1. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Modeling techniques to study CO 2 -injection induced micro-seismicity by José M. Carcione 1, Federico Da Col 1, Gilda Currenti 2, Barbara Cantucci 2 and Juan E. Santos 3 1 OGS, 2 INGV, 3 Buenos Aires University Published in 2015: International Journal of Greenhouse Gas Control, vol 42, pp 246-257

2. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) CO 2 geological storage Geological storage of CO 2 via injection in the subsurface is one solution to reduce the emissions of this greenhouse gas. An important problem is to provide confident predictions of the long-term behavior of the injected CO 2 and identify potentially hazardous leakages to the surface. Seismic technology provides a valuable tool to monitor CO 2 during the injection and post-injection stages.

3. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Distance (m) Distance (m) Depth (m) Computation of synthetic seismograms for a preliminary evaluation of the reliability of a survey.

4. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Gas density : Peng-Robinson equation of state Experimental and computed densities for CO 2 (Data from Klimeck et al. (2001) and Wang and Nur (1989)) Experimental and computed densities for CH 4 (Data from Klimeck et al. (2001)) Different T

5. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Experimental and computed bulk modulus for CO 2 (data from Wang and Nur (1989)). Bulk modulus of CH 4 computed with the Peng-Robinson EoS and with Batzle and Wang (1992) semi- empirical formulas. Gas bulk modulus (inverse of adiabatic compressibility) Different T

6. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) CO 2 Phase diagram It is assumed that CO 2 is injected in the A4 formation, and the presence of 3 leakages, caused by degradation of the casing of an old well. Leakage 1 (L1) is located at 480 m depth, with the CO 2 in the gaseous state. Leakage 2 (L2) is located at 950 m depth, with the CO 2 in the liquid state. Leakage 3 (L3) is located at 1440 m depth, with the CO 2 in the supercritical state. The reservoir (R) is located at 1788 m depth, with the CO 2 in the supercritical state. Geological model with leakages Reservoir Depth(km)

7. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Plane-wave numerical simulation - Leakages 1,2 and 3 The modeling algorithm is based on a 4th-order Runge-Kutta time-integration scheme and the staggered Fourier method to compute the spatial derivatives. The equations of motion of the equivalent viscoelastic isotropic medium were solved in the space-time domain using a velocity- stress formulation. Time (s)

8. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Analytical and TOUGH solutions for a layer CO 2 injection in sand. Pressure (a) and saturation (b).

9. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Failure criterion Random distribution of the failure criterion (in MPa) based on the Young modulus. The medium is divided into 375 x 375 cells.

10. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Seismic sources Cloud of tensile (a) and shear (b) events

11. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Location of the events Location of the tensile (a) and shear (b) events as a function of the emission time, where the solid lines corresponds to a simple equation, with D = 0.137 m 2 /s.

12. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Seismic sources Snapshot of the vertical component of the particle velocity at 0.4 s, showing the radiation patterns of the tensile and shear sources. The maximum value is 8.4 mm/s

13. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Emission Model and snapshot at 3000 s, where three shear sources and one tensile source are active. The star indicates the injection point and the dashed line represents the receivers.

14. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Seismic properties Unrelaxed wave velocities and density as a function of the radial distance at 3000 s from the onset of injection.

15. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Seismograms Synthetic seismogram (a) and time history at the two receivers indicated with a V letter (b); receiver 1 (solid line) and receiver 2 (dashed line).

16. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Time histories Time histories recorded at receiver 1. The media are lossless (left) and lossy (right).

17. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Snaps and events Snapshot (a) and seismogram (b) corresponding to three sources activated at different onsets. The location of the sources are indicated by stars and the seismic events are labeled by the source that has generated them. The dashed line are the receivers.

18. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Reverse time migration Reverse-time migration images at different back propagation times, where the wave field has been focused at each source location. The numbers indicate the sources.

19. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Event time Wave field maxima at the images obtained by reverse-time migration as a function of the back propagation time. The numbers correspond to the previous figure.

20. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Conclusions We propose a simple analytical model to describe the pore-pressure build-up in a layer due to the injection and describe the emission of seismic events due to the generation of micro-cracks based on a criterion that takes into account the stiffness moduli of the host rock. A poroelastic model allows us to obtain the hydraulic diffusivity and permeability of the formation on the basis of the location and onset time of the seismic events. We then introduce a realistic forward modeling algorithm to simulate P- and S-wave propagation, where each source strength and radiation pattern is determined by the pore pressure and a generalized moment-tensor theory, respectively. Finally, we propose an algorithm to map the location of the multiple sources, approximating the CO 2 cloud, based on a reverse-time migration algorithm and an imaging condition, where optimal focusing (maximum amplitude) of the wave field back propagated in time occurs.

21. Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS ( Italy) Conclusions