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In horizontal displacement, mobility control is very important. Because the viscosity of CO2 at reservoir conditions is much lower than that of most oils, viscous instability will limit the sweep efficiency of the displacement and, therefore, oil recovery. In addition, reservoir rock is heterogeneous, exhibiting zones of high permeability in close proximity to those of low permeability.
Reservoir heterogeneity and the adverse effects of CO2 viscosity must be contended with to optimize oil recovery. Two basic strategies have been developed by the petroleum to mitigate these effects:
- 1. Alternate injection cycles of CO2 and water (water alternating gas or WAG process). This technique forms sequential banks of fluids in the reservoir rock: oil, CO2 and water, that migrate from the injection to the production wells (see Figure 4). Reservoir management may include the following steps:
- Ensure reservoir pressure exceeds the MMP in all areas of the field.
- Ensure fluid injection rates balance (or exceed) fluid withdrawal rates at both pattern and field levels.
- Start WAG process when first breakthrough of CO2 is observed.
- Reduce CO2 injection and increase the WAG ratio as the flood matures.
- Continually optimize the WAG process at a pattern level.
Note: In low permeability reservoirs, alternate injection of CO2 and water may seriously reduce injection rate.
2. Addition of chemical agents, such as: ethoxylated and/or unethoxylated species, fluroacrylate-styrene copolymers, lignosulfonates, etc, to CO2 to form stable foams that increase its viscosity without compromising its efficacy (Meyer, 2007).
It is interesting to note that because of high CO2 costs and lack of process control, older CO2-EOR projects injected relatively small quantities of CO2, typically 0.20-0.4 hydrocarbon pore volumes. Today, up to 1.5 pore volumes are being planned to improve sweep efficiency and mobilize more residual oil.
Figure 5 is a plot of areal sweep efficiency, EA versus mobility ration, M, for various displaceable reservoir pore volumes, VpD, injected. Some useful observations can be made here:
- Reservoir sweep efficiency at CO2 breakthrough (lower boundary of the plot) increases as the mobility ratio decreases. This shows the importance of WAG and viscosity enhancers for mobility control.
- For any given mobility ratio, sweep efficiency continues to improve (i.e. addition oil is recovered) after breakthrough, as more pore volumes of CO2 are injected. It is expected, therefore, that provisions must be made for recycling of produced CO2.