Measurement of Immiscible CO2 Flooding Processes and Permeability Reduction due to Asphaltene Precipitation by X-ray CT Imaging

Abstract

Carbon dioxide tertiary or secondary oil recovery becomes increasingly significant to the petroleum industry. CO2 flooding can not only enhance or improve oil recovery clearly but also considerably reduce greenhouse gas emission. After CO2 was injected into an oil reservoir, it contacted the reservoir oil and, consequently, changed the equilibrium conditions and fluid properties, which may lead to the precipitation of organic solids, primarily asphaltenes. In this case, the bulk porosity reduces obviously because of asphaltene deposition, which can cause permeability reduction of a tight sandstone reservoir after immiscible CO2 flooding. In this paper, a total of three CO2 coreflood tests under immiscible conditions were performed through the so-called dry, secondary, and tertiary oil recovery processes, respectively. After or before each process, the distribution of crude oil and brine (3wt% NaCl) in the core was scanned by employing X-ray CT scanning. Comparing two CT images before and after recovery processes in one test, it is found that asphaltenes deposited onto the sand grains and thus left in the reservoir, which caused permeability reduction of a tight sandstone reservoir after immiscible CO2 flooding.