Estimation of reservoir fluid properties from seismic data is one of the central issues in petroleum exploration. For isotropic porous reservoirs this is accomplished through the use of Gassmann equations. However, such explicit yet general expressions have not been developed for porous reservoirs with aligned fractures. On the other hand, traditional models of fractured media ignore the background porosity of the rock, and thus do not account for the wave induced fluid flow between pores and fractures.
A major effort of the rock physics group is directed towards modelling attenuation, dispersion and frequency dependent anisotropy of porous reservoirs permeated by aligned fractures. In 2001-2003 we have developed a methodology of fluid substitution in fractured reservoirs, which is based on the combination of anisotropic Gassmann equations and Schoenberg’s linear slip parameterisation of the effect of fractures on rock properties. In 2003-2006 we developed a model for attenuation and dispersion of P-waves propagating perpendicular to a periodic system of parallel planar fractures, and validated this model with numerical simulations using a poroelastic extension of reflectivity method. These simulations helped to extend the attenuation/dispersion model to randomly spaced fractures and to oblique incidence.
More recently we developed a model for seismic attenuation and dispersion caused by the presence of sparsely distributed finite fractures in the porous reservoirs. The model is based on the combination of Biot’s theory of poroelasticity with the ideas of a multiple scattering theory. The current effort in this area is focused on the deeper understanding of the implications of this theory, and its extensions to:
- Oblique incidence
- Shear waves
- Higher fracture densities
- Arbitrary aspect ratios.
While all of the above models are designed for a single set of aligned fractures, real reservoirs often contain multiple fracture sets. Moreover similar phenomena (fluid flow between pores and fractures) lead to frequency dependent attenuation and dispersion in isotropic rocks with microcracks, compliant grain contacts, etc. These effects will be examined by extending the aligned fracture models to arbitrary angular distributions of fractures. Using this approach, we will be developing new models of isotropic microcracked rocks in order to gain new insight into squirt-flow attenuation and dispersion, and their dependency on stress and pressure.