Reservoir heterogeneity reflected by permeability variation in the vertical direction is expected to significantly impact on the subsurface multiphase flow behaviour. In this context, we have shown previously that during immiscible flooding the crossflow between low and high permeability zones plays a significant role in determining the reservoir performance in terms of the hydrocarbon yield. In this manuscript, the contribution of crossflow to oil recovery in layered sandstone porous media during miscible CO2 flooding is explored. We conducted core flooding experiments using a core sample constructed by attaching two axially split half sandstone plugs each with a different permeability (0.008 and 0.1 (μm)2). The crossflow between the two layers was controlled by placing either a lint-free tissue paper or an impermeable Teflon sheet to represent a layered heterogeneity with and without communication, respectively. Additionally, to better understand the underpinning mechanisms influencing the flood performance, we imaged the samples during flooding using a high-resolution medical X-Ray computed tomography (XCT) scanner.

Our results show that core-scale heterogeneity would indeed play an important role in determining the spatial distribution of the injected CO2during miscible flooding, consequently the oil recovery factor. For instance, our results confirm that permeability heterogeneity in vertical direction would lead to CO2 establishing a prefrential flow path through the high permeability layer leading to its early breakthrough. The above-mentioned CO2 channeling is clearly evident from the X-ray images captured during flooding. However, a reasonble amount of CO2 would still enter the low permeability layer contributing positively to the ultimate oil recovery factor. In fact, the post-processing of the XCT data confirmed the above to take place when cross-layer communication was allowed. The diversion of CO2 from the high to low permeablity layer is believed to be due to the crossflow phenomenon (induced by the viscous and dispersion forces) resulting in a subtle increase (i.e. 1.7%) in the ultimate oil recovery. In a similar study we have done about immiscible flooding, the contribution of crossflow to the overall recovery was found to be about 5%. The less pronounced effect of crossflow under miscible conditions is believed to be due to the absence of capillarity as a more effective driving force behind crossflow. To the best of our knowledge, our core-flooding results as presented in this manuscript and backed by X-ray CT visualisation, are the first set of their kind. They are insightful and would be of interest to the scientific community in revealing how crossflow may control flow behaviour in heterogeneous sandstone reservoirs, with important implications for numerical modelling of CO2 injection.

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