Model-based amplitude versus offset and azimuth inversion for estimating fracture parameters and fluid content

Abstract

The normal-to-shear fracture compliance ratio is commonly used as a fluid indicator. In the seismic frequency range, the fluid indicator lies between the values for isolated fluid-filled fractures and dry fractures, and it is not easy to discriminate the fluid content. Assuming that the fracture surfaces are smooth, we use [Formula: see text], with [Formula: see text] and [Formula: see text] representing the normal fracture weakness of the saturated and dry rock, to indicate fluid types, and to define a fluid influencing factor. The fluid influencing factor is sensitive to the fluid properties, the aspect ratio of the fractures, and the frequency. Conventionally, the amplitude versus offset and azimuth (AVOA) inversion is formulated in terms of the contrasts of the fracture weaknesses across the interface, assuming that the fractures are vertical with the same symmetry axis. We consider fractures with arbitrary azimuths, and develop a method to estimate fracture parameters from wide-azimuth seismic data. The proposed AVOA inversion algorithm is tested on real 3D prestack seismic data from the Tarim Basin, China, and the inverted fracture density show good agreement with well log data, except that there are some discrepancies for one of the fractured reservoir sections. The discrepancies can be ascribed to neglect of the dip angle for the tilted fractures and the conjugate fracture sets, and to the validity of the linear-slip model. The fractured reservoirs are expected to be liquid saturated, under the assumption of smooth fractures. Overall, the inverted fracture density and fluid influencing factor can be potentially used for better well planning in fractured reservoirs and quantitatively estimating the fluid effects.

Fluid Discrimination in Ultra-Deep Reservoirs Based on a Double Double-Porosity Theory

“…The Gassmann fluid term and its product with mass density were used as a fluid discriminator by Russell et al (2003Russell et al ( , 2006, and Quakenbush et al (2006) used Poisson's ratio for the same purpose. Quintal (2012) suggested that frequency-dependent attenuation due to waveinduced fluid flow is a potential index of saturation, and Xue et al (2017) used a combination of the normal crack compliance of the saturated and dry rocks as a fluid factor.…”

Fluid pressure diffusion effects on the excess compliance matrix of porous rocks containing aligned fractures

“…The seismic method is a valuable tool for the detection and characterization of fractures, due to the fact that seismic waves experience strong directional dependence, and are also significantly attenuated and delayed when propagating through fractures (Maultzsch et al 2003). For this reason, there is currently great interest in improving the understanding of seismic wave propagation through fractured rocks in general, and in presence of subvertical fractures in particular (Foord et al 2015;Xue et al 2017). Since seismic methods usually lack the resolution needed to directly image individual fractures, most efforts in this direction focus on exploiting the information encoded in the effective seismic properties of the probed fractured material.…”

Azimuthal elastic impedance-based Fourier coefficient variation with angle inversion for fracture weakness

“…AVAZ inversion is an ill condition; a large number of unknown parameters and the cross talk between the elastic and fracture parameters inevitably make several-term AVAZ inversion unstable (Downton et al 2006;Bachrach 2015). One tentative approach to reduce the number of unknown parameters is to implement azimuthal seismic amplitude difference inversion for fracture weakness estimation (Chen et al 2017a;Pan et al 2017;Xue et al 2017). In addition, Downton andRoure (2011, 2015) rewrote azimuthal P-wave reflectivity in terms of Fourier coefficients (FCs) and proposed an azimuthal Fourier coefficient elastic inversion for fracture parameters estimation.…”