The efficiency of carbon dioxide (CO2)-flooding in highly heterogeneous reservoirs may be low due to poor macroscopic sweep. One of the possible ways to increase macroscopic sweep efficiency is by reducing the CO2 mobility by foam. Retention of the foaming agent during CO2-foam flooding is an important aspect affecting both technical and economical potential of the process.

This paper presents both experimental and numerical simulation studies performed to investigate the retention of CO2-foaming agents in heterogeneous carbonate reservoirs. These reservoir types consist of high permeable "thief zones" - fractures or high permeable layers and the rest of the reservoir - matrix or low permeable layers. The retention of CO2-foaming agent was studied in flow-through laboratory experiments with CO2-foaming agents; branched ethoxylated sulphonates with different ethoxylation degree in outcrop Liege chalk rock at 55°C. The retention of the foaming agents was determined by breakthrough curves based on chromatographic separation between the foaming agent and a tracer. The flow-through retention experiments were reproduced in a numerical simulator.

The results indicate that the retention (the amount of foaming agent lost per rock weight) depends on the type of foaming agent and oil saturation. It is concluded based on simulation results that the amount of the foaming agents retained in heterogeneous reservoirs would strongly depend on its transfer mechanism from high into low permeable zones. The diffusion process is slow and therefore the foaming agent might penetrate only into a certain fraction of the matrix blocks or low permeability layers during the project life time. The retention of the foaming agent in these heterogeneous carbonate reservoirs may therefore be much lower than estimated based on viscous floods.

A numerical model describing diffusion and retention by adsorption was established. A good match between experimental and modeling results was obtained without individually tuning the numerical model for each specific experiment. Additional simulation results showed that on the field scale the amount of foaming agent retained in heterogeneous reservoirs will depend on the number of high permeable zones and the size of the matrix blocks.

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