Insights into immiscible supercritical CO2 EOR: An XCT scanner assisted flow behaviour in layered sandstone porous media

Abstract

Reservoir heterogeneity and possible associated crossflow between low and high permeability zones may trigger intrinsic uncertainties to manage and predict reservoir performance. To understand the contribution of crossflow to oil recovery during immiscible CO2 flooding, we conducted core flooding experiments using layered core samples with the aid of a high-resolution medical X-Ray computed tomography (XCT) scanner. Our results showed that core scale heterogeneity plays a significant role in dictating the spatial distribution of injected CO2 during flooding thus the oil recovery factor. The experimental results reveal fluid distributions during flooding to be correlated reasonably well with the permeability of each layer. In other words, the higher permeability region of the rock sample contained a higher CO2 saturation at all times compared with the lower permeability one. X-ray images have facilitated and visually demonstrated the channelling of CO2 into the high permeability layer leaving a considerable amount of bypassed oil in the low permeability one. We also observed that crossflow indeed arises between layers of the heterogeneous cores suggesting that capillarity plays a noticeable role in increasing oil recovery (i.e. 4.8%). Three dimensional XCT reconstruction of cores together with 2D cross sectional images validated the occurrence of crossflow that would divert CO2 flow from the high permeability to low permeability zone enhancing oil recovery. The images show that the diverted CO2 into the low permeability layer would be trapped by associated trapping mechanisms (e.g. snapped-off) and would not form a continuous phase along the length of the sample.