Field trials and laboratory tests of CO2-foam have shown some success. A number of CO2-EOR field trials with either miscible or near miscible gas injection have shown that foam can lower the mobility of CO2 and improve the macroscopic sweep efficiency. However, not much work about CO2-foam flooding in fractured chalk reservoirs with low matrix permeability have been published in the literature. During CO2-foam flooding of fractured chalk media, it is expected that the injected fluids will flow in the high permeability fractures where foam will preferentially be formed. This will increase the local resistance to flow and divert more of the CO2 and CO2-foaming agent solution into the low permeable matrix.

This paper presents results from laboratory experiments and simulation studies carried out to investigate the effect of mode of injection on oil recovery during CO2-foam flooding of water flooded fractured chalk models at reservoir conditions. The fractured model was created by drilling a hole through the centre of the plug and homogeneously packing it with glass beads. The CO2-foam process was conducted by injecting pre-formed foam either horizontally or vertically into the fractured models at 340 bar and 55°C. Pure CO2 and WAG processes were also carried out on similar fractured models to establish base case behaviours at the same conditions.

Laboratory results show that additional oil after water flooding is recoverable by injecting pre-formed foam either horizontally or vertically (from the top) in the CO2-foam processes when compared to the injection of pure CO2 and CO2-WAG at the same effective permeability and CO2 volume. In the simulation studies carried out with CMG's compositional reservoir simulator STARS on a similar fractured model used in the laboratory experiments, the results from the core floods were quantitatively confirmed. Additional simulation carried out in large matrix blocks on the field scale showed that CO2-foam can recover additional oil at reservoir conditions. However, the adsorption of the foaming agent can make the CO2-foam process inefficient.

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