Correlations between the effective permeability and seismic anisotropy of fractured reservoirs

Abstract

The successful exploitation of many hydrocarbon reservoirs may depend on proper characterization of fractured or composite porous media. However, due to anisotropy and heterogeneity, the characterization of such kind of reservoirs is a complicated task which needs proper attention and care. There are often strong correlations between the effective permeability and seismic anisotropy of fractured reservoirs. The exploitation of such correlations is extremely important in understanding of the spatial variation of anisotropic permeability. The purpose of this thesis work was to develop novel workflows and methodologies in order to exploit the correlations between effective permeability and seismic anisotropy of fractured reservoirs. The correlations between the effective mechanical and transport properties of fractured reservoirs within the context of a joint inversion of seismic and production data have been exploited to obtain improved hydrocarbon reservoir characterization. In this work, the effective mechanical and transport properties of fractured reservoirs and related systems are estimated using an efficient and consistent permeabilitystiffness model from the same parameters of microstructure. The estimated effective mechanical properties in combination with anisotropic Gassmann relations have been used to calculate the seismic amplitude versus angle and azimuth AVAZ) data i.e. reflection coefficients as a function of incidence angle and azimuth from the top of the reservoir. Similarly from the estimated effective hydraulic properties, the reservoir simulator can calculate the production data such as well bottom hole pressure, well oil production, well water cut, saturation and pressure for all grid blocks at specific time steps. The seismic AVAZ and/or production data is used in the Bayesian inversion scheme to estimate the parameters of factures (e.g. fracture density, fracture aperture and fracture orientation) required to predict permeability. This work shows that joint inversion of seismic and production data give the best results due to the less sensitivity of seismic AVAZ data to fracture aperture which is one of the most important parameter of the microstructure predicting the permeability. This workflow of joint inversion of seismic and production data has also been applied for the characterization of faulted reservoirs (e.g., fault damage zone with deformation bands oriented parallel to the fault core). The transmissibility multiplier is used to model the effect of the fault core and the consistent rock physics model has been used to model fractures and deformation bands in the damaged zone. Analogies between different physical phenomena and coupled processes such as wave-induced fluid flow have been given a special attention to infer more useful information about fractured reservoirs containing single or multiple sets of fractures. In this context, implications of the unified theory for the relative importance of global and squirt flow characterized by different microstructures and fluid mobilities are invesigated. This work shows that global flow effects are not so important at seismic frequencies for more realistic models of microstructure and needs very high permeability and low viscosity to have an effect. For the case of a fractured reservoir containing a single set of fractures, it is demonstrated in this work that an improved characterization can indeed be obtained from frequency-dependent seismic AVAZ data, provided that the anisotropic Gassmann relations are replaced by a theory for seismic attenuation and dispersion due to wave-induced fluid flow. The reflection coefficients are no longer assumed to be real-valued and frequency-independent. The information about all the three fracture parameters can be obtained that determine the effective permeability tensor. For the case of complex fractured reservoirs (with multiple fracture sets), it is demonstrated in this work that measurements of velocity anisotropy data corresponding to different seismic frequencies and azimuths alone cannot recover the parameters related with multiple sets of fractures i.e. the fracture densities and the azimuthal fracture orientations. Joint inversion of measurements of seismic velocity and attenuation anisotropy data corresponding to different seismic frequencies and azimuths leads to improved estimates of fracture parameters and better management of fractured reservoirs containing multiple sets of fractures. A satisfactory characterization of complex fractured reservoirs requires a model accounting for frequency-dependent anisotropy. The results obtained form these workflows and methods can help in better management of fractured reservoirs and optimum field development.