Inversion Of Small-scale Vertical Fracture Density By Using P Wave AVAZ Method

Nowadays, fractured reservoir has already been a big picture of oil and gasexploration, at the same time, description of fractured reservoir by using geophysicalexploration method is getting more and more attention. In fact, the overlying stratapressure may cause high-angle fracture and vertical fracture can be easily kept. Theexisting development zone of vertical fracture may cause azimuthal anisotropy ofseismic wave propagation characteristics which means measured value of physicalfeatures varies with the direction or polarization of wave propagation. What’s more Pwave exploration is the most economical and practical exploration technology atpresent, so we can research on the development zone of vertical fracture by gettingazimuthal anisotropy of seismic characterizations, such as P wave velocity, amplitudeand attenuation. However, descriptions of fracture development zone is qualitative sofar and there is only a few methods for quantitative description of fracture density. Onthe basis of predecessors’ work, the research in this paper focus on inversion offracture density by using the difference information of P wave reflection amplitudebetween two crossing line. The main study objects are HTI media and the media withtwo vertical scale fractures.Firstly，based on constitutive relationship of anisotropic media, we can getexpressions of phase velocity and group velocity by solving Christoffel equation fortransversely isotropic media and study on seismic wave propagation law. Thenintroduce and compare three theories of equivalent transversely isotropic media with ahorizontal symmetry axis including Hudson theory, linear slip deformation theory andThomsen fracture theory. The changes of fracture characteristics will play influenceon elastic parameters, then Thomsen parameters will be also influenced. What’s more, according to the analysis on P wave reflection coefficient variation with incident angleand azimuth for HTI media with different anisotropy, we can choose the mosesensitive Thomsen parameter to reflection amplitude.Secondly, AVAZ response can reveal fracture strike very well in theory, but westill need face the limitation of observing system，geological conditions and so on.Aiming at Emilio area, we reasonably choose a bin and use P wave AVAZ responsegetting from it to estimate fracture strike in this paper. The fracture strike got by usingAVAZ response and from well log result are coincide and that verifies the feasibilityof estimating fracture strike by using P wave AVAZ response in Emilio area.Thirdly, we analysis reflection characteristics form three component seismicrecord for HTI media in this paper. Then come up with a fracture density inversionmethod based on multiple group difference of AVAZ response from crossing lines.What’s more, we use SVD on the multiple group difference of AVAZ response fromcrossing lines to reduce noise before inversion using this method. Through comparethe results of reducing noise between directly using SVD on AVAZ response andusing SVD on AVAZ response difference, we testify the advantage of studying onfracture characteristics based on AVAZ response difference. The inversion value offracture density based on AVAZ response from numerical simulation approaches themodel value which testify the feasibility of this algorithm.At last, assume that background of the media is isotropic homogeneous and themedia contain two sets of vertical fractures with different scales. Then analysis theinfluence of small-scale fracture density which is inversion result estimated by usingthe new algorithm cased by large-scale fracture when fracture compliance of verticallarge-scale fracture or what filled in small-scale fracture is changing. The erroranalysis of inversion result got by applying the new algorithm and the value offracture density reveals the range of application for multi-scale fracture model by using the new algorithm.