Retention of CO2-Foaming Agent in CO2-Foam Flooding of HeterogeneousCarbonate Reservoirs

摘要

The efficiency of carbon dioxide (CO2)-flooding in highly heterogeneous reservoirs may be low due to poor macroscopicsweep. One of the possible ways to increase macroscopic sweep efficiency is by reducing the CO2 mobility by foam. Retentionof the foaming agent during CO2-foam flooding is an important aspect affecting both technical and economical potential of theprocess. This paper presents both experimental and numerical simulation studies performed to investigate the retention of CO2-foamingagents in heterogeneous carbonate reservoirs. These reservoir types consist of high permeable "thief zones" - fractures or highpermeable layers and the rest of the reservoir - matrix or low permeable layers. The retention of CO2-foaming agent wasstudied in flow-through laboratory experiments with CO2-foaming agents; branched ethoxylated sulphonates with differentethoxylation degree in outcrop Liege chalk rock at 55oC. The retention of the foaming agents was determined by breakthroughcurves based on chromatographic separation between the foaming agent and a tracer. The flow-through retention experimentswere 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 agentand oil saturation. It is concluded based on simulation results that the amount of the foaming agents retained in heterogeneousreservoirs would strongly depend on its transfer mechanism from high into low permeable zones. The diffusion process is slowand therefore the foaming agent might penetrate only into a certain fraction of the matrix blocks or low permeability layersduring the project life time. The retention of the foaming agent in these heterogeneous carbonate reservoirs may therefore bemuch lower than estimated based on viscous floods. A numerical model describing diffusion and retention by adsorption was established. A good match between experimental andmodeling results was obtained without individually tuning the numerical model for each specific experiment. Additionalsimulation results showed that on the field scale the amount of foaming agent retained in heterogeneous reservoirs will dependon the number of high permeable zones and the size of the matrix blocks.