Fracture Detection Of The Jurassic Tight Reservoir In Centre Szechwan: A Physical Modeling Study

The tight fractured shell limestone reservoir in Da'anzhai group of Jurassic system in central Szechwan,has gradually become the most important research area on unconventional alternative oil and gas resources of China National Petroleum Corporation(CNPC).It is characterized by poor physical properties,strong anisotropy as well as ultralow permeability and porosity.The study of distributing characteristics and patterns of fractures is of great importance for tight hydrocarbon reservoir.Combining the seismic physical model technology,small-scale model materials with variable fracture parameters are constructed to simulate the equivalent characteristics of all fractures in certain regions,the paper sheets are pretend to be fractures according to fracture characteristics of reservoir.The fracture density is changed by controlling the superimposed intensity of the paper sheets,and the bulk velocity of the fractured zone is affected by the changing matrix velocity.Then the rock phsyical parameters are tested to compare with embedded controllable rock physical model,and they have the similar results,so the model materials can simulate fractures in reservoir and provide the basic material for fractured reservoir model.Besides,A corresponding relationship between the linear fracture density and the volume fracture density are established by taking velocity anisotropy as a benchmark.Three-dimentional compounded physical model of fractured reservoir-the real fractured reservoir model and the simplified fractured model-are constructed to capture the actual geology structure of the target area of Longgang in central Szechwan,according to the geological features of the reservoir and the corresponding fracture parameters.A regular pattern distribution of fracture areas in the simplified fractured model is in the form of fractured zones and takes 8 fracture parameters into account.There are some fratured zones with different widths and fracture swarms with different intervals which are randomly distributed in the real fractured reservoir model besides faults,4-6 thin-beds and other geological structures.After that,the 3D data of the fractured reservoir physical model are obtained for the first time by using wide-azimuth seismic data acqusition and processing and can provide reliable data support for study of fractured response characteristics and optimization of fracture detection methods.Based on the multi-azimuthal seismic data of the fractured reservoir physical model,we obtained the azimuthal seismic response characteristics of the fractured zones in the simplified fractured model and of the fracture swarms in the real fractured reservoir model through observing the seismic sections and slices from various azimuths.The results indicate that the fractured zones appear to have strong reflectance signatures,and the fracture swarms have weak reflection characteristics.We have also given out the relationships between the fracture parameters and the widths of the reflection imaging,the configuration of seismic reflections,as well as the reflection amplitudes.In addition,the multiple post-stack attributes are analyzed to find out the prominent seismic attribute which is more sensitive to fractures.And on this basis,the attribute fusion pattern is proposed to highlight the major faults and reduce multiple solutions based on the multi-attribute fusion technology.Based on the pre-stack azimuthal seismic data of the fractured reservoir physical model,the meso-and micro-scale fractures are detected,and the pre-stack fracture prediction methods are optimized.Comparing the azimuthal anisotropy of amplitude,attenuation and spectral decomposition,the azimuthal anisotropy of frequency attenuation gradient attributes is sensitive to meso-and micro-scale fractured zones,fracture swarms,and is of high precision.