Rock Physics Inversion for CO2 Saturation Estimation at Sleipner - Sensitivity Tests and Baseline Application
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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.