1. Impact of capillary trapping on geological CO2 storage
Martin Blunt, Branko Bijeljic, Tara C LaForce, Stefan Iglauer, Ran Qi, Saleh Al-Mansoori, Chris Pentland and Erica Thompson

2. Outline Field scale: Streamline Simulation
Core scale: Column Experiment
Pore scale: CT scan

3. Background
Long term fate, how can you be sure that the CO2 stays underground?

4. Field scale - The streamline method
Permeability field
Initial saturation
Pressure solve
SL tracing
Saturation along SL
Saturation for the next time step

5. Streamline method for CO2 transport
Phases (3)
Components (4)
Hydrocarbon
Aqueous
Solid
CO2
Oil
Water
Salt +
Hydrocarbon phase
Aqueous phase
Todd&Longstaff
Fingering model for CO2 in oil

6. Streamline method for CO2 transport
Trapping model
Pore-scale model matches experimental data.
Kr is from Berea sandstone, which matches Oak (1990)’s
experiments.
CO2/water system is weakly water-wet (Chiquet et al., 2007)
contact angle (θ) = 65º.
New trapping model (Juanes et al., 2006)

7. Design of carbon dioxide storage
Mobility ratio between carbon dioxide/brine mixture and formation brine
Mobility ratio between chase brine and carbon dioxide/brine mixture during chase brine injection
Mobility ratio = 1.0
fgi
The ratio of the mobility of injected brine and CO2 to the formation brine as a function of the injected CO2-phase volume fraction, fgi.

8. Design of carbon dioxide storage
1D analysis: Numerical simulation vs. analytical solution
fgi = 0.5
fgi = 0.85

9. Design of carbon dioxide storage
Mobile CO2 saturation Z
170m X
3200m Y
2280m
Trapped CO2 saturation
Injector
Producer
SPE 10 reservoir model, 1,200,000 grid cells (60X220X85), 7.8 Mt CO2 injected.
Two years after chase water injection for fgi=0.85.

10. Design of carbon dioxide storage
3D simulation: Storage efficiency vs. trapping efficiency
Trapping efficiency =
the fraction of the injected mass of CO2 that is either trapped or dissolved
Storage efficiency =
the fraction of the reservoir pore volume filled with CO2
The storage efficiency is highest for fgi = 0.85, which also requires minimum mass of chase brine to trap 95% of CO2.

11. Design Criterion Inject CO2+brine where mobility ratio = 1.0
(fgi=0.85 in this example).
Inject chase brine that is 25% of the initially injected CO2 mass.
90-95% of the CO2 is trapped.

12. Issues arising from field scale simulation
Streamline-based simulator has been extended to model CO2 storage in aquifers and oil reservoir by incorporating a Todd-Longstaff model, equilibrium transfer between phases (dissolution) and rate-limited reaction;
Trapping is an important mechanism to store CO2 as an immobile phase. Our study showed that WAG CO2 injection into aquifer can trap more than 90% of the CO2 injected;
We have proposed a design strategy for CO2 storage in aquifers, in which CO2 and formation brine are injected simultaneously followed by chase brine.
Streamline-based simulation combined with pore-scale network modeling can capture both the large-scale heterogeneity of the reservoir and the pore-scale effects of trapping.

13. Future work Injection strategy design
Require better experimental data, since the trapping model used has a significant impact on the results.
Design of an injection strategy to maximize CO2 storage and oil recovery.

14. CT Scanning
A homogeneous sandpack was compressed and the porosity was determined via mass balance (Φ = 38,93).
n-Heptane was injected; when no more brine was produced, another CT scan was performed at the irreducible water saturation, Swi.
CO2 was injected again. Gas injection was stopped when no more liquid production was observed. Another CT scan was taken.
30 pore volumes (PV) of brine were injected and a final CT scan was taken at the residual gas saturation Sgr .
resolution 9 µm

15. Sandpack at irreducible water saturation
Brine – blue
Sand – red
Oil - orange
Oil penetrates on average mainly into the larger
pores as expected by capillary pressure
considerations.
Thin water layer is visible on the rock surface as
expected for quartz.
Oil has penetrated into the middle of some pores.

16. Sandpack at residual gas saturation
Brine –blue
Sand – red
CO2 - yellow
The largest CO2 ganglia is continuously spread over the
largest available pore.
Though overall gas accumulates in the larger pores, a random
distribution between large and medium size pores is
observable.
Several tiny gas bubbles are randomly distributed in
the pore volume. Though they might originate from
the segmentation process, it is thought that they are real.

17. Vertical column experiments – Sor vs. Soi
Sand-packed columns were oriented vertically.
5 pore volumes of de-aired brine were injected to reach full saturation.
Decane reservoir connected to top of columns and brine allowed to drain under gravity from the base. Decane enters the top of the column. No pumping.
Equilibrium reached where both columns have a (theoretically) identical oil saturation profile versus height.
One column removed for slicing and sampling – Soi.
Second column has brine injected from the base, Brine sweeps oil leaving an Sor. Coulmn sliced and sampled.
brine flow
Oil flow
COLUMN A - Soi
COLUMN B - Sor

18. Vertical column experiments – Sor vs. Soi - results