1. Seismic Plume Evolution in a
Heterogeneous Sandstone Reservoir:
Role and Impact of Patchy CO2 Saturation
Rami Eid, Anton Ziolkowski, Mark Naylor, Gillian Pickup
3. Introduction
CO2 monitorability
Monitor, measure and validate injected CO2
Identify and quantify any movement
Ability to detect structurally trapped CO2 successfully demonstrated
Ability to image a free-phase migrating front is not well understood
Uncertainties regarding the pore-scale distribution of fluids and the most
appropriate rock-physics model to simulate this
Seismic response depends not only on fluid-type but also on spatial
distribution of phases. End-member models are used
Understanding the range of pore-fluid saturation scales and the
phase distributions which could be encountered is key
4. Methodology
Investigate range of seismic responses due to injected CO2
Three-stage model-driven workflow
Heterogeneous sandstone reservoir
5. Geological model
Bunter Sandstone Formation
UK sector of the North Sea
Previously identified as having potential to
store large amounts of CO2
Heterogeneity - geophysical log analysis
Dome A
Saline reservoir, 1200m deep
4-way dip closed structure
Formation partitioned into five zones
Obstacles to migration
(Modified from Williams et al., 2013)
6. Fluid-flow modelling
Permedia’s BOS
Single well, 20m perforation interval, 0.1MT/year, 20 years
Bennion and Bachu (2006) Cardium Sandstone rel-perm curves
Duan and Sun (2003) EOS
8. Rock physics modelling
Predict the change in elastic properties as a result of the injected CO2
Gassmann’s equation: assumes immiscible and homogeneously distributed
phases throughout
CO2 injection disrupts reservoir equilibrium, partial fluid saturation
Two fluid-saturation end-members; patchy and uniform
Related to hydraulic diffusivity and diffusion length
suggests the spatial scales over which pore-pressure can equilibrate during a
seismic period
𝐿 𝑐 =
𝑘𝐾𝑓𝑙
𝑓𝜂
Critical length scale
• Length scale over which fluid
phases interconnect
9. Rock physics modelling
Uniform saturation
Microscopic scale fluid distribution, d < 𝐿 𝑐
Sufficient time for wave induced pressure
oscillations to flow and relax
Less stiff porous rock
Assumes homogeneous saturated region, Reuss
average
Patchy saturation
Mesoscopic scale heterogeneity, d > 𝐿 𝑐 < λ
Not enough time for wave induced pressure
oscillations to flow and relax
Patches of rock at different pressures
Increase material stiffness, higher velocities
Unrelaxed state, Hills constant shear modulus
equation
Modified-patchy
Saturations constrained by rel-perm curves,
limits for Swir
10. Rock physics modelling
Uniform saturation
Microscopic scale fluid distribution, d < 𝐿 𝑐
Sufficient time for wave induced pressure
oscillations to flow and relax
Less stiff porous rock
Assumes homogeneous saturated region, Reuss
average
Patchy saturation
Mesoscopic scale fluid distribution, d > 𝐿 𝑐
Not enough time for wave induced pressure
oscillations to flow and relax
Patches of rock at different pressures
Increased material stiffness, higher velocities
Unrelaxed state, Hills constant shear modulus
equation
Modified-patchy
Saturations constrained by rel-perm curves,
limits for Swir
11. Rock physics modelling
Uniform saturation
Microscopic scale fluid distribution, d < 𝐿 𝑐
Sufficient time for wave induced pressure
oscillations to flow and relax
Less stiff porous rock
Assumes homogeneous saturated region, Reuss
average
Patchy saturation
Mesoscopic scale fluid distribution, d > 𝐿 𝑐
Not enough time for wave induced pressure
oscillations to flow and relax
Patches of rock at different pressures
Increased material stiffness, higher velocities
Unrelaxed state, Hills constant shear modulus
equation
Modified-patchy
Saturations constrained by rel-perm curves,
limits for Swir
12. Rock physics modelling
Represent upper and lower bounds of
seismic velocity as function of SCO2
Assume single rock facies, with
homogeneous lithology
𝐾 𝑚, 𝐾 𝑑𝑟𝑦 and 𝜑 are uniform in space
Not valid for all reservoir conditions
Heterogeneous models
Wide range of possible velocity distributions
Important to model range which could be
encountered
16. Time-lapse seismic
Negative impedance contrast corresponding to structurally trapped CO2
Three main reflectors – three main CO2 accumulations
Patchy model – subtle change in amplitude – related to saturation of CO2
Important implications for CO2 quantification
Free-phase migrating front – very minimal to no change in amplitude
Not only dependent on pore-fluid distribution, but also on spatial geometry of
the front
17. Conclusions
Synthetics highlight key differences between patchy and uniform saturation
distribution models
Comparison shows clear difference in amplitude and time-shift
Plume detected using both, each accumulation below baffles interpreted
Migrating front difficult to detect – geometry threshold?
Patchy model
Easy to distinguish CO2 accumulations within each zone
Subtle changes in amplitude related to CO2 saturation
Great implications for CO2 detectability and quantification
Factors affecting detectability
Phase and distribution of CO2
Relative-permeability curves
Heterogeneity
Site specific variations:
Application of detectability workflow
Initial storage-site assessment stage
Provide valuable information regarding
CO2 detectability
Each interpreted zone plays an important role in the growth and evolution of the injected CO2. The zones provide obstacles to migration, allowing for accumulations beneath each barrier and allowing for an assessment of the potential of seismic techniques to detect them.
Valid in systems which have come to equilibrium over geological timescales. CO2 injection disrupts this and results in nonuniform phase distributions with spatially varying saturations.
Choice of model depends on Lc.
