Seismic Wave Attenuation in Partially Saturated Sandstone and AVO Analysis for Carbon Dioxide Quantification at Sleipner

Sleipner field (North Sea) is the first largest CO₂ storage site, where the Carbon Dioxide is reinjected into the Utsira Sandstone. The main aim of this work is to obtain an estimate of the CO₂ saturation by using AVO techniques. Some other effects responsible for the energy loss, amplitude reduction and phase shift of the wave due to scattering and intrinsic attenuation, might mislead quantitative results if they are not considered. In order to account those effects. The study of reflection and transmission coefficients for elastic/elastic and elastic/viscoelastic models are performed. Secondly, several attenuation models are simulated increasing their complexity from a three layers model with a thick partially saturated sandstone (viscoelastic and poroelastic) embedded into shales, to a 28 layers model of sandstones with variable thickness and CO₂ saturations interbedded with thinner shales. Results show that viscoelastic and poroelastic sandstones are equivalent to elastic ones. Therefore, attenuation effects can be neglected. Elastic AVO analysis is carried out using the OptAVO tool. First, a feasibility study is performed based on well log and rock physics information, where three different fluid distributions are tested using Biot’s theory and Brie’s equation (e exponent equal to 1, 5 and 40). Then, the AVO inversion is performed via least squares fitting of the seismic amplitudes with the optimal reference curves. As expected, the AVO results are strongly influenced by the type of fluid distribution. For instance, patchy saturation (e=1) shows the highest saturations levels, while an average mixing law (e=5) suggests much lower saturations. The CO₂ estimation ambiguity from the elastic reflectivity results need to be addressed in the future and better constraints must be included into the modeling stage in order to mitigate it.