Estimation of components of elastic scattering and intrinsic attenuation via well control at the Buena Vista Hills Field

Abstract

Attenuation of seismic energy and velocity anisotropy consists of an intrinsic component and an apparent component caused by strictly elastic scattering from velocity and density heterogeneities. The intrinsic component of attenuation is believed to be related to fluid properties in the porous portions of a reservoir. However, it may be difficult to distinguish between the apparent (layer-induced) effects caused by elastic scattering and the intrinsic effects when both mechanisms are operative. In this case, a detailed velocity model of the formation is required in order to predict the elastic scattering component of velocity. We have implemented such a scattering correction technique to determine the magnitude of the intrinsic attenuation from observed sonic velocity data. We constructed a high-resolution velocity model with layers one inch thick based on FMI logs and quality factors derived from sonic log data recorded at the Buena Vista Hills field. Quality factors for intervals were derived using the two station spectral ratio method applied to the full wave sonic log data at the Buena Vista Hills field. The wave response for intervals at different frequencies was obtained using a viscoelastic plane-wave modeling code to model a finely layered region. Intrinsic attenuation is determined in one way by measuring amplitudes of sonic full wave seismograms. The amplitudes at two adjacent stations are processed using the spectral ratio method to obtain an estimate of the attenuation. A second approach to computing the intrinsic attenuation of the formation is to make an elastic scattering correction, based upon the contribution of the reflection coefficients that is caused by thin layers, to the total velocity measured for the interval at sonic frequencies. The resulting difference between the observed velocity and the velocity predicted as a result of elastic scattering is assumed to be due to intrinsic attenuation. The applicability of this method is demonstrated by comparing the predicted intrinsic attenuation results, based on the layered model, with the intrinsic attenuation determined from sonic logs.