Numerical analysis of wave-induced fluid flow effects on seismic data: Application to monitoring of CO<SUB>2</SUB> storage at the Sleipner field

Abstract

In this work we analyze how patchy distributions of CO<SUB>2</SUB> and brine within sand reservoirs may lead to significant attenuation and velocity dispersion effects, which in turn may have a profound impact on surface seismic data. The ultimate goal of this paper is to contribute to the understanding of these processes within the framework of the seismic monitoring of CO<SUB>2</SUB> sequestration, a key strategy to mitigate global warming. We first carry out a Monte Carlo analysis to study the statistical behavior of attenuation and velocity dispersion of compressional waves traveling through rocks with properties similar to those at the Utsira Sand, Sleipner field, containing quasi-fractal patchy distributions of CO<SUB>2</SUB> and brine. These results show that the mean patch size and CO<SUB>2</SUB> saturation play key roles in the observed wave-induced fluid flow effects. The latter can be remarkably important when CO<SUB>2</SUB> concentrations are low and mean patch sizes are relatively large. To analyze these effects on the corresponding surface seismic data, we perform numerical simulations of wave propagation considering reservoir models and CO<SUB>2</SUB> accumulation patterns similar to the CO<SUB>2</SUB> injection site in the Sleipner field. These numerical experiments suggest that wave-induced fluid flow effects may produce changes in the reservoir's seismic response, modifying significantly the main seismic attributes usually employed in the characterization of these environments. Consequently, the determination of the nature of the fluid distributions as well as the proper modeling of the seismic data constitute important aspects that should not be ignored in the seismic monitoring of CO<SUB>2</SUB> sequestration problems.