Abstract
Residual oil distribution plays a critical role in understanding of the CO2flooding processes, but its quantitative research for reservoirs with different permeability levels rarely has been comprehensively conducted in the laboratory. This article presents the results of an experimental study on the immiscible CO2displacement efficiency in different permeability core samples and various oil distribution patterns prior to and after immiscible CO2flooding. Experiments were conducted on four core samples extracted from the selected oil field with a permeability range from 0.210–66.077 mD. The experimental results show that the immiscible CO2can mobilize oil in ultralow-permeability environment and achieve a reasonable displacement efficiency (40.98%). The contribution of different oil distribution patterns to displacement efficiency varies in reservoirs with different permeabilities. With the increase of core permeability, the contribution of cluster and intergranular pore oil distribution patterns to displacement efficiency increases. However, the oil displacement efficiency of corner and oil film patterns tends to increase with lower permeability. Therefore, immiscible CO2flooding is recommended for ultralow-permeability case, especially for reservoirs with larger amount of oil in corner and oil film distribution patterns. The oil displacement efficiency calculated by immiscible CO2flooding experiment results agrees reasonably well with the core frozen slices observation. The results of this study have practical significance that refers to the effective development of low-permeability reservoirs.
Highlights
The onshore proven undeveloped oil reserves in China are mainly low-permeability and ultralowpermeability reservoirs (Zhao et al, 2011; Duan et al, 2014)
Residual oil distribution plays a critical role in understanding of the processes which take place within a reservoir during and after CO2 flooding, and many laboratory studies have been performed to evaluate the residual oil distribution, very limited effort has been focused on its quantitative research at the core scale considering different permeability levels
Wei et al (2021) applied the nuclear magnetic resonance (NMR) technology to carry out experimental research, in which the microscopic oil displacement mechanism in low-permeability reservoirs was discussed, but they focused on five different enhanced oil recovery (EOR) methods including CO2 flooding
Summary
The onshore proven undeveloped oil reserves in China are mainly low-permeability and ultralowpermeability reservoirs (Zhao et al, 2011; Duan et al, 2014). Druetta and Picchioni (2020) developed a numerical model to investigate the mechanisms and effectiveness of polymer flooding on the residual oil after waterflooding, and they validated their results by comparing them with the experiments carried out by other authors They conducted numerical simulation, it still provided some perspectives on the experimental investigation of residual oil distribution. Wang et al (2020) conducted an investigation on the multiphase flow characteristics and EOR mechanism of water-alternating gas (WAG) injection after continuous CO2 injection at the microscale using CT scanning and microelectronic photolithography They concluded that WAG injection after continuous CO2 injection increased oil recovery by 23.15%, which was dominated by the first and second WAG injection cycles. Wei et al (2021) applied the NMR technology to carry out experimental research, in which the microscopic oil displacement mechanism in low-permeability reservoirs was discussed, but they focused on five different EOR methods including CO2 flooding. This research is dedicated to bridge the existing gap in our knowledge by following a systematic experimental approach and improve fundamental understanding with which it will be possible to develop better production techniques and improved CO2 EOR models
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