A new model for fluid substitution in fractured reservoirs

Abstract

One of the main issues in the characterisation of fractured reservoirs is the ability to predict the effect of fluid properties on seismic characteristics. Background porosity can significantly affect the elastic properties of fractured rocks. This effect is studied using the model of fractures as linear-slip interfaces in an isotropic porous background. Such a medium represents a particular case of a transversely isotropic (TI) porous medium, and can be described by equations of anisotropic poroelasticity. An analysis based on these equations yields explicit analytical expressions for the low-frequency elastic constants and anisotropy parameters of the fractured porous medium saturated with a given fluid. The five elastic constants of the resultant TI medium are derived as a function of the properties of the dry (isotropic) background porous matrix, fracture properties (normal and shear excess compliances), and fluid bulk modulus. Analysis shows that: (1) for penny-shaped cracks in a non-porous host medium the results reduce to the classical equations for isolated cracks; (2) for the same case of penny-shaped cracks but with background porosity, the expression for P-wave anisotropy parameter e has the form similar but not identical to that given by the model of Thomsen; (3) the compliance matrix of the fluid-saturated fractured medium with considerable background porosity is not equal to the compliance matrix of any solid medium with a single set of parallel fractures. This effect is caused by the wave-induced flow of fluid between pores and fractures. These results can be used for fluid substitution in porous rocks with parallel fractures, which is important, in particular, for AVO analysis in naturally fractured reservoirs.