Rock physics inversion for CO₂ saturation estimation at Sleipner - Sensitivity tests and baseline application

We demonstrate the use of rock physics inversion for estimating CO₂ saturation and rock frame properties at the Sleipner CO₂ storage pilot in the North Sea. We investigate the relation between rock physics properties and elastic attributes for the Utsira unconsolidated sandstone. An effective fluid phase plugged into Biot theory is used together with the Brie mixing theory for the calculation of effective bulk modulus. We use the estimated viscoelastic properties under different brine and CO₂ distributions to invert selected poroelastic parameters from various input data parametrizations. By analysis of the sensitivity tests, we can conclude that CO₂ saturation can be well estimated from only VP input, especially for high brine saturation. The quality factors of both P-wave and S-wave velocities help better estimate CO₂ saturation and reduce the uncertainties. In a second part, the method is applied to well log data acquired prior to CO₂ injection. The S-wave velocities are derived using empirical relations from the P-wave velocities and density and used to estimate the rock frame moduli of the Utsira sand before CO₂ injection. We found that the S-wave velocities are crucial to help the estimation of frame moduli.