Analysis of fluid substitution in a porous and fractured medium

Abstract

To improve quantitative interpretation of seismic data, we analyze the effect of fluid substitution in a porous and fractured medium on elastic properties and reflection coefficients. This analysis uses closed-form expressions suitable for fluid substitution in transversely isotropic media with a horizontal symmetry axis (HTI). For the HTI medium, the effect of changing porosity and water saturation on (1) P-wave moduli, (2) horizontal and vertical velocities, (3) anisotropic parameters, and (4) reflection coefficients are examined. The effects of fracture density on these four parameters are also studied. For the model used in this study, a 35% increase in porosity lowers the value of P-wave moduli by maximum of 45%. Consistent with the reduction in P-wave moduli, P-wave velocities also decrease by maximum of 17% with a similar increment in porosity. The reduction is always larger for the horizontal P-wave modulus than for the vertical one and is nearly independent of fracture density. The magnitude of the anisotropic parameters of the fractured medium also changes with increased porosity depending on the changes in the value of P-wave moduli. The reflection coefficients at an interface of the fractured medium with an isotropic medium change in accordance with the above observations and lead to an increase in anisotropic amplitude variation with offset (AVO) gradient with porosity. Additionally, we observe a maximum increase in P-wave modulus and velocity by 30% and 8%, respectively, with a 100% increase in water saturation. Water saturation also changes the anisotropic parameters and reflection coefficients. Increase in water saturation considerably increases the magnitude of the anisotropic AVO gradient irrespective of fracture density. From this study, we conclude that porosity and water saturation have a significant impact on the four studied parameters and the impacts are seismically detectable.

Water saturation effects onP-wave anisotropy in synthetic sandstone with aligned fractures

“…It can be seen that as the effective isotropic background rock property (in this case as a result of changing water saturation) changes, this has an effect on the anisotropy of the fractured rock, a result which may not be obvious. A similar observation was made in the numerical study (although dispersion was not considered) by Sil et al (2011) where they showed that changes in the isotropic background properties as a result of changes in water saturation and porosity had an effect on P-wave anisotropy of the fractured rock considered. It then follows that if the effective isotropic background property is frequency dependent, then the effect on P-wave anisotropy would be frequency dependent.…”

Linearized amplitude variation with offset and azimuth and anisotropic poroelasticity

“…In this paper, we extend the linearized AVO and poroelasticity proposed by Russell et al (2011) to the case of linearized AVOA and anisotropic poroelasticity. Gassmann's (1951) equations formulated the effects of fluid properties on seismic response, and Gurevich (2003) studied the elastic properties of saturated porous rocks permeated with aligned vertical fractures incorporating anisotropic Gassmann's equation and linear-slip model (Sil, Sen and Gurevich 2011). Huang et al (2015) derived analogical expressions for anisotropic fluid substitution in fracture-induced media.…”

Azimuthal Attenuation Elastic Impedance Inversion for Fluid and Fracture Characterization Based on Modified Linear-Slip Theory

“…To overcome the pennyshaped fracture model assumption, Gurevich [13] combined the anisotropic Gassmann's [12] equation with the more generally linear-slip (LS) theory to propose a saturated fractured porous model, who derived the exact analytical expressions for the stiffness matrix of a fluid-saturated fractured porous rock, related to the dry elastic properties of isotropic background, matrix moduli, porosity, dry fracture compliances, and saturated fluid modulus. On this basis, Sil et al [14] and Huang et al [15] analyzed the effects of fluid substitution on elastic properties and reflection coefficients in saturated fractured porous media with HTI symmetry and orthorhombic symmetry, respectively. Using the anisotropic Gassmann equation and the linear-slip theory model, Pan and Zhang [16] derived the weakly anisotropic approximations of fluid substitutions and reflection coefficients for a set of parallel, aligned vertical fractures (i.e., an equivalent HTI medium) and two sets of orthogonal vertical fractures (i.e., an equivalent orthorhombic medium), respectively.…”