Research On Seismic Response Of Anisotropic Shale Reservoirs

Shale reservoirs prone to form discontinuous fractures when subjected to tectonic deformation or diagenesis.The characters and distribution of fractures play an important role in exploration.This paper provide evidence for regional inversion of shale and geological interpretation with seismic response simulation analysis.The main process of seismic response simulation includes building rock physical model and implementing forward algorithm.In this paper,the equivalent medium theory is used to construct the fractured shale reservoir model,calculate its anisotropic seismic response and analyze its characteristics.We analyzed the rock physical characteristics and seismic responses of elastic and inelastic reservoirs to study the correlation between the velocity of vertical and horizontal waves and the properties of fractures.Base on rock physics in this paper,we designed corresponding HTI model and orthorhombic anisotropic model for shale reservoirs respectively,analyzed seismic response calculated by Ruger theory,reflectivity method and propagation matrix algorithm.According to the results,reflection coefficient calculated by the theory of Ruger,using traditional convolution algorithm in computing synthetic seismogram in time domain,needs to consider the effect of sampling time,while the result of reflectivity method and transmission matrix algorithm with time domain calculation of reflection coefficient sequence transformed to frequency is more accurate,and is not affected by time of sampling rate;for reservoirs with multiple fractures,azimuth angle between fractures will complicate the seismic response,the spindle of azimuth amplitude curve for PP wave,PSV wave and PSH wave will vary with azimuth angle between fractures.Orthorhombic anisotropic media containing vertical fractures,due to the affected by horizontal bedding,are weaker than HTI media in azimuth anisotropy,and its azimuth amplitude anisotropy curve is more difficult to see the orientation of fractures,which requires further use of inversion and other methods for auxiliary identification.For inelastic medium,we established fracture reservoir model by Chapman's rock physical theory,simulated and analyzed seismic response using reflectivity method.The reservoir behaves inelastic anisotropy when HTI medium fracture is filled with fluid.Seismic wave velocity has strong dispersion and attenuation in inelastic medium.The characteristics of azimuth anisotropic response for reservoirvaries with properties of surrounding rocks,fluid properties and fracture parameters.When the fluid density in fracture or pore increases,P wave attenuates obviously in vertical fracture direction and decreases in the parallel.The fracture density increases,P wave velocity decreases,the amplitude azimuth curve trend remains unchanged but the amplitude decreases.