A model for CO2 storage and seismic monitoring combining multiphase fluid flow and wave propagation simulators. The Sleipner-field case

The main objective of this paper is to use a flow simulator to represent the CO2 storage and combine it with a wave propagation simulator in order to obtain syn- thetic seismograms qualitatively matching time-lapse real field data. The procedure is applied to the Utsira forma- tion at Sleipner field. The field data at the site available to us is a collection of seismic sections (time-lapse seis- mics) used to monitor the CO2 storage. An estimate of the CO2 injection rate and the location of the injection point are known. Using these data, we build a geologi- cal model, including intramudstone layers with openings, whose coordinates are defined by performing a qualita- tive match of the field seismic data. The flow simulator parameters and the petrophysical properties are updated to obtain CO2 saturation maps, including CO2 plumes, so that the synthetic seismic images resemble the real data. The geological model is based on a porous-media constitu- tive equation. It considers a poroelastic description of the Utsira formation (a shaly sandstone), based on porosity and clay content, and takes into account the variation of the properties with pore pressure and fluid saturation. More- over, the model considers the geometrical features of the formations, including the presence of shale seals and frac- tures. We also assume fractal variations of the petrophysical properties. The numerical simulation of the CO2-brine flow is based on the Black-Oil formulation, which uses the pressure-volume-temperature (PVT) behavior as a simpli- fied thermodynamic model. The corresponding equations are solved using a finite difference IMPES formulation. Using the resulting saturation and pore-pressure maps, we determine an equivalent viscoelastic medium at the macroscale, formulated in the space-frequency domain. Wave attenuation and velocity dispersion, caused by het- erogeneities formed of gas patches, are described with White’s mesoscopic model. The viscoelastic wave equation is solved in the space-frequency domain for a collection of frequencies of interest using a finite-element iterative domain decomposition algorithm. The space-time solution is recovered by a discrete inverse Fourier transform, allow- ing us to obtain our synthetic seismograms. In the numerical examples, we determine a set of flow and petrophysical parameters allowing us to obtain synthetic seismograms resembling actual field data. In particular, this approach yields CO2 accumulations below the mudstone layers and synthetic seismograms which successfully reproduce the typical pushdown effect.