Effects of patch size changes on the time-lapse seismic reflections of highly attenuating reservoirs

Reservoir fluid depletion and injection inevitably induce changes in pore pressure and saturation, leading to a varying fluid patch size and hence perturbations in seismic velocities and observable time-lapse seismic attributes. The patch size, which may exhibit spatial variations, can possess a characteristic size at each scenario, and therefore another variable impacting time-lapse seismic signatures. In this study, we extend previous research efforts of time-lapse seismic interpretation by considering the fluid patch size effect on time-lapse seismic signals to investigate uncertainties in time-lapse seismic observations. We predicted and analyzed the influence of reservoir permeability and gas saturation on seismic acoustic properties, i.e., attenuation and velocity dispersion, reflectivity and phase, on the basis of a generalized White's spherical model for varied patch sizes on millimeters to centimeters. A stratified model consisting of a fluid-saturated sandstone reservoir showing viscoelastic properties sandwiched between two elastic half-spaces is employed, so as to give insights into the impacts of fluid extraction induced changes of velocity dispersion and attenuation, as well as acoustic impedance contrast, on time-lapse seismic reflections. Analysis of time-lapse seismic travel-time and reflection amplitude differences indicates that ignoring the time-dependent patch size behaviour can give rise to underestimation or overestimation of the reservoir fluid saturation and permeability change. •We extend previous efforts by assessing the significance of patch size variation on reservoir change estimates.•We estimate the influence of reservoir permeability and gas saturation on seismic acoustic properties.•Time-lapse seismic AVA signatures are useful to indicate the reservoir fluid distributions.