Quantitative seismic characterization of CO₂ at the Sleipner storage site, North Sea

Abstract

Reliable quantification of carbon dioxide (CO 2 ) properties and saturation is crucial in the monitoring of CO 2 underground storage projects. We have focused on quantitative seismic characterization of CO 2 at the Sleipner storage pilot site. We evaluate a methodology combining high-resolution seismic waveform tomography, with uncertainty quantification and rock physics inversion. We use full-waveform inversion (FWI) to provide highresolution estimates of P-wave velocity V P and perform an evaluation of the reliability of the derived model based on posterior covariance matrix analysis. To get realistic estimates of CO 2 saturation, we implement advanced rock physics models taking into account effective fluid phase theory and patchy saturation. We determine through sensitivity tests that the estimation of CO 2 saturation is possible even when using only the P-wave velocity as input. After a characterization of rock frame properties based on log data prior to the CO 2 injection at Sleipner, we apply our two-step methodology. The FWI result provides clear indications of the injected CO 2 plume being observed as low-velocity zones corresponding to thin CO 2 filled layers. Several tests, varying the rock physics model and CO 2 properties, are then performed to estimate CO 2 saturation. The results suggest saturations reaching 30%-35% in the thin sand layers and up to 75% when patchy mixing is considered. We have carried out a joint estimation of saturation with distribution type and, even if the inversion is not wellconstrained due to limited input data, we conclude that the CO 2 has an intermediate pattern between uniform and patchy mixing, which leads to saturation levels of approximately 25% AE 15%. It is worth noting that the 2D section used in this work is located 533 m east of the injection point. We also conclude that the joint estimation of CO 2 properties with saturation is not crucial and consequently that knowing the pressure and temperature state of the reservoir does not prevent reliable estimation of CO 2 saturation.

CO2 saturation estimates at Sleipner (North Sea) from seismic tomography and rock physics inversion

“…The values of CO 2 saturation obtained for inline 1836 are higher than those obtained in Dupuy et al . (). The inline data used in our study is located very close to the injection point, while the one‐dimensional section shown in Dupuy et al .…”

“…Otherwise, sensitivity tests have been carried out to estimate CO 2 properties in addition to saturation (Dupuy et al . ). The tests show that the CO 2 saturation estimates are only slightly affected if CO 2 properties are unknown.…”

“…Estimation of brine saturation S W (top panels) and estimation of brine saturation and patchiness exponent (bottom panels) from P‐wave velocity (Figures from Dupuy et al. ). The blue cross indicates the model with the lowest misfit and the red cross indicates the true model.…”

“…Finally, Biot–Gassmann theory combined with the effective fluid phase rock physics model have been shown to be relevant for fluid substitution in brine–CO 2 saturated sands but its applicability for shales is less straightforward (Queiber and Singh ; Bergmann and Chadwick ; Dupuy et al . ; Falcon‐Suarez et al . ).…”

“…At Sleipner, Dupuy et al . () show however that the estimation of CO 2 saturations from P‐wave velocity models derived from FWI is still possible.…”