This paper draws together basic rock physics, AVO, and seismic amplitude inversion to discuss how fluid discrimination can be performed using pre-stack seismic data. From both Biot and Gassmann theories for porous, fluid-saturated rocks, a general formula is first derived for fluid-property discrimination given that both the P and S impedances are available. In essence, an AVO inversion is transformed into the elastic properties of the pore space. This formula provides a more sensitive discriminator of the pore-fluid saturant than the acoustic impedance and is especially applicable in hard-rock environments. The formulation can be expressed with either the Lamé constants and density, or the bulk and shear moduli and density. Numerical and well-log examples illustrate the applicability of this approach. The combination of an AVO inversion and the parameters of the formula are then discussed to show how this technique can be implemented using pre-stack seismic data. Finally, a shallow gas-sand example from Alberta and a well-log example from Eastern Canada are shown to illustrate the techniques.

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The possible use of attenuation measurements for time-lapse seismic monitoring of an EOR steam flood project in Saskatchewan, Canada is investigated. A VSP survey was used to calculate Q. These values were input to a synthetic seismogram attenuation modeling program that showed there should be an observable increase in attenuation after steam injection. Two seismic lines shot at the same location nine years apart were analyzed to see if attenuation anomalies were apparent. The results indicate a strong anomaly on the recent seismic line that is consistent with the location of steam injection. A weaker anomaly on the older line is consistent with the amount of steam injected at that time. Theoretical and laboratory analyses of compressional velocity as a function of changes in temperature, pressure, fluid type and fluid phase suggest there should be a measurable effect on compressional wave amplitude, isochron, and frequency response.

Linearized approximations to the P-wave reflectivity as a function of the incidence angle (called amplitude variation with offset) involve the extraction of band-limited reflectivity terms that are a function of changes in the elastic constants of the earth across each lithologic interface. The most common of these extracted reflectivities are the intercept and gradient, usually labeled [Formula: see text] and [Formula: see text], respectively. The extended elastic impedance (EEI) method uses a rotation angle [Formula: see text] to map [Formula: see text] and [Formula: see text] into a new reflectivity corresponding to a particular elastic parameter. The success of EEI depends on finding an optimum value for the angle [Formula: see text]. This value is usually calculated by correlating the EEI result over a range of [Formula: see text] angles with various elastic parameters and then finding the best correlation coefficient. We have developed a new approach for the interpretation of the EEI method, which incorporates the Biot-Gassmann poroelastic theory and attaches a physical meaning to the [Formula: see text] angle. We call this method extended poroelastic impedance (EPI). The main advantage of the EPI method is that the [Formula: see text] angle is now interpreted as a parameter that is dependent on the dry-rock properties of the reservoir, rather than a parameter whose value is estimated empirically. The method is evaluated by numerical and synthetic seismic examples and by application to field data from a gas sand reservoir.

Early Arrival Waveform Tomography (EWT) reveals subtle sub-wavelength perturbations in the velocity model, given a sufficiently accurate starting model. Time-lapse seismic surveys over reservoirs are typically intended to detect small changes in a relatively well-known overall velocity field due to localized effects such as carbon dioxide injection, steam injection, petroleum production, and more. EWT is ideally suited to detect and reveal the extent of these effects, both spatially and in terms of the magnitude of the effect on the velocity. Our investigations strongly suggest that this method deserves serious consideration for time-lapse seismic analysis, though many challenging questions remain. Chiefly, acquisition limitations may present the greatest hurdle to broader acceptance of the technique, and the optimal configuration of sources, receivers, and bandwidth remains an open question.