Effect of calculated dry rock bulk modulus on uncertainty in compressional wave velocities derived from Gassmann fluid substitution theory and fluid mixing law in a CO2 sequestration case

This study examines the effect of variations in dry rock bulk modulus (Kdry) on compressional wave velocity (Vp) using Gassmann fluid substitution theory and various fluid mixing laws in the context of CO2 sequestration in the Utsira formation at the Sleipner Field. Significant changes in Vp, exceeding 600 m/s, were observed, particularly when applying the Gassmann-Wood law, which demonstrated heightened sensitivity to saturation variations. In contrast, the Gassmann-Brie approach, utilizing an empirically selected constant, produced more stable and reliable results. This study emphasizes that relying solely on the Gassmann-Wood equation for Kdry calculations may result in inaccurate predictions, particularly for CO2-saturated formations. To mitigate this, laboratory measurements of Kdry are recommended to enhance the reliability of predictions. Furthermore, initial saturation conditions critically affect the Vp calculations, underlining the importance of selecting an appropriate mixing law to ensure accurate assessments during CO2 injection and storage.