1. 100 years of living science Date Location of Event Development of Gas/Oil Miscibility in Water and Gas Injection Tara LaForce, and Franklin M. Orr, Jr. (Stanford) Second Nature Sample Poster Dec. 3, 2008

2. Abstract We use analytical solutions for a one-dimensional model to study water and gas injection strategies into an oil reservoir. The model accounts for three-phase flow of four components through porous rocks and uses a cubic equation of state to model gas/oil phase behavior. In oil reservoirs super-critical CO 2 may generate a multi-contact miscible displacement of hydrocarbons, so both miscible and immiscible injection of CO 2 is studied. We consider the implications of using CO 2 in order to improve oil recovery, and find that this is often a competing goal with storing CO 2 far from production wells. –Mixtures which create efficient miscible displacements result in earlier breakthrough of injected CO 2, while mixtures that have delayed arrival of CO 2 in production wells have less favorable oil recovery.

3. Background Depleted oil reservoirs are excellent candidates for CO 2 storage. When CO 2 is stored in oil reservoirs it is of critical importance to maximize the reservoir volume containing CO 2. CO 2 can behave as a miscible gas in many reservoirs, which means displacement efficiency of hydrocarbons is optimal Injecting water simultaneously with CO 2 in aquifers has been shown to increase the volume of CO 2 that can be stored as a trapped phase because injected water inhibits the ability of CO 2 to rise under buoyancy forces (SPE109905). In an oil reservoir, a similar result is likely. Moreover, if there are active wells in the reservoir then injection of water to trap CO 2 will prevent excessive gas cycling. But can we store CO 2 securely and improve oil recovery at once?

4. Displacement Efficiency Can be thought of as flow along a streamline in the reservoir This is a 1D problem From SPE 97270 D Gas Saturation 1 0

5. CO 2 Injection in 2D and 3D The drawback in injecting CO 2 alone is that the sweep efficiency is poor By injecting water and gas (WAG) simultaneously, it is possible to get good sweep efficiency AND optimal displacement efficiency … or is it? From SPE 97270 From Al-Shuraiqi et al. 12th European Symposium on IOR

6. Phase behavior in ternary subsystems at MMP (three-phase region shown in light blue) Phase behavior in full system at MMP (only three-phase regions are shown) Vaporizing Gas Drive Phase Behavior Peng-Robinson EOS is used to model the gas/oil phase behavior Henrys Law is used to model hydrocarbon/aqueous phase partitioning Water does not partition into gas or oil

7. A B C D E Initial Condition Injection Conditions shock rarefaction Vaporizing Gas Drive at MMP Injection of CO 2 into a mixture of C 3 and C 16 results in a miscible vaporizing gas drive. Water/CO 2 mixtures C-E behave like miscible gas drives, but A and B are immiscible. Injection of water/CO 2 mixtures have miscible displacements as long as injected CO 2, is more mobile than water and flows ahead of injected water. Mobility of the CO 2 is determined by the relative permeabilities of the water and CO 2 phases. Volume of CO 2 injected: E=100% D=25% C=19.84% B=15% A=0% A B C D E

8. shock rarefaction D /t D Tertiary Vaporizing Gas Drive JC = 75% water Water in the initial condition causes the miscible front to accelerate: If no water is present initially then the MGF = 19.84% If 12% residual water is present initially then the MGF = 20.77% If 25% water is present initially then the MGF is about 22.17% Initial Conditions

9. Results and Conclusions In SWAG displacements a minimum gas fraction of CO 2 must be injected to create a miscible gas bank Miscible displacements that have optimal displacement efficiency result in an unstable displacement and early gas breakthrough Relative permeabilities determine whether miscibility will be achieved The failure to achieve miscibility will allow the design for injection strategies that will effectively trap the CO 2 in the reservoir A high fraction of CO 2 can be injected in a water-flooded reservoir without creating a fast-moving miscible gas bank that will quickly reach production wells This represents directly competing goals between optimizing oil recovery with a miscible gas flood and storing the CO 2 far from production wells

10. Many Thanks To: Grantham Institute for Climate Change at Imperial College London Shell Grand Challenge on Clean Fossil Fuels UK Engineering and Physical Science Research Council