Elastic wave AVO using borehole seismic data

Abstract

In situ elastic measurements are used to investigate the amplitude variation with offset (AVO) response of a gas-saturated carbonate reservoir in Alberta, Canada. The in situ measurements include three VSP surveys (a zero offset, an offset, and a multioffset), full waveform sonic measurements, and bulk density measurements. The reservoir parameters from the well-log measurements are Vp=5186 m/s, k’ s=2969 m/s (Vp/Vs=1.75), Poisson’s ratio a=O.256 , and p=2550 kg/m3. The P-wave traveltimes from the zero-offset VSP and the S-wave traveltimes from the offset VSP were used in a traveltime inversion algorithm to determine the seismic velocities. The reservoir values from the traveltime inversion are Vp=4817 m/s, Vs=2833 m/s (Vp/Vs=1.70), and cH.236. Tbe multioffset VSP data were processed to show the true seismic P-P and P-Sv AVO response of the reservoir. The P-Sv data were found to have higher vertical resolution than the P-P data. This is attributed to the shorter wavelength of the propagating shear wave given the same frequency band as the compressional wave. The AVO response of the reservoir was forward modeled using a velocity model from the VSP traveltime inversions. Both the P-P and P-Sv forward models compare well with the seismic data, with the P-P data showing the most favorable comparison. It was found that the P-P AVO response could be better matched by perturbing the P-wave velocity of the reservoir. The perturbed reservoir parameters giving rise to a better match of the P-P AVO response are Vp=4333 m/s, Vs=2833 m/s (Vp/Vs=I.60), 0=0.179, and p=2550 kg/m3.