Anouar Romdhane

Shallow-structure characterization by 2D elastic full-waveform inversion

Assessing the effectiveness of elastic full-waveform-inversion (FWI) algorithms when applied to shallow 2D structures in the presence of a complex topography is critically important. By using FWI, we overcome inherent limitations of conventional seismic methods used for near-surface prospecting (acoustic tomography and multichannel spectral analysis of surface waves). The elastic forward problem, formulated in the frequency domain, is based on a mixed finite-element P0P1 discontinuous Galerkin method to ensure accurate modeling of complex topography effects at a reasonable computing cost. The inversion problem uses an FWI algorithm to minimize the misfit between observed and calculated data. Based on results from a numerical experiment performed on a realistic landslide model inspired from the morphostructure of the Super-Sauze earthflow, we analyzed the effect of using a hierarchical preconditioning strategy, based on a simultaneous multifrequency inversion of damped data, to mitigate the strong nonlinearities coming from the surface waves. This strategy is a key point in alleviating the strong near-surface effects and avoiding convergence toward a local minimum. Using a limited-memory quasi-Newton method improved the convergence level. These findings are analogous to recent applications on large-scale domains, although limited sourcereceiver offset ranges, low-frequency content of the source, and domination of surface waves on the signal led to some difficulties. Regarding the impact of data decimation on the inversion results, we have learned that an inversion restricted to the vertical data component can be successful without significant loss in terms of parameter imagery resolution. In our investigations of the effect of increased source spacing, we found that a sampling of 4 m (less than three times the theoretical maximum of one half-wavelength) led to severe aliasing.

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

CO2 saturations are estimated at Sleipner using a two-step imaging workflow. The workflow combines seismic tomography (full-waveform inversion) and rock physics inversion and is applied to a two-dimensional seismic line located near the injection point at Sleipner. We use baseline data (1994 vintage, before CO2 injection) and monitor data that was acquired after 12 years of CO2 injection (2008 vintage). P-wave velocity models are generated using the Full waveform inversion technology and then, we invert selected rock physics parameters using an rock physics inversion methodology. Full waveform inversion provides high-resolution P-wave velocity models both for baseline and monitor data. The physical relations between rock physics properties and acoustic wave velocities in the Utsira unconsolidated sandstone (reservoir formation) are defined using a dynamic rock physics model based on well-known Biot–Gassmann theories. For data prior to injection, rock frame properties (porosity, bulk and shear dry moduli) are estimated using rock physics inversion that allows deriving physically consistent properties with related uncertainty. We show that the uncertainty related to limited input data (only P-wave velocity) is not an issue because the mean values of parameters are correct. These rock frame properties are then used as a priori constraint in the monitor case. For monitor data, the Full waveform inversion results show nicely resolved thin layers of CO2–brine saturated sandstones under intra-reservoir shale layers. The CO2 saturation estimation is carried out by plugging an effective fluid phase in the rock physics model. Calculating the effective fluid bulk modulus of the brine–CO2 mixture (using Brie equation in our study) is shown to be the key factor to link P-wave velocity to CO2 saturation. The inversion tests are done with several values of Brie/patchiness exponent and show that the CO2 saturation estimates are varying between 0.30 and 0.90 depending on the rock physics model and the location in the reservoir. The uncertainty in CO2 saturation estimation is usually lower than 0.20. When the patchiness exponent is considered as unknown, the inversion is less constrained and we end up with values of exponent varying between 5 and 20 and up to 33 in specific reservoir areas. These estimations tend to show that the CO2–brine mixing is between uniform and patchy mixing and variable throughout the reservoir.

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

AbstractReliable quantification of carbon dioxide (CO2) properties and saturation is crucial in the monitoring of CO2 underground storage projects. We have focused on quantitative seismic characterization of CO2 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 high-resolution estimates of P-wave velocity VP and perform an evaluation of the reliability of the derived model based on posterior covariance matrix analysis. To get realistic estimates of CO2 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 CO2 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 CO2 injection at Sleipner, we apply our t...

Carbon Dioxide Saturation Estimates at Sleipner Using Seismic Imaging and Rock Physics Inversion

We present an integrated approach combining Full-Waveform Inversion and rock physics inversion to estimate CO2 saturation at Sleipner. Acoustic FWI provides high-resolution P-wave velocity images where the uncertainty is quantified using posterior covariance matrix analysis. The seismic properties and their associated uncertainties are used as input to the rock physics inversion. At the fairly large distance of the injection point (533m), we derive CO2 saturations lower than 30% and which are following a patchy mixing distribution.

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.