Abstract Oil recovery during CO2 injection into a thick and/or fractured reservoir will be limited as a result of viscous fingering and gravity override. Due to density differences between the injected CO2 and resident fluids in the reservoir, the CO2, being lighter, tends to rise to the top of the reservoir thereby bypassing some of the remaining oil. In order to study the impact of reservoir heterogeneity on oil recovery by seawater and CO2 flooding, this paper, for the first time, investigates the use of a dual core flooding apparatus to investigate the effect of both CO2 gravity override and permeability contrast on oil recovery performance by CO2 injection. Experimental investigation of different oil recovery schemes, including secondary and tertiary oil recovery processes, was conducted using dual core holders with different permeable carbonate composite stacks. The core holders were placed in parallel horizontally and contained a high permeable core plug (HPCP) and low permeable core plug (LPCP). The permeability ratio of HPCP to LPCP was 50 to 1 with the HPCP core holder placed above the LPCP core holder. Core flooding experiment was conducted at reservoir conditions with live reservoir fluids at a pore pressure of 3200 psi, temperature of 102oC and confining pressure of 4500 psi. Using this experimental setup, various experiments were conducted to determine the oil recovery performance as a function of injection rates, seawater/CO2 injection modes, slug volume, and diversion of CO2 by plugging the HPCP. Experimental procedures are provided for conducting these experiments that has the potential to become a gold standard for such studies. Results based on this study have shown that CO2 injection following waterflooding resulted in additional oil recovery, as expected. However, the amount of this recovered additional oil was dependent on initial core plug permeability, injection mode and CO2/seawater slug volume. It was observed that waterflooding recovered more oil from high permeable core plugs, compared to the tighter core plug. On average, seawater left considerable more remaining oil in LPCP, which indicated the poor performance of waterflooding in formations with high permeability contrast. The remaining oil in the LPCP was mobilized by plugging the HPCP using a diversion technique and a subsequent CO2 flood. This paper provides detailed description of the effect of different mechanisms of flooding with seawater and supercritical CO2 on recovering this additional oil from LPCP. The results bode well for CO2-EOR projects and will lead to further oil recovery potential beyond the normal CO2 flood.


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