With the increasing demand for energy production and environmental concerns, the focus has been shifting towards new green recovery techniques for heavy oil complex reservoirs such as naturally fractured carbonate reservoirs (NFCRs). After the success of VAPEX as an alternate to SAGD for heavy oil recovery, it has been studied for fractured reservoirs in recent years. Moreover, CO2 has been a successful EOR agent in fractured light-medium oil reservoirs and thus can be used as a higher soluble, low cost, and environment friendly alternate to solvent. Our aim is to show that CO2 with VAPEX could be a feasible choice for NFCRs. This work is focused on investigating the applicability and optimization of CO2 based solvent extraction process for heavy oil recovery and greenhouse gas sequestration from NFCRs.

A series of numerical experiments were performed to study the role of a) solvent type, b) operating condition, c) oil and gas diffusion, and d) extraction process; in various single/multiple fracture configurations while injecting CO2 - solvent in different ratios. The optimal injection rate and CO2 - solvent concentration ratio were obtained considering maximum oil recovery and CO2 storage. A parametric study was also performed to analyze the influence of various operational and reservoir parameters such as fracture properties, fracture configurations, oil viscosity, solvent type, and injection/production constrains. The results suggested that CO2 or conventional VAPEX alone may not be able to achieve higher recovery in high pressure NFCRs, but CO2 based VAPEX can provide efficient technique to recover heavy oil with emphasis on sequestration.

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