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Title: Processing and modelling of shear-wave VSPs in anisotropic structures : case studies
Author: Yardley, Gareth S.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1993
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Recent work has shown that the fractures, which control productivity and fluid flow in some reservoirs, can cause the rockmass to be anisotropic to shear-wave propagation. The aim of this thesis is to extract information about fracture orientation and density from shear-waves recorded in producing formations. I examine VSP data from two areas (where productivity is fracture related): the Lost Hills field, California; and from three sites along the Austin Chalk trend, Texas. I use anisotropy estimation techniques to determine instrument polarities and the anisotropy parameters at each site. I produce anisotropic models for both areas. I am not able to resolve reservoir anisotropy using transmitted shear-waves. To determine reservoir anisotropy, I adapt reflected amplitude techniques and apply them to the Austin Chalk VSP data. At all four sites I find that the leading split shear-wave is polarized parallel to known fracture and stress directions. The polarization direction of the rockmass changes with depth in the Lost Hills anticline leading to multiply split shear-waves. Application of estimation techniques in the presence of multiple splitting has lead to incorrect interpretations of this data set in recent publications. I modelled the multioffset data from the BP test site, Texas, with a combination of vertical aligned cracks and horizontal thin layer anisotropy. This study demonstrates that analysis of shearwave anisotropy can be used to determine fracture orientation for use in oil recovery projects. Reflected amplitude studies show that the Austin Chalk in the Burleson County VSP, which contains a producing reservoir, is anisotropic, whereas the Austin Chalk at the other two sites, which do not contain reservoirs, is isotropic. I conclude that analysis of reflected shear-wave amplitudes represents an important tool for identifying fractured reservoirs. Also, reflection studies can be used in cases where the reservoir is too thin for delays to build up in transmitted shear-waves.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available