Azimuthally anisotropic elastic impedance inversion for fluid indicator driven by rock physics

Abstract

The normal-to-tangential fracture compliance ratio is usually used as a fracture fluid indicator (FFI) for fluid identification in fractured reservoirs. With a new parameterization for fracture weaknesses, we have defined a new FFI based on azimuthally anisotropic elastic impedance (EI) inversion and fractured anisotropic rock-physics models. First, we derived a new azimuthally anisotropic EI equation with a similar expression for the isotropic and anisotropic EI parts to remove the exponential correction of EI that is attributable to weak anisotropy. Then, we built a fractured anisotropic rock-physics model used for the estimation of well-log parameters for the normal and tangential fracture weaknesses, which built the initial background low-frequency trend of fracture weaknesses. Finally, based on the azimuthally anisotropic EI inversion method with the Cauchy-sparse and low-frequency information regularization, we estimated an FFI applied to fluid identification in fractured reservoirs. Tests on the synthetic and real data demonstrate that the anisotropic parameters related to fracture weaknesses can be estimated reasonably and stably and that our method appears to provide an alternative available for fluid identification in fractured reservoirs.

Amplitude variation with incident angle and azimuth inversion for Young's impedance, Poisson's ratio and fracture weaknesses in shale gas reservoirs

“…Implementing the decorrelation of model parameters, the forward equation can be re‐expressed using the new matrix and the new vector as where , and represents the inverse of the decorrelation matrix U (Pan et al . ).…”

“…In our case, we do not have anisotropic well log data, and build a fractured anisotropic rock‐physics model to estimate the well log information of fracture weaknesses as the complementary background low‐frequency trends (Pan et al . ). Therefore, we use the conventional acoustic velocity curves, density curves and other well‐log interpretation information, such as the mineral component, porosity curve, clay content curves and water saturation curve to calculate the fracture weakness parameters based on the fractured anisotropic rock‐physics model (see Pan et al .…”

“…To evaluate the fracture properties, the normal and tangential fracture weaknesses of fracture system are usually used in the linear slip model to replace the Thomsen's anisotropic parameters (Shaw and Sen ; Downton and Roure ; Pan, Zhang and Yin ). In a penny‐shaped fracture‐induced HTI model, the tangential weakness relates to the fracture density directly (Bakulin et al .…”

Linearized amplitude variation with offset and azimuth and anisotropic poroelasticity

“…We develop new parameterization expressions for fracture weaknesses, that is, and — termed as quasi‐weaknesses (Pan et al . ): and where the subscript 0 indicates the average quantities of the upper and lower layers.…”

“…We show the comparisons between the inverted (red) and original (black) curves of fluid, rigidity, density and fracture weaknesses calculated using well logs and rock‐physics analysis results (Pan et al . ,c,d) in Figs . Figure (a–c) shows the relative errors of the estimated model parameters, and from the results we conclude that the estimated model parameters agree well with the original curves even when with moderate random noise.…”