This document outlines a study to assess the detectability of migrating carbon dioxide (CO2) using seismic modeling. It finds that the detectability depends on factors like the saturation distribution of CO2 and rock properties. Uniform CO2 saturation leads to a velocity decrease of 200 m/s while patchy saturation results in a smaller 50 m/s decrease. Forward modeling shows structurally trapped CO2 is clearly detectable but migrating CO2 fronts are only weakly visible. Detectability is also influenced by the reservoir properties, relative permeability curves, and heterogeneity from capillary trapping. Careful site characterization of factors impacting CO2 migration and storage is important for monitoring survey design to enable detection of containment loss.

1. Detectability of free-phase
migrating CO2
A rock physics and seismic modelling feasibility study
Rami Eid, Anton Ziolkowski, Mark Naylor, Gillian Pickup

2. Outline
 Introduction
 Motivation
 Methodology
 Fluid flow modelling
 Rock physics modelling
 Seismic forward modelling
 Conclusions

3. Introduction
 CO2 monitorability
 Ability to detect structurally trapped CO2
successfully demonstrated
 Detection of a migrating front is
moderately understood
 Results in multi-phase fluid distributions
of different compressibilities
 Seismic response depends on fluid type,
saturation and distribution
Primary seal
Secondary reservoir
Secondary seal
Overburden
Primary reservoir
Storage complex
Reservoir-seal pair
Storage site
Free-phase
migration
Migration
Leakage

4. Motivation
 Understanding the range of variables
which affect the detectability of a
migrating front of CO2
 Interplay between geology, geophysics
and petrophysics
 Develop a workflow to aid in the
detection a loss of containment
 Provide operators with an early warning
system
Primary seal
Secondary reservoir
Secondary seal
Overburden
Primary reservoir
Storage complex
Reservoir-seal pair
Storage site
Free-phase
migration
Migration
Leakage

5. Methodology
Assess the expected range of velocities
 Patchy vs Uniform saturation distribution
2D elastic finite-difference wave
equation modelling
Front of vertically
ascending plume

6. Numerical flow modelling
CO2 Saturation

7. Rock physics modelling - theory
 Predict the change in elastic properties of the migrating
front
 Gassmann’s equation: assumes immiscible and
homogeneously distributed phases throughout
 Migrating CO2 is spatially heterogeneous, resulting in
partial fluid saturation
 Result in two fluid-saturation end-members;
patchy and uniform distribution
 Related to hydraulic diffusivity and diffusion length
 suggests the spatial scales over which pore-pressure can
equilibrate during a seismic period
 Obvious consequences for seismic velocity and impedance

8. Rock physics modelling - theory
Compressibility of CO2 directly affects reservoir
seismic velocity
 Vp is heavily dependent on the model used.
 Uniform saturation, Gassmann-Reuss
 Sufficient time for wave induced pressure oscillations to
flow and relax – less stiff porous rock

9. Rock physics modelling - theory
Compressibility of CO2 directly affects reservoir
seismic velocity
 Vp is heavily dependent on the model used.
 Uniform saturation, Gassmann-Reuss
 Sufficient time for wave induced pressure oscillations to
flow and relax – less stiff porous rock
 Patchy saturation, Gassmann-Hill
 Not enough time for wave-induced pore pressure
equilibrium during seismic period
 patches of rock remain at different pressures
 increase in material stiffness
 predicts higher velocities

10. Rock physics modelling - theory
Compressibility of CO2 directly affects reservoir
seismic velocity
 Vp is heavily dependent on the model used.
 Uniform saturation, Gassmann-Reuss
 Sufficient time for wave induced pressure oscillations to
flow and relax – less stiff porous rock
 Patchy saturation, Gassmann-Hill
 Not enough time for wave-induced pore pressure
equilibrium during seismic period
 patches of rock remain at different pressures
 increase in material stiffness
 predicts higher velocities
 Modified-patchy
 Saturations constrained by rel-perm curves, limits for Swir

11. Compressibility of CO2 directly affects reservoir
seismic velocity
 Vp is heavily dependent on the model used.
 Uniform saturation, Gassmann-Reuss
 Sufficient time for wave induced pressure oscillations to
flow and relax – less stiff porous rock
 Patchy saturation, Gassmann-Hill
 Not enough time for wave-induced pore pressure
equilibrium during seismic period
 patches of rock remain at different pressures
 increase in material stiffness
 predicts higher velocities
 Modified-patchy
 Saturations constrained by rel-perm curves, limits for Swir
Highlights
 range of velocities which could be encountered
 importance of determining the most
appropriate model
Rock physics modelling - theory
Migrating
plume front

12. Rock physics modelling - application
 Elastic properties of a migrating front
 Assume both Modified-patchy and Uniform saturation
Uniform Saturation
 Change in velocity
of -200 m/s at the
migrating front
Mod-patchy saturation
 Change in velocity of
-50 m/s

13. Seismic forward modelling
 2D finite-difference wave equation modelling
 Single line towed streamer survey
Baseline survey
Monitor survey
Time-lapse

14. Seismic forward modelling
Secondary reservoir
Intraformational seal
Primary reservoir
Vp [m/s]
Baseline survey

15. Seismic forward modelling
 Monitor survey
 Amplitude changes  Time shifts
 Structurally trapped CO2
 Migrating CO2 front
 High amplitude change at 1780 m
 Velocity push-down at 2200 m
 No obvious change in amplitude
Uniform saturation model Modified-patchy saturation model

16. Seismic forward modelling
 Geometry of structurally trapped CO2
 Clear push-down below plume
 Weak amplitude difference at the migrating
front
 Time-lapse seismic sections
 Reveal details not easily observed in
monitor section alone
 Slight push-down below plume
 Very weak amplitude change at the
migrating front
Time-lapse uniform saturation model Time-lapse modified-patchy saturation model

17. Seismic forward modelling
 Geometry of structurally trapped CO2
 Clear push-down below plume
 Weak amplitude difference at the migrating
front
 Time-lapse seismic sections
 Reveal details not easily observed in
monitor section alone
 Slight push-down below plume
 Very weak amplitude change at the
migrating front
Time-lapse uniform saturation model Time-lapse modified-patchy saturation model
Clean sandstone model
 Potential for detecting a migrating front?

18. Conclusions
 Whilst this may seem like a negative result, CO2 plume rising in clean sandstone is the
hardest end-member to detect
 Factors impacting on detectability:
 Phase of CO2
 Relative-permeability curves
 Heterogeneity: capillary trapping
 Knowledge of migration hotspots
 For site appraisal, it is important to assess factors that
1. affect not only storage security and capacity, but
2. factors that will lead to more favourable detection of a loss of containment
Design and construction of monitoring
surveys – aiding in the detection of a
loss of containment