Modelling and analysis of attenuation anisotropy in multi‐azimuth VSP data from the Clair field

Abstract

A B S T R A C TAnisotropic variations in attenuation are of interest since they can give information on the fracture system and may be more amenable to measurement than absolute attenuation values. We examine methods for detecting changes in relative attenuation with azimuth from VSP data, and validate the techniques on synthetic data. Analysis of a multi-azimuth walkaway VSP data set from a fractured hydrocarbon reservoir indicates that such azimuthal variations in P-wave attenuation are observable. The effects are localized in the reservoir, and analysis allows the prediction of a fracture strike direction, which agrees with geological information. The observed effects can be modelled under reasonable assumptions, which suggests the validity of the link between the anisotropic attenuation and the fracturing. I N T R O D U C T I O NMeasurements of seismic attenuation from laboratory and field data, as well as theoretical work, indicate a link between attenuation, the presence of fractures and fluid saturation. Nevertheless, the difficulty of measuring attenuation in band-limited seismic data has impeded the routine application of attenuation-based techniques. The measurement of anisotropic attenuation is of great potential interest both because of the information it could provide about fracture systems and also because there is a suggestion that measuring directional variations of attenuation could perhaps be more robust than attempting to measure absolute values of attenuation.Fracture-related anisotropic attenuation has been theoretically predicted (Hudson, Liu and Crampin 1996), observed in the laboratory (Rathore et al. 1995;Luo et al. 2006) and its effects have been noted in surface reflection data (Lynn 2004). The quantitative interpretation of anisotropic attenuation in terms of theoretical models remains problematic, as the underlying physics is relatively poorly understood. * In previous work (Maultzsch et al. 2003), we showed that frequency dependence of shear-wave splitting could be first observed in vertical seismic profile (VSP) data and then modelled and interpreted in terms of a dynamic equivalent medium theory (Chapman 2003). The goal of this paper is to determine to what extent the concepts can also be maintained for P-wave propagation.Our theoretical expectations with respect to P-wave attenuation are outlined in Fig. 1. In addition to the expected increase of attenuation with fracture density, attenuation is predicted to increase with polar angle and azimuth away from the fracture strike. This means that the 'low attenuation azimuth' is likely to correspond to the fracture strike direction. An important point is that the magnitude of attenuation is sensitive to the fluid bulk modulus, such that, all other parameters being held constant, the attenuation is increased if the fluid bulk modulus is lowered from values typical of water to those typical of gas.In this study, we show that azimuthal variations of attenuation can be observed in a multi-azimuth walkaway VSP survey. As expected, the effec...

Anisotropy of Seismic Attenuation in Fractured Media: Theory and Ultrasonic Experiment

“…Liu et al (1993), MacBeth (1999), Clark et al (2001), Chapman (2003), Vasconcelos and Jenner (2005), Varela et al (2006), Chichinina et al (2006a,b), Maultzsch et al (2007), Liu et al (2007) reveal different aspects of this problem.…”

Attenuation anisotropy in the linear-slip model: Interpretation of physical modeling data

“…Besides ultrasonic experiments, there are several field experiments on fracture-direction estimation from azimuthally varying P-wave attenuation in surface seismic reflection data and VSPs ͑Clark et al, 2001;Vasconselos and Jenner, 2005;Varela et al, 2006;Liu et al, 2007;Maultzsch et al, 2007͒. In laboratory experiments involving velocity and attenuation anisotropy study, it is preferable to simulate an ideal transversely isotropic ͑TI͒ medium using synthetic specimens formed by thin microlayers, or models with embedded parallel, penny-shaped cracks ͑e.g., Rathore et al, 1994;Sothcott et al, 2007;. A model constructed from thin layers ͑a plate-stack model͒ can be considered the simplest way to represent TI media ͑e.g., Hsu and Schoenberg, 1993;Zhu et al, 2007a͒.…”

Traveltime inversion and error analysis for layered anisotropy

“…2D layered model TTI parameter estimation using VSP first arrivals VSP has been good acquisition geometry to detect anisotropy (Maultzsch et al, 2007;Slawinski et al, 2003). A major challenge is to distinguish the effect of depth variations of velocity interfaces from that caused by anisotropy in the layer velocities, especially if only the first arrivals are used.…”