Acoustic, elastic and poroelastic simulations of CO2 sequestration crosswell monitoring based on spectral-element and adjoint methods

Abstract

S U M M A R YThe key issues in CO 2 sequestration involve accurate monitoring, from the injection stage to the prediction and verification of CO 2 movement over time, for environmental considerations. '4-D seismics' is a natural non-intrusive monitoring technique which involves 3-D time-lapse seismic surveys. Successful monitoring of CO 2 movement requires a proper description of the physical properties of a porous reservoir. We investigate the importance of poroelasticity by contrasting poroelastic simulations with elastic and acoustic simulations. Discrepancies highlight a poroelastic signature that cannot be captured using an elastic or acoustic theory and that may play a role in accurately imaging and quantifying injected CO 2 . We focus on timelapse crosswell imaging and model updating based on Fréchet derivatives, or finite-frequency sensitivity kernels, which define the sensitivity of an observable to the model parameters. We compare results of time-lapse migration imaging using acoustic, elastic (with and without the use of Gassmann's formulae) and poroelastic models. Our approach highlights the influence of using different physical theories for interpreting seismic data, and, more importantly, for extracting the CO 2 signature from seismic waveforms. We further investigate the differences between imaging with the direct compressional wave, as is commonly done, versus using both direct compressional (P) and shear (S) waves. We conclude that, unlike direct P-wave traveltimes, a combination of direct P-and S-wave traveltimes constrains most parameters. Adding P-and S-wave amplitude information does not drastically improve parameter sensitivity, but it does improve spatial resolution of the injected CO 2 zone. The main advantage of using a poroelastic theory lies in direct sensitivity to fluid properties. Simulations are performed using a spectral-element method, and finite-frequency sensitivity kernels are calculated using an adjoint method.

On the Field Variables of the Biot Theory and Modeling of Seismic Wave Propagation

“…So complexities in computation are bound to augment in the poroelastic full waveform inversion. It is commendable that a few research groups have begun looking into the poroelastic inversion of seismic data using Biot's framework [4] [5] [11]. This warrants immediate attention to address an unsettled issue: how the observed seismic wave field, which is a single vector quantity, relates to the two vector field variables, namely, the average solid and fluid displacement vector fields of the Biot theory?…”

Simulation study on seismic monitoring of aquifers

“…The properties of the media are summarized in Table 1. In the model, the porous layers are modeled with same parameter choices in grain and matrix as used in [8]. One must note that attenuation is ignored in this experiment.…”

“…In seismology, the concept of an adjoint field was first introduced in [7]. The iteration procedure used here is motivated by the work done in [8], where SE and adjoint methods are used as a modeling tool for monitoring injected CO 2 .…”

Use of global sensitivity analysis to assess the soil poroelastic parameter influence

“…• it is of interest to applications in the field of underwater acoustics, material wave characterization, or civil engineering for natural or artificial media [1][2][3][4][5][6][7][8],…”

“…Obtaining structural images of a porous material, and estimating its physical properties, is a topic of considerable interest in many branches of activities in geophysics, underwater acoustics, civil engineering, and biomechanics [1][2][3][4][5][6][7][8].…”