New insight into CO2 huff-n-puff process for extraheavy oil recovery via viscosity reducer agents: An experimental study

In this paper, extraheavy oil recovery assisted by CO2 and an oil-soluble viscosity reducer under deep reservoir conditions was experimentally investigated. Specifically, the viscosity reduction efficiency of the oil-soluble viscosity reducer was measured, and the morphological characteristics of the asphaltenes in the heavy oil were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). CO2 huff-n-puff experiments were carried out to determine the oil recovery factor and the oil/gas exchange ratio. The properties of the produced oil were tested, and the oil saturation in cores was scanned through nuclear magnetic resonance (NMR). The experimental results show that the oil-soluble viscosity reducer decreases the viscosity of extraheavy oil by decreasing the asphaltene concentration and the size of the asphaltene aggregations in the extraheavy oil. CO2 huff-n-puff assisted by an oil-soluble viscosity reducer can considerably improve the oil recovery of extraheavy oil. When the concentration of the viscosity reducer increases from 0 to 10 wt%, the oil recovery factor after five cycles increases from 18.74 % to 34.44 %, and the foamy oil effect is stimulated. An economic concentration range of 1-3 wt% is suggested for field CO2 huff-n-puff applications. With increasing CO2 huff-n-puff cycles, the cyclic oil recovery factor and oil/gas exchange ratio sharply decrease. The oil recovery and oil/gas exchange ratio of the first two cycles are considerably higher than those of the last three cycles. The oil saturation tested by NMR shows that the sweep area of CO2 huff-n-puff is limited to the first twothirds of the core, indicating that the injection volume of CO2 should be increased with each successive cycle, and the well distance should be suitably decreased to improve the oil recovery of the whole reservoir.