium model of shale gas reservoirs,which improves the conventional modeling process based on DEM model and Gassmann model.And the characterization of elastic parameters is presented as well.The shear wave velocity prediction in a shale gas reservoir of Sichuan exploration area verifies the validity of the model constructed in this paper.Combined with the effective medium model of shale gas reservoir,the elastic parameters characterizing rock's brittleness are analyzed and selected to identify fragile shale and provide reference for favor hydraulic region.By introduction of azimuth with relationship between stress and strain,this paper derives the anisotropy formula of brittleness parameters.Then,the development of shale's brittleness under different formation's stress conditions has been simulated numerically.Brittleness analysis illustrates that the brittleness in direction of parallel to strike and perpendicular to the bedding would be the biggest,and brittleness in direction of perpendicular to strike and inclined to bedding at a 45 degree would be the least.Thereby,through the analysis of the anisotropy of shale's brittleness,we could predict the favorable fracture direction in shale reservoir.By calculated the rhetorical velocity of equivalent medium model in shale reservoir,this paper analyzes the seismic wave propagation velocity varying with different compositions volumes and fracture parameters.The wave propagation characteristics of equivalent medium model with different parameters is analyzed by numerical modeling method using the rotated staggered grid finite-difference method.In numerical implementation,non-splitting PML boundary condition is introduced as the absorbing boundary condition.Then AVO response characteristics in shale gas reservoir are analyzed as well.Based on the above analysis,it is recognized that the brittle shale shows a seismic response mode of a weak reflection events amplitude and bottom boundary events shifting up;organic-rich shale presents a seismic response mode of continuous reflection events,strong amplitude and bottom boundary events shifting down;fractured shale reservoirs demonstrates response mode of strong steep dip angle long offset amplitude in inspection surface of vertical fracture surface.Thus,a shale reservoir seismic response mode of brittleness and organic-rich and fracture is established,presenting the theoretical foundation for development of identifying of shale gas reservoir parameters.